<?php

use Drupal\Component\Utility\Crypt;
use Drupal\Component\Utility\Json;
use Drupal\Component\Utility\String;
use Drupal\Component\Utility\Tags;
use Drupal\Component\Utility\UrlValidator;
use Drupal\Component\Utility\Xss;
use Drupal\Core\Cache\Cache;
use Drupal\Core\Language\Language;
use Symfony\Component\DependencyInjection\Container;
use Symfony\Component\HttpFoundation\RedirectResponse;
use Symfony\Component\Yaml\Parser;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpFoundation\Request;
use Drupal\Component\PhpStorage\PhpStorageFactory;
use Drupal\Component\Utility\MapArray;
use Drupal\Component\Utility\NestedArray;
use Drupal\Component\Utility\Unicode;
use Drupal\Core\Cache\CacheBackendInterface;
use Drupal\Core\Datetime\DrupalDateTime;
use Drupal\Core\Database\Database;
use Drupal\Core\Routing\GeneratorNotInitializedException;
use Drupal\Core\SystemListingInfo;
use Drupal\Core\Template\Attribute;
use Zend\Feed\Writer\Writer;
use Zend\Feed\Reader\Reader;

/**
 * @file
 * Common functions that many Drupal modules will need to reference.
 *
 * The functions that are critical and need to be available even when serving
 * a cached page are instead located in bootstrap.inc.
 */

/**
 * @defgroup php_wrappers PHP wrapper functions
 * @{
 * Functions that are wrappers or custom implementations of PHP functions.
 *
 * Certain PHP functions should not be used in Drupal. Instead, Drupal's
 * replacement functions should be used.
 *
 * For example, for improved or more secure UTF8-handling, or RFC-compliant
 * handling of URLs in Drupal.
 *
 * For ease of use and memorizing, all these wrapper functions use the same name
 * as the original PHP function, but prefixed with "drupal_". Beware, however,
 * that not all wrapper functions support the same arguments as the original
 * functions.
 *
 * You should always use these wrapper functions in your code.
 *
 * Wrong:
 * @code
 *   $my_substring = substr($original_string, 0, 5);
 * @endcode
 *
 * Correct:
 * @code
 *   $my_substring = drupal_substr($original_string, 0, 5);
 * @endcode
 *
 * @}
 */

/**
 * Return status for saving which involved creating a new item.
 */
const SAVED_NEW = 1;

/**
 * Return status for saving which involved an update to an existing item.
 */
const SAVED_UPDATED = 2;

/**
 * Return status for saving which deleted an existing item.
 */
const SAVED_DELETED = 3;

/**
 * The default aggregation group for CSS files added to the page.
 */
const CSS_AGGREGATE_DEFAULT = 0;

/**
 * The default aggregation group for theme CSS files added to the page.
 */
const CSS_AGGREGATE_THEME = 100;

/**
 * The default weight for CSS rules that style HTML elements ("base" styles).
 */
const CSS_BASE = -200;

/**
 * The default weight for CSS rules that layout a page.
 */
const CSS_LAYOUT = -100;

/**
 * The default weight for CSS rules that style design components (and their associated states and skins.)
 */
const CSS_COMPONENT = 0;

/**
 * The default weight for CSS rules that style states and are not included with components.
 */
const CSS_STATE = 100;

/**
 * The default weight for CSS rules that style skins and are not included with components.
 */
const CSS_SKIN = 200;

/**
 * The default group for JavaScript settings added to the page.
 */
const JS_SETTING = -200;

/**
 * The default group for JavaScript and jQuery libraries added to the page.
 */
const JS_LIBRARY = -100;

/**
 * The default group for module JavaScript code added to the page.
 */
const JS_DEFAULT = 0;

/**
 * The default group for theme JavaScript code added to the page.
 */
const JS_THEME = 100;

/**
 * @defgroup block_caching Block Caching
 * @{
 * Constants that define each block's caching state.
 *
 * Modules specify how their blocks can be cached in their hook_block_info()
 * implementations. Caching can be turned off (DRUPAL_NO_CACHE), managed by the
 * module declaring the block (DRUPAL_CACHE_CUSTOM), or managed by the core
 * Block module. If the Block module is managing the cache, you can specify that
 * the block is the same for every page and user (DRUPAL_CACHE_GLOBAL), or that
 * it can change depending on the page (DRUPAL_CACHE_PER_PAGE) or by user
 * (DRUPAL_CACHE_PER_ROLE or DRUPAL_CACHE_PER_USER). Page and user settings can
 * be combined with a bitwise-binary or operator; for example,
 * DRUPAL_CACHE_PER_ROLE | DRUPAL_CACHE_PER_PAGE means that the block can change
 * depending on the user role or page it is on.
 *
 * The block cache is cleared when the 'content' cache tag is invalidated,
 * following the same pattern as the page cache (node, comment, user, taxonomy
 * added or updated...).
 *
 * Note that user 1 is excluded from block caching.
 */

/**
 * The block should not get cached.
 *
 * This setting should be used:
 * - For simple blocks (notably those that do not perform any db query), where
 *   querying the db cache would be more expensive than directly generating the
 *   content.
 * - For blocks that change too frequently.
 */
const DRUPAL_NO_CACHE = -1;

/**
 * The block is handling its own caching in its hook_block_view().
 *
 * This setting is useful when time based expiration is needed or a site uses a
 * node access which invalidates standard block cache.
 */
const DRUPAL_CACHE_CUSTOM = -2;

/**
 * The block or element can change depending on the user's roles.
 *
 * This is the default setting for blocks, used when the block does not specify
 * anything.
 */
const DRUPAL_CACHE_PER_ROLE = 0x0001;

/**
 * The block or element can change depending on the user.
 *
 * This setting can be resource-consuming for sites with large number of users,
 * and thus should only be used when DRUPAL_CACHE_PER_ROLE is not sufficient.
 */
const DRUPAL_CACHE_PER_USER = 0x0002;

/**
 * The block or element can change depending on the page being viewed.
 */
const DRUPAL_CACHE_PER_PAGE = 0x0004;

/**
 * The block or element is the same for every user and page that it is visible.
 */
const DRUPAL_CACHE_GLOBAL = 0x0008;

/**
 * @} End of "defgroup block_caching".
 */

/**
 * The delimiter used to split plural strings.
 *
 * This is the ETX (End of text) character and is used as a minimal means to
 * separate singular and plural variants in source and translation text. It
 * was found to be the most compatible delimiter for the supported databases.
 */
const LOCALE_PLURAL_DELIMITER = "\03";

/**
 * Adds content to a specified region.
 *
 * @param $region
 *   Page region the content is added to.
 * @param $data
 *   Content to be added.
 */
function drupal_add_region_content($region = NULL, $data = NULL) {
  static $content = array();

  if (isset($region) && isset($data)) {
    $content[$region][] = $data;
  }
  return $content;
}

/**
 * Gets assigned content for a given region.
 *
 * @param $region
 *   A specified region to fetch content for. If NULL, all regions will be
 *   returned.
 * @param $delimiter
 *   Content to be inserted between imploded array elements.
 */
function drupal_get_region_content($region = NULL, $delimiter = ' ') {
  $content = drupal_add_region_content();
  if (isset($region)) {
    if (isset($content[$region]) && is_array($content[$region])) {
      return implode($delimiter, $content[$region]);
    }
  }
  else {
    foreach (array_keys($content) as $region) {
      if (is_array($content[$region])) {
        $content[$region] = implode($delimiter, $content[$region]);
      }
    }
    return $content;
  }
}

/**
 * Gets the name of the currently active installation profile.
 *
 * When this function is called during Drupal's initial installation process,
 * the name of the profile that's about to be installed is stored in the global
 * installation state. At all other times, the standard Drupal systems variable
 * table contains the name of the current profile, and we can call
 * variable_get() to determine what one is active.
 *
 * @return $profile
 *   The name of the installation profile.
 */
function drupal_get_profile() {
  global $install_state;

  if (isset($install_state['parameters']['profile'])) {
    $profile = $install_state['parameters']['profile'];
  }
  else {
    $profile = variable_get('install_profile', 'standard');
  }

  return $profile;
}


/**
 * Sets the breadcrumb trail for the current page.
 *
 * @param $breadcrumb
 *   Array of links, starting with "home" and proceeding up to but not including
 *   the current page.
 *
 * @deprecated This will be removed in 8.0. Instead, register a new breadcrumb
 *   builder service.
 *
 * @see Drupal\Core\Breadcrumb\BreadcrumbBuilderInterface
 */
function drupal_set_breadcrumb($breadcrumb = NULL) {
  $stored_breadcrumb = &drupal_static(__FUNCTION__);

  if (isset($breadcrumb)) {
    $stored_breadcrumb = $breadcrumb;
  }
  return $stored_breadcrumb;
}

/**
 * Adds output to the HEAD tag of the HTML page.
 *
 * This function can be called as long as the headers aren't sent. Pass no
 * arguments (or NULL for both) to retrieve the currently stored elements.
 *
 * @param $data
 *   A renderable array. If the '#type' key is not set then 'html_tag' will be
 *   added as the default '#type'.
 * @param $key
 *   A unique string key to allow implementations of hook_html_head_alter() to
 *   identify the element in $data. Required if $data is not NULL.
 *
 * @return
 *   An array of all stored HEAD elements.
 *
 * @see drupal_pre_render_html_tag()
 */
function drupal_add_html_head($data = NULL, $key = NULL) {
  $stored_head = &drupal_static(__FUNCTION__);

  if (!isset($stored_head)) {
    // Make sure the defaults, including Content-Type, come first.
    $stored_head = _drupal_default_html_head();
  }

  if (isset($data) && isset($key)) {
    if (!isset($data['#type'])) {
      $data['#type'] = 'html_tag';
    }
    $stored_head[$key] = $data;
  }
  return $stored_head;
}

/**
 * Returns elements that are always displayed in the HEAD tag of the HTML page.
 */
function _drupal_default_html_head() {
  // Add default elements. Make sure the Content-Type comes first because the
  // IE browser may be vulnerable to XSS via encoding attacks from any content
  // that comes before this META tag, such as a TITLE tag.
  $elements['system_meta_content_type'] = array(
    '#type' => 'html_tag',
    '#tag' => 'meta',
    '#attributes' => array(
      'charset' => 'utf-8',
    ),
    // Security: This always has to be output first.
    '#weight' => -1000,
  );
  // Show Drupal and the major version number in the META GENERATOR tag.
  // Get the major version.
  list($version, ) = explode('.', VERSION);
  $elements['system_meta_generator'] = array(
    '#type' => 'html_tag',
    '#tag' => 'meta',
    '#attributes' => array(
      'name' => 'Generator',
      'content' => 'Drupal ' . $version . ' (http://drupal.org)',
    ),
  );
  // Also send the generator in the HTTP header.
  $elements['system_meta_generator']['#attached']['drupal_add_http_header'][] = array('X-Generator', $elements['system_meta_generator']['#attributes']['content']);
  return $elements;
}

/**
 * Retrieves output to be displayed in the HEAD tag of the HTML page.
 */
function drupal_get_html_head() {
  $elements = drupal_add_html_head();
  drupal_alter('html_head', $elements);
  return drupal_render($elements);
}

/**
 * Adds a feed URL for the current page.
 *
 * This function can be called as long the HTML header hasn't been sent.
 *
 * @param $url
 *   An internal system path or a fully qualified external URL of the feed.
 * @param $title
 *   The title of the feed.
 */
function drupal_add_feed($url = NULL, $title = '') {
  $stored_feed_links = &drupal_static(__FUNCTION__, array());

  if (isset($url)) {
    $stored_feed_links[$url] = theme('feed_icon', array('url' => $url, 'title' => $title));

    drupal_add_html_head_link(array(
      'rel' => 'alternate',
      'type' => 'application/rss+xml',
      'title' => $title,
      // Force the URL to be absolute, for consistency with other <link> tags
      // output by Drupal.
      'href' => url($url, array('absolute' => TRUE)),
    ));
  }
  return $stored_feed_links;
}

/**
 * Gets the feed URLs for the current page.
 *
 * @param $delimiter
 *   A delimiter to split feeds by.
 */
function drupal_get_feeds($delimiter = "\n") {
  $feeds = drupal_add_feed();
  return implode($feeds, $delimiter);
}

/**
 * @defgroup http_handling HTTP handling
 * @{
 * Functions to properly handle HTTP responses.
 */

/**
 * Processes a URL query parameter array to remove unwanted elements.
 *
 * @param $query
 *   (optional) An array to be processed. Defaults to $_GET.
 * @param $exclude
 *   (optional) A list of $query array keys to remove. Use "parent[child]" to
 *   exclude nested items.
 * @param $parent
 *   Internal use only. Used to build the $query array key for nested items.
 *
 * @return
 *   An array containing query parameters, which can be used for url().
 */
function drupal_get_query_parameters(array $query = NULL, array $exclude = array(), $parent = '') {
  // Set defaults, if none given.
  if (!isset($query)) {
    $query = $_GET;
  }
  // If $exclude is empty, there is nothing to filter.
  if (empty($exclude)) {
    return $query;
  }
  elseif (!$parent) {
    $exclude = array_flip($exclude);
  }

  $params = array();
  foreach ($query as $key => $value) {
    $string_key = ($parent ? $parent . '[' . $key . ']' : $key);
    if (isset($exclude[$string_key])) {
      continue;
    }

    if (is_array($value)) {
      $params[$key] = drupal_get_query_parameters($value, $exclude, $string_key);
    }
    else {
      $params[$key] = $value;
    }
  }

  return $params;
}

/**
 * Splits a URL-encoded query string into an array.
 *
 * @param $query
 *   The query string to split.
 *
 * @return
 *   An array of URL decoded couples $param_name => $value.
 */
function drupal_get_query_array($query) {
  $result = array();
  if (!empty($query)) {
    foreach (explode('&', $query) as $param) {
      $param = explode('=', $param);
      $result[$param[0]] = isset($param[1]) ? rawurldecode($param[1]) : '';
    }
  }
  return $result;
}

/**
 * Parses an array into a valid, rawurlencoded query string.
 *
 * @see \Drupal\Core\Routing\PathBasedGeneratorInterface::httpBuildQuery()
 * @see drupal_get_query_parameters()
 * @deprecated as of Drupal 8.0. Use
 *   Drupal::urlGenerator()->httpBuildQuery() instead.
 * @ingroup php_wrappers
 */
function drupal_http_build_query(array $query, $parent = '') {
  return Drupal::urlGenerator()->httpBuildQuery($query, $parent);
}

/**
 * Prepares a 'destination' URL query parameter for use with url().
 *
 * Used to direct the user back to the referring page after completing a form.
 * By default the current URL is returned. If a destination exists in the
 * previous request, that destination is returned. As such, a destination can
 * persist across multiple pages.
 *
 * @return
 *   An associative array containing the key:
 *   - destination: The path provided via the destination query string or, if
 *     not available, the current path.
 *
 * @see current_path()
 */
function drupal_get_destination() {
  $destination = &drupal_static(__FUNCTION__);

  if (isset($destination)) {
    return $destination;
  }

  if (isset($_GET['destination'])) {
    $destination = array('destination' => $_GET['destination']);
  }
  else {
    $path = current_path();
    $query = Drupal::urlGenerator()->httpBuildQuery(drupal_get_query_parameters());
    if ($query != '') {
      $path .= '?' . $query;
    }
    $destination = array('destination' => $path);
  }
  return $destination;
}

/**
 * Parses a system URL string into an associative array suitable for url().
 *
 * This function should only be used for URLs that have been generated by the
 * system, such as via url(). It should not be used for URLs that come from
 * external sources, or URLs that link to external resources.
 *
 * The returned array contains a 'path' that may be passed separately to url().
 * For example:
 * @code
 *   $options = drupal_parse_url($_GET['destination']);
 *   $my_url = url($options['path'], $options);
 *   $my_link = l('Example link', $options['path'], $options);
 * @endcode
 *
 * This is required, because url() does not support relative URLs containing a
 * query string or fragment in its $path argument. Instead, any query string
 * needs to be parsed into an associative query parameter array in
 * $options['query'] and the fragment into $options['fragment'].
 *
 * @param $url
 *   The URL string to parse, f.e. $_GET['destination'].
 *
 * @return
 *   An associative array containing the keys:
 *   - 'path': The path of the URL. If the given $url is external, this includes
 *     the scheme and host.
 *   - 'query': An array of query parameters of $url, if existent.
 *   - 'fragment': The fragment of $url, if existent.
 *
 * @see url()
 * @ingroup php_wrappers
 */
function drupal_parse_url($url) {
  $options = array(
    'path' => NULL,
    'query' => array(),
    'fragment' => '',
  );

  // External URLs: not using parse_url() here, so we do not have to rebuild
  // the scheme, host, and path without having any use for it.
  if (strpos($url, '://') !== FALSE) {
    // Split off everything before the query string into 'path'.
    $parts = explode('?', $url);
    $options['path'] = $parts[0];
    // If there is a query string, transform it into keyed query parameters.
    if (isset($parts[1])) {
      $query_parts = explode('#', $parts[1]);
      parse_str($query_parts[0], $options['query']);
      // Take over the fragment, if there is any.
      if (isset($query_parts[1])) {
        $options['fragment'] = $query_parts[1];
      }
    }
  }
  // Internal URLs.
  else {
    // parse_url() does not support relative URLs, so make it absolute. E.g. the
    // relative URL "foo/bar:1" isn't properly parsed.
    $parts = parse_url('http://example.com/' . $url);
    // Strip the leading slash that was just added.
    $options['path'] = substr($parts['path'], 1);
    if (isset($parts['query'])) {
      parse_str($parts['query'], $options['query']);
    }
    if (isset($parts['fragment'])) {
      $options['fragment'] = $parts['fragment'];
    }
  }

  return $options;
}

/**
 * Encodes a Drupal path for use in a URL.
 *
 * For aesthetic reasons slashes are not escaped.
 *
 * Note that url() takes care of calling this function, so a path passed to that
 * function should not be encoded in advance.
 *
 * @param $path
 *   The Drupal path to encode.
 */
function drupal_encode_path($path) {
  return str_replace('%2F', '/', rawurlencode($path));
}

/**
 * Determines if an external URL points to this Drupal installation.
 *
 * @param $url
 *   A string containing an external URL, such as "http://example.com/foo".
 *
 * @return
 *   TRUE if the URL has the same domain and base path.
 */
function _external_url_is_local($url) {
  $url_parts = parse_url($url);
  $base_host = parse_url($GLOBALS['base_url'], PHP_URL_HOST);

  if (!isset($url_parts['path'])) {
    return ($url_parts['host'] == $base_host);
  }
  else {
    // When comparing base paths, we need a trailing slash to make sure a
    // partial URL match isn't occuring. Since base_path() always returns with
    // a trailing slash, we don't need to add the trailing slash here.
    return ($url_parts['host'] == $base_host && stripos($url_parts['path'], base_path()) === 0);
  }
}

/**
 * Helper function for determining hosts excluded from needing a proxy.
 *
 * @return
 *   TRUE if a proxy should be used for this host.
 */
function _drupal_http_use_proxy($host) {
  $proxy_exceptions = settings()->get('proxy_exceptions', array('localhost', '127.0.0.1'));
  return !in_array(strtolower($host), $proxy_exceptions, TRUE);
}

/**
 * @} End of "defgroup http_handling".
 */

/**
 * @defgroup validation Input validation
 * @{
 * Functions to validate user input.
 */

/**
 * Verifies the syntax of the given e-mail address.
 *
 * This uses the
 * @link http://php.net/manual/filter.filters.validate.php PHP e-mail validation filter. @endlink
 *
 * @param $mail
 *   A string containing an e-mail address.
 *
 * @return
 *   TRUE if the address is in a valid format.
 */
function valid_email_address($mail) {
  return (bool)filter_var($mail, FILTER_VALIDATE_EMAIL);
}

/**
 * Verifies the syntax of the given URL.
 *
 * This function should only be used on actual URLs. It should not be used for
 * Drupal menu paths, which can contain arbitrary characters.
 * Valid values per RFC 3986.
 * @param $url
 *   The URL to verify.
 * @param $absolute
 *   Whether the URL is absolute (beginning with a scheme such as "http:").
 *
 * @return
 *   TRUE if the URL is in a valid format.
 *
 * @see \Drupal\Component\Utility\UrlValidator::isValid()
 *
 * @deprecated as of Drupal 8.0. Use UrlValidator::isValid() instead.
 */
function valid_url($url, $absolute = FALSE) {
  return UrlValidator::isValid($url, $absolute);
}

/**
 * Verifies that a number is a multiple of a given step.
 *
 * The implementation assumes it is dealing with IEEE 754 double precision
 * floating point numbers that are used by PHP on most systems.
 *
 * This is based on the number/range verification methods of webkit.
 *
 * @param $value
 *   The value that needs to be checked.
 * @param $step
 *   The step scale factor. Must be positive.
 * @param $offset
 *   (optional) An offset, to which the difference must be a multiple of the
 *   given step.
 *
 * @return bool
 *   TRUE if no step mismatch has occured, or FALSE otherwise.
 *
 * @see http://opensource.apple.com/source/WebCore/WebCore-1298/html/NumberInputType.cpp
 */
function valid_number_step($value, $step, $offset = 0.0) {
  $double_value = (double) abs($value - $offset);

  // The fractional part of a double has 53 bits. The greatest number that could
  // be represented with that is 2^53. If the given value is even bigger than
  // $step * 2^53, then dividing by $step will result in a very small remainder.
  // Since that remainder can't even be represented with a single precision
  // float the following computation of the remainder makes no sense and we can
  // safely ignore it instead.
  if ($double_value / pow(2.0, 53) > $step) {
    return TRUE;
  }

  // Now compute that remainder of a division by $step.
  $remainder = (double) abs($double_value - $step * round($double_value / $step));

  // $remainder is a double precision floating point number. Remainders that
  // can't be represented with single precision floats are acceptable. The
  // fractional part of a float has 24 bits. That means remainders smaller than
  // $step * 2^-24 are acceptable.
  $computed_acceptable_error = (double)($step / pow(2.0, 24));

  return $computed_acceptable_error >= $remainder || $remainder >= ($step - $computed_acceptable_error);
}

/**
 * @} End of "defgroup validation".
 */

/**
 * @defgroup sanitization Sanitization functions
 * @{
 * Functions to sanitize values.
 *
 * See http://drupal.org/writing-secure-code for information
 * on writing secure code.
 */

/**
 * Strips dangerous protocols (e.g. 'javascript:') from a URI.
 *
 * This function must be called for all URIs within user-entered input prior
 * to being output to an HTML attribute value. It is often called as part of
 * check_url() or filter_xss(), but those functions return an HTML-encoded
 * string, so this function can be called independently when the output needs to
 * be a plain-text string for passing to t(), l(),
 * Drupal\Core\Template\Attribute, or another function that will call
 * check_plain() separately.
 *
 * @param $uri
 *   A plain-text URI that might contain dangerous protocols.
 *
 * @return
 *   A plain-text URI stripped of dangerous protocols. As with all plain-text
 *   strings, this return value must not be output to an HTML page without
 *   check_plain() being called on it. However, it can be passed to functions
 *   expecting plain-text strings.
 *
 * @see \Drupal\Component\Utility\Url::stripDangerousProtocols()
 */
function drupal_strip_dangerous_protocols($uri) {
  return UrlValidator::stripDangerousProtocols($uri);
}

/**
 * Strips dangerous protocols from a URI and encodes it for output to HTML.
 *
 * @param $uri
 *   A plain-text URI that might contain dangerous protocols.
 *
 * @return
 *   A URI stripped of dangerous protocols and encoded for output to an HTML
 *   attribute value. Because it is already encoded, it should not be set as a
 *   value within a $attributes array passed to Drupal\Core\Template\Attribute,
 *   because Drupal\Core\Template\Attribute expects those values to be
 *   plain-text strings. To pass a filtered URI to
 *   Drupal\Core\Template\Attribute, call drupal_strip_dangerous_protocols()
 *   instead.
 *
 * @see \Drupal\Component\Utility\Url::stripDangerousProtocols()
 * @see \Drupal\Component\Utility\String::checkPlain()
 */
function check_url($uri) {
  return String::checkPlain(UrlValidator::stripDangerousProtocols($uri));
}

/**
 * Applies a very permissive XSS/HTML filter for admin-only use.
 *
 * Use only for fields where it is impractical to use the
 * whole filter system, but where some (mainly inline) mark-up
 * is desired (so check_plain() is not acceptable).
 *
 * Allows all tags that can be used inside an HTML body, save
 * for scripts and styles.
 *
 * @param string $string
 *   The string to apply the filter to.
 *
 * @return string
 *   The filtered string.
 *
 * @see \Drupal\Component\Utility\Xss::filterAdmin()
 */
function filter_xss_admin($string) {
  return Xss::filterAdmin($string);
}

/**
 * Filters HTML to prevent cross-site-scripting (XSS) vulnerabilities.
 *
 * Based on kses by Ulf Harnhammar, see http://sourceforge.net/projects/kses.
 * For examples of various XSS attacks, see: http://ha.ckers.org/xss.html.
 *
 * This code does four things:
 * - Removes characters and constructs that can trick browsers.
 * - Makes sure all HTML entities are well-formed.
 * - Makes sure all HTML tags and attributes are well-formed.
 * - Makes sure no HTML tags contain URLs with a disallowed protocol (e.g.
 *   javascript:).
 *
 * @param $string
 *   The string with raw HTML in it. It will be stripped of everything that can
 *   cause an XSS attack.
 * @param $allowed_tags
 *   An array of allowed tags.
 *
 * @return
 *   An XSS safe version of $string, or an empty string if $string is not
 *   valid UTF-8.
 *
 * @see \Drupal\Component\Utility\Xss::filter()
 *
 * @ingroup sanitization
 */
function filter_xss($string, $allowed_tags = array('a', 'em', 'strong', 'cite', 'blockquote', 'code', 'ul', 'ol', 'li', 'dl', 'dt', 'dd')) {
  return Xss::filter($string, $allowed_tags);
}

/**
 * Processes an HTML attribute value and strips dangerous protocols from URLs.
 *
 * @param string $string
 *   The string with the attribute value.
 *
 * @return string
 *   Cleaned up and HTML-escaped version of $string.
 *
 * @see \Drupal\Component\Utility\Url::filterBadProtocol()
 */
function filter_xss_bad_protocol($string) {
  return UrlValidator::filterBadProtocol($string);
}

/**
 * @} End of "defgroup sanitization".
 */

/**
 * @defgroup format Formatting
 * @{
 * Functions to format numbers, strings, dates, etc.
 */

/**
 * Formats an RSS channel.
 *
 * Arbitrary elements may be added using the $args associative array.
 */
function format_rss_channel($title, $link, $description, $items, $langcode = NULL, $args = array()) {
  $langcode = $langcode ? $langcode : language(Language::TYPE_CONTENT)->id;

  $output = "<channel>\n";
  $output .= ' <title>' . check_plain($title) . "</title>\n";
  $output .= ' <link>' . check_url($link) . "</link>\n";

  // The RSS 2.0 "spec" doesn't indicate HTML can be used in the description.
  // We strip all HTML tags, but need to prevent double encoding from properly
  // escaped source data (such as &amp becoming &amp;amp;).
  $output .= ' <description>' . check_plain(decode_entities(strip_tags($description))) . "</description>\n";
  $output .= ' <language>' . check_plain($langcode) . "</language>\n";
  $output .= format_xml_elements($args);
  $output .= $items;
  $output .= "</channel>\n";

  return $output;
}

/**
 * Formats a single RSS item.
 *
 * Arbitrary elements may be added using the $args associative array.
 */
function format_rss_item($title, $link, $description, $args = array()) {
  $output = "<item>\n";
  $output .= ' <title>' . check_plain($title) . "</title>\n";
  $output .= ' <link>' . check_url($link) . "</link>\n";
  $output .= ' <description>' . check_plain($description) . "</description>\n";
  $output .= format_xml_elements($args);
  $output .= "</item>\n";

  return $output;
}

/**
 * Formats XML elements.
 *
 * @param $array
 *   An array where each item represents an element and is either a:
 *   - (key => value) pair (<key>value</key>)
 *   - Associative array with fields:
 *     - 'key': element name
 *     - 'value': element contents
 *     - 'attributes': associative array of element attributes
 *
 * In both cases, 'value' can be a simple string, or it can be another array
 * with the same format as $array itself for nesting.
 */
function format_xml_elements($array) {
  $output = '';
  foreach ($array as $key => $value) {
    if (is_numeric($key)) {
      if ($value['key']) {
        $output .= ' <' . $value['key'];
        if (isset($value['attributes']) && is_array($value['attributes'])) {
          $output .= new Attribute($value['attributes']);
        }

        if (isset($value['value']) && $value['value'] != '') {
          $output .= '>' . (is_array($value['value']) ? format_xml_elements($value['value']) : check_plain($value['value'])) . '</' . $value['key'] . ">\n";
        }
        else {
          $output .= " />\n";
        }
      }
    }
    else {
      $output .= ' <' . $key . '>' . (is_array($value) ? format_xml_elements($value) : check_plain($value)) . "</$key>\n";
    }
  }
  return $output;
}

/**
 * Formats a string containing a count of items.
 *
 * This function ensures that the string is pluralized correctly. Since t() is
 * called by this function, make sure not to pass already-localized strings to
 * it.
 *
 * For example:
 * @code
 *   $output = format_plural($node->comment_count, '1 comment', '@count comments');
 * @endcode
 *
 * Example with additional replacements:
 * @code
 *   $output = format_plural($update_count,
 *     'Changed the content type of 1 post from %old-type to %new-type.',
 *     'Changed the content type of @count posts from %old-type to %new-type.',
 *     array('%old-type' => $info->old_type, '%new-type' => $info->new_type));
 * @endcode
 *
 * @param $count
 *   The item count to display.
 * @param $singular
 *   The string for the singular case. Make sure it is clear this is singular,
 *   to ease translation (e.g. use "1 new comment" instead of "1 new"). Do not
 *   use @count in the singular string.
 * @param $plural
 *   The string for the plural case. Make sure it is clear this is plural, to
 *   ease translation. Use @count in place of the item count, as in
 *   "@count new comments".
 * @param $args
 *   An associative array of replacements to make after translation. Instances
 *   of any key in this array are replaced with the corresponding value.
 *   Based on the first character of the key, the value is escaped and/or
 *   themed. See format_string(). Note that you do not need to include @count
 *   in this array; this replacement is done automatically for the plural case.
 * @param $options
 *   An associative array of additional options. See t() for allowed keys.
 *
 * @return
 *   A translated string.
 *
 * @see t()
 * @see format_string()
 */
function format_plural($count, $singular, $plural, array $args = array(), array $options = array()) {
  $args['@count'] = $count;
  // Join both forms to search a translation.
  $tranlatable_string = implode(LOCALE_PLURAL_DELIMITER, array($singular, $plural));
  // Translate as usual.
  $translated_strings = t($tranlatable_string, $args, $options);
  // Split joined translation strings into array.
  $translated_array = explode(LOCALE_PLURAL_DELIMITER, $translated_strings);

  if ($count == 1) {
    return $translated_array[0];
  }

  // Get the plural index through the gettext formula.
  // @todo implement static variable to minimize function_exists() usage.
  $index = (function_exists('locale_get_plural')) ? locale_get_plural($count, isset($options['langcode']) ? $options['langcode'] : NULL) : -1;
  if ($index == 0) {
    // Singular form.
    return $translated_array[0];
  }
  else {
    if (isset($translated_array[$index])) {
      // N-th plural form.
      return $translated_array[$index];
    }
    else {
      // If the index cannot be computed or there's no translation, use
      // the second plural form as a fallback (which allows for most flexiblity
      // with the replaceable @count value).
      return $translated_array[1];
    }
  }
}

/**
 * Parses a given byte count.
 *
 * @param $size
 *   A size expressed as a number of bytes with optional SI or IEC binary unit
 *   prefix (e.g. 2, 3K, 5MB, 10G, 6GiB, 8 bytes, 9mbytes).
 *
 * @return
 *   An integer representation of the size in bytes.
 */
function parse_size($size) {
  $unit = preg_replace('/[^bkmgtpezy]/i', '', $size); // Remove the non-unit characters from the size.
  $size = preg_replace('/[^0-9\.]/', '', $size); // Remove the non-numeric characters from the size.
  if ($unit) {
    // Find the position of the unit in the ordered string which is the power of magnitude to multiply a kilobyte by.
    return round($size * pow(DRUPAL_KILOBYTE, stripos('bkmgtpezy', $unit[0])));
  }
  else {
    return round($size);
  }
}

/**
 * Generates a string representation for the given byte count.
 *
 * @param $size
 *   A size in bytes.
 * @param $langcode
 *   Optional language code to translate to a language other than what is used
 *   to display the page.
 *
 * @return
 *   A translated string representation of the size.
 */
function format_size($size, $langcode = NULL) {
  if ($size < DRUPAL_KILOBYTE) {
    return format_plural($size, '1 byte', '@count bytes', array(), array('langcode' => $langcode));
  }
  else {
    $size = $size / DRUPAL_KILOBYTE; // Convert bytes to kilobytes.
    $units = array(
      t('@size KB', array(), array('langcode' => $langcode)),
      t('@size MB', array(), array('langcode' => $langcode)),
      t('@size GB', array(), array('langcode' => $langcode)),
      t('@size TB', array(), array('langcode' => $langcode)),
      t('@size PB', array(), array('langcode' => $langcode)),
      t('@size EB', array(), array('langcode' => $langcode)),
      t('@size ZB', array(), array('langcode' => $langcode)),
      t('@size YB', array(), array('langcode' => $langcode)),
    );
    foreach ($units as $unit) {
      if (round($size, 2) >= DRUPAL_KILOBYTE) {
        $size = $size / DRUPAL_KILOBYTE;
      }
      else {
        break;
      }
    }
    return str_replace('@size', round($size, 2), $unit);
  }
}

/**
 * Formats a time interval with the requested granularity.
 *
 * @param $interval
 *   The length of the interval in seconds.
 * @param $granularity
 *   How many different units to display in the string.
 * @param $langcode
 *   Optional language code to translate to a language other than
 *   what is used to display the page.
 *
 * @return
 *   A translated string representation of the interval.
 */
function format_interval($interval, $granularity = 2, $langcode = NULL) {
  $units = array(
    '1 year|@count years' => 31536000,
    '1 month|@count months' => 2592000,
    '1 week|@count weeks' => 604800,
    '1 day|@count days' => 86400,
    '1 hour|@count hours' => 3600,
    '1 min|@count min' => 60,
    '1 sec|@count sec' => 1
  );
  $output = '';
  foreach ($units as $key => $value) {
    $key = explode('|', $key);
    if ($interval >= $value) {
      $output .= ($output ? ' ' : '') . format_plural(floor($interval / $value), $key[0], $key[1], array(), array('langcode' => $langcode));
      $interval %= $value;
      $granularity--;
    }

    if ($granularity == 0) {
      break;
    }
  }
  return $output ? $output : t('0 sec', array(), array('langcode' => $langcode));
}

/**
 * Formats a date, using a date type or a custom date format string.
 *
 * @param $timestamp
 *   A UNIX timestamp to format.
 * @param $type
 *   (optional) The format to use, one of:
 *   - One of the built-in formats: 'short', 'medium',
 *     'long', 'html_datetime', 'html_date', 'html_time',
 *     'html_yearless_date', 'html_week', 'html_month', 'html_year'.
 *   - The name of a date type defined by a module in
 *     hook_date_format_types(), if it's been assigned a format.
 *   - The machine name of an administrator-defined date format.
 *   - 'custom', to use $format.
 *   Defaults to 'medium'.
 * @param $format
 *   (optional) If $type is 'custom', a PHP date format string suitable for
 *   input to date(). Use a backslash to escape ordinary text, so it does not
 *   get interpreted as date format characters.
 * @param $timezone
 *   (optional) Time zone identifier, as described at
 *   http://php.net/manual/timezones.php Defaults to the time zone used to
 *   display the page.
 * @param $langcode
 *   (optional) Language code to translate to. Defaults to the language used to
 *   display the page.
 *
 * @return
 *   A translated date string in the requested format.
 *
 * @see \Drupal\Component\Datetime\Date::format()
 */
function format_date($timestamp, $type = 'medium', $format = '', $timezone = NULL, $langcode = NULL) {
  return Drupal::service('date')->format($timestamp, $type, $format, $timezone, $langcode);
}

/**
 * Returns an ISO8601 formatted date based on the given date.
 *
 * @param $date
 *   A UNIX timestamp.
 *
 * @return string
 *   An ISO8601 formatted date.
 */
function date_iso8601($date) {
  // The DATE_ISO8601 constant cannot be used here because it does not match
  // date('c') and produces invalid RDF markup.
  return date('c', $date);
}

/**
 * Translates a formatted date string.
 *
 * Callback for preg_replace_callback() within format_date().
 */
function _format_date_callback(array $matches = NULL, $new_langcode = NULL) {
  // We cache translations to avoid redundant and rather costly calls to t().
  static $cache, $langcode;

  if (!isset($matches)) {
    $langcode = $new_langcode;
    return;
  }

  $code = $matches[1];
  $string = $matches[2];

  if (!isset($cache[$langcode][$code][$string])) {
    $options = array(
      'langcode' => $langcode,
    );

    if ($code == 'F') {
      $options['context'] = 'Long month name';
    }

    if ($code == '') {
      $cache[$langcode][$code][$string] = $string;
    }
    else {
      $cache[$langcode][$code][$string] = t($string, array(), $options);
    }
  }
  return $cache[$langcode][$code][$string];
}

/**
 * Retrieves the correct datetime format type for this system.
 *
 * This value is sometimes required when the format type needs to be determined
 * before a date can be created.
 *
 * @return string
 *   A string as defined in \DrupalComponent\Datetime\DateTimePlus.php: either
 *   'intl' or 'php', depending on whether IntlDateFormatter is available.
 */
function datetime_default_format_type() {
  static $drupal_static_fast;
  if (!isset($drupal_static_fast)) {
    $drupal_static_fast['format_type'] = &drupal_static(__FUNCTION__);
  }
  $format_type = &$drupal_static_fast['format_type'];

  if (!isset($format_type)) {
    $date = new DrupalDateTime();
    $format_type = $date->canUseIntl() ? DrupalDateTime::INTL : DrupalDateTime::PHP;
  }
  return $format_type;
}

/**
 * @} End of "defgroup format".
 */

/**
 * Generates an internal or external URL.
 *
 * When creating links in modules, consider whether l() could be a better
 * alternative than url().
 *
 * @see \Drupal\Core\Routing\PathBasedGeneratorInterface::generateFromPath().
 */
function url($path = NULL, array $options = array()) {
  $generator = Drupal::urlGenerator();
  try {
    $url = $generator->generateFromPath($path, $options);
  }
  catch (GeneratorNotInitializedException $e) {
    // Fallback to using globals.
    // @todo Remove this once there is no code that calls url() when there is
    //   no request.
    global $base_url, $base_path, $script_path;
    $generator->setBasePath($base_path);
    $generator->setBaseUrl($base_url . '/');
    $generator->setScriptPath($script_path);
    $url = $generator->generateFromPath($path, $options);
  }
  return $url;
}

/**
 * Returns TRUE if a path is external to Drupal (e.g. http://example.com).
 *
 * If a path cannot be assessed by Drupal's menu handler, then we must
 * treat it as potentially insecure.
 *
 * @param $path
 *   The internal path or external URL being linked to, such as "node/34" or
 *   "http://example.com/foo".
 *
 * @return
 *   Boolean TRUE or FALSE, where TRUE indicates an external path.
 */
function url_is_external($path) {
  $colonpos = strpos($path, ':');
  // Avoid calling drupal_strip_dangerous_protocols() if there is any
  // slash (/), hash (#) or question_mark (?) before the colon (:)
  // occurrence - if any - as this would clearly mean it is not a URL.
  return $colonpos !== FALSE && !preg_match('![/?#]!', substr($path, 0, $colonpos)) && drupal_strip_dangerous_protocols($path) == $path;
}

/**
 * Formats an attribute string for an HTTP header.
 *
 * @param $attributes
 *   An associative array of attributes such as 'rel'.
 *
 * @return
 *   A ; separated string ready for insertion in a HTTP header. No escaping is
 *   performed for HTML entities, so this string is not safe to be printed.
 *
 * @see drupal_add_http_header()
 */
function drupal_http_header_attributes(array $attributes = array()) {
  foreach ($attributes as $attribute => &$data) {
    if (is_array($data)) {
      $data = implode(' ', $data);
    }
    $data = $attribute . '="' . $data . '"';
  }
  return $attributes ? ' ' . implode('; ', $attributes) : '';
}

/**
 * Formats an internal or external URL link as an HTML anchor tag.
 *
 * This function correctly handles aliased paths and adds an 'active' class
 * attribute to links that point to the current page (for theming), so all
 * internal links output by modules should be generated by this function if
 * possible.
 *
 * However, for links enclosed in translatable text you should use t() and
 * embed the HTML anchor tag directly in the translated string. For example:
 * @code
 * t('Visit the <a href="@url">settings</a> page', array('@url' => url('admin')));
 * @endcode
 * This keeps the context of the link title ('settings' in the example) for
 * translators.
 *
 * @param string|array $text
 *   The link text for the anchor tag as a translated string or render array.
 * @param string $path
 *   The internal path or external URL being linked to, such as "node/34" or
 *   "http://example.com/foo". After the url() function is called to construct
 *   the URL from $path and $options, the resulting URL is passed through
 *   check_plain() before it is inserted into the HTML anchor tag, to ensure
 *   well-formed HTML. See url() for more information and notes.
 * @param array $options
 *   An associative array of additional options. Defaults to an empty array. It
 *   may contain the following elements.
 *   - 'attributes': An associative array of HTML attributes to apply to the
 *     anchor tag. If element 'class' is included, it must be an array; 'title'
 *     must be a string; other elements are more flexible, as they just need
 *     to work as an argument for the constructor of the class
 *     Drupal\Core\Template\Attribute($options['attributes']).
 *   - 'html' (default FALSE): Whether $text is HTML or just plain-text. For
 *     example, to make an image tag into a link, this must be set to TRUE, or
 *     you will see the escaped HTML image tag. $text is not sanitized if
 *     'html' is TRUE. The calling function must ensure that $text is already
 *     safe.
 *   - 'language': An optional language object. If the path being linked to is
 *     internal to the site, $options['language'] is used to determine whether
 *     the link is "active", or pointing to the current page (the language as
 *     well as the path must match). This element is also used by url().
 *   - Additional $options elements used by the url() function.
 *
 * @return string
 *   An HTML string containing a link to the given path.
 *
 * @see url()
 */
function l($text, $path, array $options = array()) {
  // Start building a structured representation of our link to be altered later.
  $variables = array(
    'text' => is_array($text) ? drupal_render($text) : $text,
    'path' => $path,
    'options' => $options,
  );

  // Merge in default options.
  $variables['options'] += array(
    'attributes' => array(),
    'query' => array(),
    'html' => FALSE,
    'language' => NULL,
  );

  // Because l() is called very often we statically cache values that require an
  // extra function call.
  static $drupal_static_fast;
  if (!isset($drupal_static_fast['active'])) {
    $drupal_static_fast['active'] = &drupal_static(__FUNCTION__);
  }
  $active = &$drupal_static_fast['active'];
  if (!isset($active)) {
    $active = array(
      'path' => current_path(),
      'front_page' => drupal_is_front_page(),
      'language' => language(Language::TYPE_URL)->id,
      'query' => Drupal::service('request')->query->all(),
    );
  }

  // Determine whether this link is "active', meaning that it links to the
  // current page. It is important that we stop checking "active" conditions if
  // we know the link is not active. This helps ensure that l() remains fast.
  // An active link's path is equal to the current path.
  $variables['url_is_active'] = ($path == $active['path'] || ($path == '<front>' && $active['front_page']))
  // The language of an active link is equal to the current language.
  && (empty($variables['options']['language']) || $variables['options']['language']->id == $active['language'])
  // The query parameters of an active link are equal to the current parameters.
  && ($variables['options']['query'] == $active['query']);

  // Add the "active" class if appropriate.
  if ($variables['url_is_active']) {
    $variables['options']['attributes']['class'][] = 'active';
  }

  // Remove all HTML and PHP tags from a tooltip, calling expensive strip_tags()
  // only when a quick strpos() gives suspicion tags are present.
  if (isset($variables['options']['attributes']['title']) && strpos($variables['options']['attributes']['title'], '<') !== FALSE) {
    $variables['options']['attributes']['title'] = strip_tags($variables['options']['attributes']['title']);
  }

  // Allow other modules to modify the structure of the link.
  Drupal::moduleHandler()->alter('link', $variables);

  // Move attributes out of options. url() doesn't need them.
  $attributes = new Attribute($variables['options']['attributes']);
  unset($variables['options']['attributes']);

  // The result of url() is a plain-text URL. Because we are using it here
  // in an HTML argument context, we need to encode it properly.
  $url = check_plain(url($variables['path'], $variables['options']));

  // Sanitize the link text if necessary.
  $text = $variables['options']['html'] ? $variables['text'] : check_plain($variables['text']);

  return '<a href="' . $url . '"' . $attributes . '>' . $text . '</a>';
}

/**
 * Forms an associative array from a linear array.
 *
 * @see \Drupal\Component\Utility\MapArray::copyValuesToKeys()
 */
function drupal_map_assoc($array, $callable = NULL) {
  return MapArray::copyValuesToKeys($array, $callable);
}

/**
 * Attempts to set the PHP maximum execution time.
 *
 * This function is a wrapper around the PHP function set_time_limit().
 * When called, set_time_limit() restarts the timeout counter from zero.
 * In other words, if the timeout is the default 30 seconds, and 25 seconds
 * into script execution a call such as set_time_limit(20) is made, the
 * script will run for a total of 45 seconds before timing out.
 *
 * It also means that it is possible to decrease the total time limit if
 * the sum of the new time limit and the current time spent running the
 * script is inferior to the original time limit. It is inherent to the way
 * set_time_limit() works, it should rather be called with an appropriate
 * value every time you need to allocate a certain amount of time
 * to execute a task than only once at the beginning of the script.
 *
 * Before calling set_time_limit(), we check if this function is available
 * because it could be disabled by the server administrator. We also hide all
 * the errors that could occur when calling set_time_limit(), because it is
 * not possible to reliably ensure that PHP or a security extension will
 * not issue a warning/error if they prevent the use of this function.
 *
 * @param $time_limit
 *   An integer specifying the new time limit, in seconds. A value of 0
 *   indicates unlimited execution time.
 *
 * @ingroup php_wrappers
 */
function drupal_set_time_limit($time_limit) {
  if (function_exists('set_time_limit')) {
    @set_time_limit($time_limit);
  }
}

/**
 * Returns the path to a system item (module, theme, etc.).
 *
 * @param $type
 *   The type of the item (i.e. theme, theme_engine, module, profile).
 * @param $name
 *   The name of the item for which the path is requested.
 *
 * @return
 *   The path to the requested item or an empty string if the item is not found.
 */
function drupal_get_path($type, $name) {
  return dirname(drupal_get_filename($type, $name));
}

/**
 * Returns the base URL path (i.e., directory) of the Drupal installation.
 *
 * base_path() adds a "/" to the beginning and end of the returned path if the
 * path is not empty. At the very least, this will return "/".
 *
 * Examples:
 * - http://example.com returns "/" because the path is empty.
 * - http://example.com/drupal/folder returns "/drupal/folder/".
 */
function base_path() {
  return $GLOBALS['base_path'];
}

/**
 * Adds a LINK tag with a distinct 'rel' attribute to the page's HEAD.
 *
 * This function can be called as long the HTML header hasn't been sent, which
 * on normal pages is up through the preprocess step of theme('html'). Adding
 * a link will overwrite a prior link with the exact same 'rel' and 'href'
 * attributes.
 *
 * @param $attributes
 *   Associative array of element attributes including 'href' and 'rel'.
 * @param $header
 *   Optional flag to determine if a HTTP 'Link:' header should be sent.
 */
function drupal_add_html_head_link($attributes, $header = FALSE) {
  $element = array(
    '#tag' => 'link',
    '#attributes' => $attributes,
  );
  $href = $attributes['href'];

  if ($header) {
    // Also add a HTTP header "Link:".
    $href = '<' . check_plain($attributes['href']) . '>;';
    unset($attributes['href']);
    $element['#attached']['drupal_add_http_header'][] = array('Link',  $href . drupal_http_header_attributes($attributes), TRUE);
  }

  drupal_add_html_head($element, 'drupal_add_html_head_link:' . $attributes['rel'] . ':' . $href);
}

/**
 * Adds a cascading stylesheet to the stylesheet queue.
 *
 * Calling drupal_static_reset('drupal_add_css') will clear all cascading
 * stylesheets added so far.
 *
 * If CSS aggregation/compression is enabled, all cascading style sheets added
 * with $options['preprocess'] set to TRUE will be merged into one aggregate
 * file and compressed by removing all extraneous white space.
 * Preprocessed inline stylesheets will not be aggregated into this single file;
 * instead, they are just compressed upon output on the page. Externally hosted
 * stylesheets are never aggregated or compressed.
 *
 * The reason for aggregating the files is outlined quite thoroughly here:
 * http://www.die.net/musings/page_load_time/ "Load fewer external objects. Due
 * to request overhead, one bigger file just loads faster than two smaller ones
 * half its size."
 *
 * $options['preprocess'] should be only set to TRUE when a file is required for
 * all typical visitors and most pages of a site. It is critical that all
 * preprocessed files are added unconditionally on every page, even if the
 * files do not happen to be needed on a page. This is normally done by calling
 * drupal_add_css() in a hook_page_build() implementation.
 *
 * Non-preprocessed files should only be added to the page when they are
 * actually needed.
 *
 * @param $data
 *   (optional) The stylesheet data to be added, depending on what is passed
 *   through to the $options['type'] parameter:
 *   - 'file': The path to the CSS file relative to the base_path(), or a
 *     stream wrapper URI. For example: "modules/devel/devel.css" or
 *     "public://generated_css/stylesheet_1.css". Note that Modules should
 *     always prefix the names of their CSS files with the module name; for
 *     example, system-menus.css rather than simply menus.css. Themes can
 *     override module-supplied CSS files based on their filenames, and this
 *     prefixing helps prevent confusing name collisions for theme developers.
 *     See drupal_get_css() where the overrides are performed. Also, if the
 *     direction of the current language is right-to-left (Hebrew, Arabic,
 *     etc.), the function will also look for an RTL CSS file and append it to
 *     the list. The name of this file should have an '-rtl.css' suffix. For
 *     example, a CSS file called 'mymodule-name.css' will have a
 *     'mymodule-name-rtl.css' file added to the list, if exists in the same
 *     directory. This CSS file should contain overrides for properties which
 *     should be reversed or otherwise different in a right-to-left display.
 *   - 'inline': A string of CSS that should be placed in the given scope. Note
 *     that it is better practice to use 'file' stylesheets, rather than
 *     'inline', as the CSS would then be aggregated and cached.
 *   - 'external': The absolute path to an external CSS file that is not hosted
 *     on the local server. These files will not be aggregated if CSS
 *     aggregation is enabled.
 * @param $options
 *   (optional) A string defining the 'type' of CSS that is being added in the
 *   $data parameter ('file', 'inline', or 'external'), or an array which can
 *   have any or all of the following keys:
 *   - 'type': The type of stylesheet being added. Available options are 'file',
 *     'inline' or 'external'. Defaults to 'file'.
 *   - 'basename': Force a basename for the file being added. Modules are
 *     expected to use stylesheets with unique filenames, but integration of
 *     external libraries may make this impossible. The basename of
 *     'core/modules/node/node.css' is 'node.css'. If the external library
 *     "node.js" ships with a 'node.css', then a different, unique basename
 *     would be 'node.js.css'.
 *   - 'group': A number identifying the aggregation group in which to add the
 *     stylesheet. Available constants are:
 *     - CSS_AGGREGATE_DEFAULT: (default) Any module-layer CSS.
 *     - CSS_AGGREGATE_THEME: Any theme-layer CSS.
 *     The aggregate group number affects load order and the CSS cascade.
 *     Stylesheets in an aggregate with a lower group number will be output to
 *     the page before stylesheets in an aggregate with a higher group number,
 *     so CSS within higher aggregate groups can take precendence over CSS
 *     within lower aggregate groups.
 *   - 'every_page': For optimal front-end performance when aggregation is
 *     enabled, this should be set to TRUE if the stylesheet is present on every
 *     page of the website for users for whom it is present at all. This
 *     defaults to FALSE. It is set to TRUE for stylesheets added via module and
 *     theme .info.yml files. Modules that add stylesheets within
 *     hook_page_build() implementations, or from other code that ensures that
 *     the stylesheet is added to all website pages, should also set this flag
 *     to TRUE. All stylesheets within the same group that have the 'every_page'
 *     flag set to TRUE and do not have 'preprocess' set to FALSE are aggregated
 *     together into a single aggregate file, and that aggregate file can be
 *     reused across a user's entire site visit, leading to faster navigation
 *     between pages.
 *     However, stylesheets that are only needed on pages less frequently
 *     visited, can be added by code that only runs for those particular pages,
 *     and that code should not set the 'every_page' flag. This minimizes the
 *     size of the aggregate file that the user needs to download when first
 *     visiting the website. Stylesheets without the 'every_page' flag are
 *     aggregated into a separate aggregate file. This other aggregate file is
 *     likely to change from page to page, and each new aggregate file needs to
 *     be downloaded when first encountered, so it should be kept relatively
 *     small by ensuring that most commonly needed stylesheets are added to
 *     every page.
 *   - 'weight': The weight of the stylesheet specifies the order in which the
 *     CSS will appear relative to other stylesheets with the same aggregate
 *     group and 'every_page' flag. The exact ordering of stylesheets is as
 *     follows:
 *     - First by aggregate group.
 *     - Then by the 'every_page' flag, with TRUE coming before FALSE.
 *     - Then by weight.
 *     - Then by the order in which the CSS was added. For example, all else
 *       being the same, a stylesheet added by a call to drupal_add_css() that
 *       happened later in the page request gets added to the page after one for
 *       which drupal_add_css() happened earlier in the page request.
 *     Available constants are:
 *     - CSS_BASE: Styles for HTML elements ("base" styles).
 *     - CSS_LAYOUT: Styles that layout a page.
 *     - CSS_COMPONENT: Styles for design components (and their associated
 *       states and skins.)
 *     - CSS_STATE: Styles for states that are not included with components.
 *     - CSS_SKIN: Styles for skins that are not included with components.
 *     The weight numbers follow the SMACSS convention of CSS categorization.
 *     See http://drupal.org/node/1887922
 *   - 'media': The media type for the stylesheet, e.g., all, print, screen.
 *     Defaults to 'all'. It is extremely important to leave this set to 'all'
 *     or it will negatively impact front-end peformance. Instead add a @media
 *     block to the included CSS file.
 *   - 'preprocess': If TRUE and CSS aggregation/compression is enabled, the
 *     styles will be aggregated and compressed. Defaults to TRUE.
 *   - 'browsers': An array containing information specifying which browsers
 *     should load the CSS item. See drupal_pre_render_conditional_comments()
 *     for details.
 *
 * @return
 *   An array of queued cascading stylesheets.
 *
 * @deprecated as of Drupal 8.0. Use the #attached key in render arrays instead.
 *
 * @see drupal_get_css()
 */
function drupal_add_css($data = NULL, $options = NULL) {
  $css = &drupal_static(__FUNCTION__, array());

  // Construct the options, taking the defaults into consideration.
  if (isset($options)) {
    if (!is_array($options)) {
      $options = array('type' => $options);
    }
  }
  else {
    $options = array();
  }

  // Create an array of CSS files for each media type first, since each type needs to be served
  // to the browser differently.
  if (isset($data)) {
    $options += array(
      'type' => 'file',
      'group' => CSS_AGGREGATE_DEFAULT,
      'weight' => 0,
      'every_page' => FALSE,
      'media' => 'all',
      'preprocess' => TRUE,
      'data' => $data,
      'browsers' => array(),
    );
    $options['browsers'] += array(
      'IE' => TRUE,
      '!IE' => TRUE,
    );

    // Files with a query string cannot be preprocessed.
    if ($options['type'] === 'file' && $options['preprocess'] && strpos($options['data'], '?') !== FALSE) {
      $options['preprocess'] = FALSE;
    }

    // Always add a tiny value to the weight, to conserve the insertion order.
    $options['weight'] += count($css) / 1000;

    // Add the data to the CSS array depending on the type.
    switch ($options['type']) {
      case 'inline':
        // For inline stylesheets, we don't want to use the $data as the array
        // key as $data could be a very long string of CSS.
        $css[] = $options;
        break;

      case 'file':
        // Local CSS files are keyed by basename; if a file with the same
        // basename is added more than once, it gets overridden.
        // By default, take over the filename as basename.
        if (!isset($options['basename'])) {
          $options['basename'] = drupal_basename($data);
        }
        $css[$options['basename']] = $options;
        break;

      default:
        // External files are keyed by their full URI, so the same CSS file is
        // not added twice.
        $css[$data] = $options;
    }
  }

  return $css;
}

/**
 * Returns a themed representation of all stylesheets to attach to the page.
 *
 * It loads the CSS in order, with 'module' first, then 'theme' afterwards.
 * This ensures proper cascading of styles so themes can easily override
 * module styles through CSS selectors.
 *
 * Themes may replace module-defined CSS files by adding a stylesheet with the
 * same filename. For example, themes/bartik/system-menus.css would replace
 * modules/system/system-menus.css. This allows themes to override complete
 * CSS files, rather than specific selectors, when necessary.
 *
 * If the original CSS file is being overridden by a theme, the theme is
 * responsible for supplying an accompanying RTL CSS file to replace the
 * module's.
 *
 * @param $css
 *   (optional) An array of CSS files. If no array is provided, the default
 *   stylesheets array is used instead.
 * @param $skip_alter
 *   (optional) If set to TRUE, this function skips calling drupal_alter() on
 *   $css, useful when the calling function passes a $css array that has already
 *   been altered.
 *
 * @return
 *   A string of XHTML CSS tags.
 *
 * @see drupal_add_css()
 */
function drupal_get_css($css = NULL, $skip_alter = FALSE) {
  global $theme_info;

  if (!isset($css)) {
    $css = drupal_add_css();
  }

  // Allow modules and themes to alter the CSS items.
  if (!$skip_alter) {
    drupal_alter('css', $css);
  }

  // Sort CSS items, so that they appear in the correct order.
  uasort($css, 'drupal_sort_css_js');

  // Allow themes to remove CSS files by basename.
  if (!empty($theme_info->stylesheets_remove)) {
    foreach ($css as $key => $options) {
      if (isset($options['basename']) && isset($theme_info->stylesheets_remove[$options['basename']])) {
        unset($css[$key]);
      }
    }
  }
  // Allow themes to conditionally override CSS files by basename.
  if (!empty($theme_info->stylesheets_override)) {
    foreach ($css as $key => $options) {
      if (isset($options['basename']) && isset($theme_info->stylesheets_override[$options['basename']])) {
        $css[$key]['data'] = $theme_info->stylesheets_override[$options['basename']];
      }
    }
  }

  // Render the HTML needed to load the CSS.
  $styles = array(
    '#type' => 'styles',
    '#items' => $css,
  );
  if (!empty($setting)) {
    $styles['#attached']['js'][] = array('type' => 'setting', 'data' => $setting);
  }

  return drupal_render($styles);
}

/**
 * Sorts CSS and JavaScript resources.
 *
 * Callback for uasort() within:
 * - drupal_get_css()
 * - drupal_get_js()
 *
 * This sort order helps optimize front-end performance while providing modules
 * and themes with the necessary control for ordering the CSS and JavaScript
 * appearing on a page.
 *
 * @param $a
 *   First item for comparison. The compared items should be associative arrays
 *   of member items from drupal_add_css() or drupal_add_js().
 * @param $b
 *   Second item for comparison.
 *
 * @see drupal_add_css()
 * @see drupal_add_js()
 */
function drupal_sort_css_js($a, $b) {
  // First order by group, so that all items in the CSS_AGGREGATE_DEFAULT group
  // appear before items in the CSS_AGGREGATE_THEME group. Modules may create
  // additional groups by defining their own constants.
  if ($a['group'] < $b['group']) {
    return -1;
  }
  elseif ($a['group'] > $b['group']) {
    return 1;
  }
  // Within a group, order all infrequently needed, page-specific files after
  // common files needed throughout the website. Separating this way allows for
  // the aggregate file generated for all of the common files to be reused
  // across a site visit without being cut by a page using a less common file.
  elseif ($a['every_page'] && !$b['every_page']) {
    return -1;
  }
  elseif (!$a['every_page'] && $b['every_page']) {
    return 1;
  }
  // Finally, order by weight.
  elseif ($a['weight'] < $b['weight']) {
    return -1;
  }
  elseif ($a['weight'] > $b['weight']) {
    return 1;
  }
  else {
    return 0;
  }
}

/**
 * Grouping callback: Groups CSS items by their types, media, and browsers.
 *
 * This function arranges the CSS items that are in the #items property of the
 * styles element into groups. Arranging the CSS items into groups serves two
 * purposes. When aggregation is enabled, files within a group are aggregated
 * into a single file, significantly improving page loading performance by
 * minimizing network traffic overhead. When aggregation is disabled, grouping
 * allows multiple files to be loaded from a single STYLE tag, enabling sites
 * with many modules enabled or a complex theme being used to stay within IE's
 * 31 CSS inclusion tag limit: http://drupal.org/node/228818.
 *
 * This function puts multiple items into the same group if they are groupable
 * and if they are for the same 'media' and 'browsers'. Items of the 'file' type
 * are groupable if their 'preprocess' flag is TRUE, items of the 'inline' type
 * are always groupable, and items of the 'external' type are never groupable.
 * This function also ensures that the process of grouping items does not change
 * their relative order. This requirement may result in multiple groups for the
 * same type, media, and browsers, if needed to accommodate other items in
 * between.
 *
 * @param $css
 *   An array of CSS items, as returned by drupal_add_css(), but after
 *   alteration performed by drupal_get_css().
 *
 * @return
 *   An array of CSS groups. Each group contains the same keys (e.g., 'media',
 *   'data', etc.) as a CSS item from the $css parameter, with the value of
 *   each key applying to the group as a whole. Each group also contains an
 *   'items' key, which is the subset of items from $css that are in the group.
 *
 * @see drupal_pre_render_styles()
 * @see system_element_info()
 */
function drupal_group_css($css) {
  $groups = array();
  // If a group can contain multiple items, we track the information that must
  // be the same for each item in the group, so that when we iterate the next
  // item, we can determine if it can be put into the current group, or if a
  // new group needs to be made for it.
  $current_group_keys = NULL;
  // When creating a new group, we pre-increment $i, so by initializing it to
  // -1, the first group will have index 0.
  $i = -1;
  foreach ($css as $item) {
    // The browsers for which the CSS item needs to be loaded is part of the
    // information that determines when a new group is needed, but the order of
    // keys in the array doesn't matter, and we don't want a new group if all
    // that's different is that order.
    ksort($item['browsers']);

    // If the item can be grouped with other items, set $group_keys to an array
    // of information that must be the same for all items in its group. If the
    // item can't be grouped with other items, set $group_keys to FALSE. We
    // put items into a group that can be aggregated together: whether they will
    // be aggregated is up to the _drupal_css_aggregate() function or an
    // override of that function specified in hook_css_alter(), but regardless
    // of the details of that function, a group represents items that can be
    // aggregated. Since a group may be rendered with a single HTML tag, all
    // items in the group must share the same information that would need to be
    // part of that HTML tag.
    switch ($item['type']) {
      case 'file':
        // Group file items if their 'preprocess' flag is TRUE.
        // Help ensure maximum reuse of aggregate files by only grouping
        // together items that share the same 'group' value and 'every_page'
        // flag. See drupal_add_css() for details about that.
        $group_keys = $item['preprocess'] ? array($item['type'], $item['group'], $item['every_page'], $item['media'], $item['browsers']) : FALSE;
        break;
      case 'inline':
        // Always group inline items.
        $group_keys = array($item['type'], $item['media'], $item['browsers']);
        break;
      case 'external':
        // Do not group external items.
        $group_keys = FALSE;
        break;
    }

    // If the group keys don't match the most recent group we're working with,
    // then a new group must be made.
    if ($group_keys !== $current_group_keys) {
      $i++;
      // Initialize the new group with the same properties as the first item
      // being placed into it. The item's 'data' and 'weight' properties are
      // unique to the item and should not be carried over to the group.
      $groups[$i] = $item;
      unset($groups[$i]['data'], $groups[$i]['weight']);
      $groups[$i]['items'] = array();
      $current_group_keys = $group_keys ? $group_keys : NULL;
    }

    // Add the item to the current group.
    $groups[$i]['items'][] = $item;
  }
  return $groups;
}

/**
 * Aggregation callback: Aggregates CSS files and inline content.
 *
 * Having the browser load fewer CSS files results in much faster page loads
 * than when it loads many small files. This function aggregates files within
 * the same group into a single file unless the site-wide setting to do so is
 * disabled (commonly the case during site development). To optimize download,
 * it also compresses the aggregate files by removing comments, whitespace, and
 * other unnecessary content. Additionally, this functions aggregates inline
 * content together, regardless of the site-wide aggregation setting.
 *
 * @param $css_groups
 *   An array of CSS groups as returned by drupal_group_css(). This function
 *   modifies the group's 'data' property for each group that is aggregated.
 *
 * @see drupal_group_css()
 * @see drupal_pre_render_styles()
 * @see system_element_info()
 */
function drupal_aggregate_css(&$css_groups) {
  // Only aggregate during normal site operation.
  if (defined('MAINTENANCE_MODE')) {
    $preprocess_css = FALSE;
  }
  else {
    $config = config('system.performance');
    $preprocess_css = $config->get('css.preprocess');
  }

  // For each group that needs aggregation, aggregate its items.
  foreach ($css_groups as $key => $group) {
    switch ($group['type']) {
      // If a file group can be aggregated into a single file, do so, and set
      // the group's data property to the file path of the aggregate file.
      case 'file':
        if ($group['preprocess'] && $preprocess_css) {
          $css_groups[$key]['data'] = drupal_build_css_cache($group['items']);
        }
        break;
      // Aggregate all inline CSS content into the group's data property.
      case 'inline':
        $css_groups[$key]['data'] = '';
        foreach ($group['items'] as $item) {
          $css_groups[$key]['data'] .= drupal_load_stylesheet_content($item['data'], $item['preprocess']);
        }
        break;
    }
  }
}

/**
 * Pre-render callback: Adds the elements needed for CSS tags to be rendered.
 *
 * For production websites, LINK tags are preferable to STYLE tags with @import
 * statements, because:
 * - They are the standard tag intended for linking to a resource.
 * - On Firefox 2 and perhaps other browsers, CSS files included with @import
 *   statements don't get saved when saving the complete web page for offline
 *   use: http://drupal.org/node/145218.
 * - On IE, if only LINK tags and no @import statements are used, all the CSS
 *   files are downloaded in parallel, resulting in faster page load, but if
 *   @import statements are used and span across multiple STYLE tags, all the
 *   ones from one STYLE tag must be downloaded before downloading begins for
 *   the next STYLE tag. Furthermore, IE7 does not support media declaration on
 *   the @import statement, so multiple STYLE tags must be used when different
 *   files are for different media types. Non-IE browsers always download in
 *   parallel, so this is an IE-specific performance quirk:
 *   http://www.stevesouders.com/blog/2009/04/09/dont-use-import/.
 *
 * However, IE has an annoying limit of 31 total CSS inclusion tags
 * (http://drupal.org/node/228818) and LINK tags are limited to one file per
 * tag, whereas STYLE tags can contain multiple @import statements allowing
 * multiple files to be loaded per tag. When CSS aggregation is disabled, a
 * Drupal site can easily have more than 31 CSS files that need to be loaded, so
 * using LINK tags exclusively would result in a site that would display
 * incorrectly in IE. Depending on different needs, different strategies can be
 * employed to decide when to use LINK tags and when to use STYLE tags.
 *
 * The strategy employed by this function is to use LINK tags for all aggregate
 * files and for all files that cannot be aggregated (e.g., if 'preprocess' is
 * set to FALSE or the type is 'external'), and to use STYLE tags for groups
 * of files that could be aggregated together but aren't (e.g., if the site-wide
 * aggregation setting is disabled). This results in all LINK tags when
 * aggregation is enabled, a guarantee that as many or only slightly more tags
 * are used with aggregation disabled than enabled (so that if the limit were to
 * be crossed with aggregation enabled, the site developer would also notice the
 * problem while aggregation is disabled), and an easy way for a developer to
 * view HTML source while aggregation is disabled and know what files will be
 * aggregated together when aggregation becomes enabled.
 *
 * This function evaluates the aggregation enabled/disabled condition on a group
 * by group basis by testing whether an aggregate file has been made for the
 * group rather than by testing the site-wide aggregation setting. This allows
 * this function to work correctly even if modules have implemented custom
 * logic for grouping and aggregating files.
 *
 * @param $element
 *   A render array containing:
 *   - '#items': The CSS items as returned by drupal_add_css() and altered by
 *     drupal_get_css().
 *   - '#group_callback': A function to call to group #items to enable the use
 *     of fewer tags by aggregating files and/or using multiple @import
 *     statements within a single tag.
 *   - '#aggregate_callback': A function to call to aggregate the items within
 *     the groups arranged by the #group_callback function.
 *
 * @return
 *   A render array that will render to a string of XHTML CSS tags.
 *
 * @see drupal_get_css()
 */
function drupal_pre_render_styles($elements) {
  // Group and aggregate the items.
  if (isset($elements['#group_callback'])) {
    $elements['#groups'] = $elements['#group_callback']($elements['#items']);
  }
  if (isset($elements['#aggregate_callback'])) {
    $elements['#aggregate_callback']($elements['#groups']);
  }

  // A dummy query-string is added to filenames, to gain control over
  // browser-caching. The string changes on every update or full cache
  // flush, forcing browsers to load a new copy of the files, as the
  // URL changed.
  $query_string = variable_get('css_js_query_string', '0');

  // For inline CSS to validate as XHTML, all CSS containing XHTML needs to be
  // wrapped in CDATA. To make that backwards compatible with HTML 4, we need to
  // comment out the CDATA-tag.
  $embed_prefix = "\n/* <![CDATA[ */\n";
  $embed_suffix = "\n/* ]]> */\n";

  // Defaults for LINK and STYLE elements.
  $link_element_defaults = array(
    '#type' => 'html_tag',
    '#tag' => 'link',
    '#attributes' => array(
      'rel' => 'stylesheet',
    ),
  );
  $style_element_defaults = array(
    '#type' => 'html_tag',
    '#tag' => 'style',
  );

  // Loop through each group.
  foreach ($elements['#groups'] as $group) {
    switch ($group['type']) {
      // For file items, there are three possibilites.
      // - The group has been aggregated: in this case, output a LINK tag for
      //   the aggregate file.
      // - The group can be aggregated but has not been (most likely because
      //   the site administrator disabled the site-wide setting): in this case,
      //   output as few STYLE tags for the group as possible, using @import
      //   statement for each file in the group. This enables us to stay within
      //   IE's limit of 31 total CSS inclusion tags.
      // - The group contains items not eligible for aggregation (their
      //   'preprocess' flag has been set to FALSE): in this case, output a LINK
      //   tag for each file.
      case 'file':
        // The group has been aggregated into a single file: output a LINK tag
        // for the aggregate file.
        if (isset($group['data'])) {
          $element = $link_element_defaults;
          $element['#attributes']['href'] = file_create_url($group['data']);
          $element['#attributes']['media'] = $group['media'];
          $element['#browsers'] = $group['browsers'];
          $elements[] = $element;
        }
        // The group can be aggregated, but hasn't been: combine multiple items
        // into as few STYLE tags as possible.
        elseif ($group['preprocess']) {
          $import = array();
          foreach ($group['items'] as $item) {
            // The dummy query string needs to be added to the URL to control
            // browser-caching. IE7 does not support a media type on the
            // @import statement, so we instead specify the media for the
            // group on the STYLE tag.
            $import[] = '@import url("' . check_plain(file_create_url($item['data']) . '?' . $query_string) . '");';
          }
          // In addition to IE's limit of 31 total CSS inclusion tags, it also
          // has a limit of 31 @import statements per STYLE tag.
          while (!empty($import)) {
            $import_batch = array_slice($import, 0, 31);
            $import = array_slice($import, 31);
            $element = $style_element_defaults;
            // This simplifies the JavaScript regex, allowing each line
            // (separated by \n) to be treated as a completely different string.
            // This means that we can use ^ and $ on one line at a time, and not
            // worry about style tags since they'll never match the regex.
            $element['#value'] = "\n" . implode("\n", $import_batch) . "\n";
            $element['#attributes']['media'] = $group['media'];
            $element['#browsers'] = $group['browsers'];
            $elements[] = $element;
          }
        }
        // The group contains items ineligible for aggregation: output a LINK
        // tag for each file.
        else {
          foreach ($group['items'] as $item) {
            $element = $link_element_defaults;
            // The dummy query string needs to be added to the URL to control
            // browser-caching.
            $query_string_separator = (strpos($item['data'], '?') !== FALSE) ? '&' : '?';
            $element['#attributes']['href'] = file_create_url($item['data']) . $query_string_separator . $query_string;
            $element['#attributes']['media'] = $item['media'];
            $element['#browsers'] = $group['browsers'];
            $elements[] = $element;
          }
        }
        break;
      // For inline content, the 'data' property contains the CSS content. If
      // the group's 'data' property is set, then output it in a single STYLE
      // tag. Otherwise, output a separate STYLE tag for each item.
      case 'inline':
        if (isset($group['data'])) {
          $element = $style_element_defaults;
          $element['#value'] = $group['data'];
          $element['#value_prefix'] = $embed_prefix;
          $element['#value_suffix'] = $embed_suffix;
          $element['#attributes']['media'] = $group['media'];
          $element['#browsers'] = $group['browsers'];
          $elements[] = $element;
        }
        else {
          foreach ($group['items'] as $item) {
            $element = $style_element_defaults;
            $element['#value'] = $item['data'];
            $element['#value_prefix'] = $embed_prefix;
            $element['#value_suffix'] = $embed_suffix;
            $element['#attributes']['media'] = $item['media'];
            $element['#browsers'] = $group['browsers'];
            $elements[] = $element;
          }
        }
        break;
      // Output a LINK tag for each external item. The item's 'data' property
      // contains the full URL.
      case 'external':
        foreach ($group['items'] as $item) {
          $element = $link_element_defaults;
          $element['#attributes']['href'] = $item['data'];
          $element['#attributes']['media'] = $item['media'];
          $element['#browsers'] = $group['browsers'];
          $elements[] = $element;
        }
        break;
    }
  }

  return $elements;
}

/**
 * Aggregates and optimizes CSS files into a cache file in the files directory.
 *
 * The file name for the CSS cache file is generated from the hash of the
 * aggregated contents of the files in $css. This forces proxies and browsers
 * to download new CSS when the CSS changes.
 *
 * The cache file name is retrieved on a page load via a lookup variable that
 * contains an associative array. The array key is the hash of the file names
 * in $css while the value is the cache file name. The cache file is generated
 * in two cases. First, if there is no file name value for the key, which will
 * happen if a new file name has been added to $css or after the lookup
 * variable is emptied to force a rebuild of the cache. Second, the cache file
 * is generated if it is missing on disk. Old cache files are not deleted
 * immediately when the lookup variable is emptied, but are deleted after a set
 * period by drupal_delete_file_if_stale(). This ensures that files referenced
 * by a cached page will still be available.
 *
 * @param $css
 *   An array of CSS files to aggregate and compress into one file.
 *
 * @return
 *   The URI of the CSS cache file, or FALSE if the file could not be saved.
 */
function drupal_build_css_cache($css) {
  $data = '';
  $uri = '';
  $map = Drupal::state()->get('drupal_css_cache_files') ?: array();
  // Create a new array so that only the file names are used to create the hash.
  // This prevents new aggregates from being created unnecessarily.
  $css_data = array();
  foreach ($css as $css_file) {
    $css_data[] = $css_file['data'];
  }
  $key = hash('sha256', serialize($css_data));
  if (isset($map[$key])) {
    $uri = $map[$key];
  }

  if (empty($uri) || !file_exists($uri)) {
    // Build aggregate CSS file.
    foreach ($css as $stylesheet) {
      // Only 'file' stylesheets can be aggregated.
      if ($stylesheet['type'] == 'file') {
        $contents = drupal_load_stylesheet($stylesheet['data'], TRUE);

        // Get the parent directory of this file, relative to the Drupal root.
        $css_base_url = substr($stylesheet['data'], 0, strrpos($stylesheet['data'], '/'));
        _drupal_build_css_path(NULL, $css_base_url . '/');
        // Anchor all paths in the CSS with its base URL, ignoring external and absolute paths.
        $data .= preg_replace_callback('/url\(\s*[\'"]?(?![a-z]+:|\/+)([^\'")]+)[\'"]?\s*\)/i', '_drupal_build_css_path', $contents);
      }
    }

    // Per the W3C specification at http://www.w3.org/TR/REC-CSS2/cascade.html#at-import,
    // @import rules must proceed any other style, so we move those to the top.
    $regexp = '/@import[^;]+;/i';
    preg_match_all($regexp, $data, $matches);
    $data = preg_replace($regexp, '', $data);
    $data = implode('', $matches[0]) . $data;

    // Prefix filename to prevent blocking by firewalls which reject files
    // starting with "ad*".
    $filename = 'css_' . Crypt::hashBase64($data) . '.css';
    // Create the css/ within the files folder.
    $csspath = 'public://css';
    $uri = $csspath . '/' . $filename;
    // Create the CSS file.
    file_prepare_directory($csspath, FILE_CREATE_DIRECTORY);
    if (!file_exists($uri) && !file_unmanaged_save_data($data, $uri, FILE_EXISTS_REPLACE)) {
      return FALSE;
    }
    // If CSS gzip compression is enabled and the zlib extension is available
    // then create a gzipped version of this file. This file is served
    // conditionally to browsers that accept gzip using .htaccess rules.
    // It's possible that the rewrite rules in .htaccess aren't working on this
    // server, but there's no harm (other than the time spent generating the
    // file) in generating the file anyway. Sites on servers where rewrite rules
    // aren't working can set css.gzip to FALSE in order to skip
    // generating a file that won't be used.
    if (config('system.performance')->get('css.gzip') && extension_loaded('zlib')) {
      if (!file_exists($uri . '.gz') && !file_unmanaged_save_data(gzencode($data, 9, FORCE_GZIP), $uri . '.gz', FILE_EXISTS_REPLACE)) {
        return FALSE;
      }
    }
    // Save the updated map.
    $map[$key] = $uri;
    Drupal::state()->set('drupal_css_cache_files', $map);
  }
  return $uri;
}

/**
 * Prefixes all paths within a CSS file for drupal_build_css_cache().
 */
function _drupal_build_css_path($matches, $base = NULL) {
  $_base = &drupal_static(__FUNCTION__);
  // Store base path for preg_replace_callback.
  if (isset($base)) {
    $_base = $base;
  }

  // Prefix with base and remove '../' segments where possible.
  $path = $_base . $matches[1];
  $last = '';
  while ($path != $last) {
    $last = $path;
    $path = preg_replace('`(^|/)(?!\.\./)([^/]+)/\.\./`', '$1', $path);
  }
  return 'url(' . file_create_url($path) . ')';
}

/**
 * Loads the stylesheet and resolves all @import commands.
 *
 * Loads a stylesheet and replaces @import commands with the contents of the
 * imported file. Use this instead of file_get_contents when processing
 * stylesheets.
 *
 * The returned contents are compressed removing white space and comments only
 * when CSS aggregation is enabled. This optimization will not apply for
 * color.module enabled themes with CSS aggregation turned off.
 *
 * @param $file
 *   Name of the stylesheet to be processed.
 * @param $optimize
 *   Defines if CSS contents should be compressed or not.
 * @param $reset_basepath
 *   Used internally to facilitate recursive resolution of @import commands.
 *
 * @return
 *   Contents of the stylesheet, including any resolved @import commands.
 */
function drupal_load_stylesheet($file, $optimize = NULL, $reset_basepath = TRUE) {
  // These statics are not cache variables, so we don't use drupal_static().
  static $_optimize, $basepath;
  if ($reset_basepath) {
    $basepath = '';
  }
  // Store the value of $optimize for preg_replace_callback with nested
  // @import loops.
  if (isset($optimize)) {
    $_optimize = $optimize;
  }

  // Stylesheets are relative one to each other. Start by adding a base path
  // prefix provided by the parent stylesheet (if necessary).
  if ($basepath && !file_uri_scheme($file)) {
    $file = $basepath . '/' . $file;
  }
  $basepath = dirname($file);

  // Load the CSS stylesheet. We suppress errors because themes may specify
  // stylesheets in their .info.yml file that don't exist in the theme's path,
  // but are merely there to disable certain module CSS files.
  if ($contents = @file_get_contents($file)) {
    // Return the processed stylesheet.
    return drupal_load_stylesheet_content($contents, $_optimize);
  }

  return '';
}

/**
 * Processes the contents of a stylesheet for aggregation.
 *
 * @param $contents
 *   The contents of the stylesheet.
 * @param $optimize
 *   (optional) Boolean whether CSS contents should be minified. Defaults to
 *   FALSE.
 *
 * @return
 *   Contents of the stylesheet including the imported stylesheets.
 */
function drupal_load_stylesheet_content($contents, $optimize = FALSE) {
  // Remove multiple charset declarations for standards compliance (and fixing Safari problems).
  $contents = preg_replace('/^@charset\s+[\'"](\S*)\b[\'"];/i', '', $contents);

  if ($optimize) {
    // Perform some safe CSS optimizations.
    // Regexp to match comment blocks.
    $comment     = '/\*[^*]*\*+(?:[^/*][^*]*\*+)*/';
    // Regexp to match double quoted strings.
    $double_quot = '"[^"\\\\]*(?:\\\\.[^"\\\\]*)*"';
    // Regexp to match single quoted strings.
    $single_quot = "'[^'\\\\]*(?:\\\\.[^'\\\\]*)*'";
    // Strip all comment blocks, but keep double/single quoted strings.
    $contents = preg_replace(
      "<($double_quot|$single_quot)|$comment>Ss",
      "$1",
      $contents
    );
    // Remove certain whitespace.
    // There are different conditions for removing leading and trailing
    // whitespace.
    // @see http://php.net/manual/regexp.reference.subpatterns.php
    $contents = preg_replace('<
      # Strip leading and trailing whitespace.
        \s*([@{};,])\s*
      # Strip only leading whitespace from:
      # - Closing parenthesis: Retain "@media (bar) and foo".
      | \s+([\)])
      # Strip only trailing whitespace from:
      # - Opening parenthesis: Retain "@media (bar) and foo".
      # - Colon: Retain :pseudo-selectors.
      | ([\(:])\s+
    >xS',
      // Only one of the three capturing groups will match, so its reference
      // will contain the wanted value and the references for the
      // two non-matching groups will be replaced with empty strings.
      '$1$2$3',
      $contents
    );
    // End the file with a new line.
    $contents = trim($contents);
    $contents .= "\n";
  }

  // Replaces @import commands with the actual stylesheet content.
  // This happens recursively but omits external files.
  $contents = preg_replace_callback('/@import\s*(?:url\(\s*)?[\'"]?(?![a-z]+:)([^\'"\()]+)[\'"]?\s*\)?\s*;/', '_drupal_load_stylesheet', $contents);
  return $contents;
}

/**
 * Loads stylesheets recursively and returns contents with corrected paths.
 *
 * This function is used for recursive loading of stylesheets and
 * returns the stylesheet content with all url() paths corrected.
 */
function _drupal_load_stylesheet($matches) {
  $filename = $matches[1];
  // Load the imported stylesheet and replace @import commands in there as well.
  $file = drupal_load_stylesheet($filename, NULL, FALSE);

  // Determine the file's directory.
  $directory = dirname($filename);
  // If the file is in the current directory, make sure '.' doesn't appear in
  // the url() path.
  $directory = $directory == '.' ? '' : $directory .'/';

  // Alter all internal url() paths. Leave external paths alone. We don't need
  // to normalize absolute paths here (i.e. remove folder/... segments) because
  // that will be done later.
  return preg_replace('/url\(\s*([\'"]?)(?![a-z]+:|\/+)/i', 'url(\1'. $directory, $file);
}

/**
 * Deletes old cached CSS files.
 */
function drupal_clear_css_cache() {
  Drupal::state()->delete('drupal_css_cache_files');
  file_scan_directory('public://css', '/.*/', array('callback' => 'drupal_delete_file_if_stale'));
}

/**
 * Deletes files modified more than a set time ago.
 *
 * Callback for file_scan_directory() within:
 * - drupal_clear_css_cache()
 * - drupal_clear_js_cache()
 */
function drupal_delete_file_if_stale($uri) {
  // Default stale file threshold is 30 days.
  if (REQUEST_TIME - filemtime($uri) > config('system.performance')->get('stale_file_threshold')) {
    file_unmanaged_delete($uri);
  }
}

/**
 * Prepares a string for use as a CSS identifier (element, class, or ID name).
 *
 * http://www.w3.org/TR/CSS21/syndata.html#characters shows the syntax for valid
 * CSS identifiers (including element names, classes, and IDs in selectors.)
 *
 * @param $identifier
 *   The identifier to clean.
 * @param $filter
 *   An array of string replacements to use on the identifier.
 *
 * @return
 *   The cleaned identifier.
 */
function drupal_clean_css_identifier($identifier, $filter = array(' ' => '-', '_' => '-', '__' => '__', '/' => '-', '[' => '-', ']' => '')) {
  // By default, we filter using Drupal's coding standards.
  $identifier = strtr($identifier, $filter);

  // Valid characters in a CSS identifier are:
  // - the hyphen (U+002D)
  // - a-z (U+0030 - U+0039)
  // - A-Z (U+0041 - U+005A)
  // - the underscore (U+005F)
  // - 0-9 (U+0061 - U+007A)
  // - ISO 10646 characters U+00A1 and higher
  // We strip out any character not in the above list.
  $identifier = preg_replace('/[^\x{002D}\x{0030}-\x{0039}\x{0041}-\x{005A}\x{005F}\x{0061}-\x{007A}\x{00A1}-\x{FFFF}]/u', '', $identifier);

  return $identifier;
}

/**
 * Prepares a string for use as a valid class name.
 *
 * Do not pass one string containing multiple classes as they will be
 * incorrectly concatenated with dashes, i.e. "one two" will become "one-two".
 *
 * @param $class
 *   The class name to clean.
 *
 * @return
 *   The cleaned class name.
 */
function drupal_html_class($class) {
  // The output of this function will never change, so this uses a normal
  // static instead of drupal_static().
  static $classes = array();

  if (!isset($classes[$class])) {
    $classes[$class] = drupal_clean_css_identifier(drupal_strtolower($class));
  }
  return $classes[$class];
}

/**
 * Prepares a string for use as a valid HTML ID and guarantees uniqueness.
 *
 * This function ensures that each passed HTML ID value only exists once on the
 * page. By tracking the already returned ids, this function enables forms,
 * blocks, and other content to be output multiple times on the same page,
 * without breaking (X)HTML validation.
 *
 * For already existing IDs, a counter is appended to the ID string. Therefore,
 * JavaScript and CSS code should not rely on any value that was generated by
 * this function and instead should rely on manually added CSS classes or
 * similarly reliable constructs.
 *
 * Two consecutive hyphens separate the counter from the original ID. To manage
 * uniqueness across multiple Ajax requests on the same page, Ajax requests
 * POST an array of all IDs currently present on the page, which are used to
 * prime this function's cache upon first invocation.
 *
 * To allow reverse-parsing of IDs submitted via Ajax, any multiple consecutive
 * hyphens in the originally passed $id are replaced with a single hyphen.
 *
 * @param $id
 *   The ID to clean.
 *
 * @return
 *   The cleaned ID.
 */
function drupal_html_id($id) {
  // If this is an Ajax request, then content returned by this page request will
  // be merged with content already on the base page. The HTML IDs must be
  // unique for the fully merged content. Therefore, initialize $seen_ids to
  // take into account IDs that are already in use on the base page.
  $seen_ids_init = &drupal_static(__FUNCTION__ . ':init');
  if (!isset($seen_ids_init)) {
    // Ideally, Drupal would provide an API to persist state information about
    // prior page requests in the database, and we'd be able to add this
    // function's $seen_ids static variable to that state information in order
    // to have it properly initialized for this page request. However, no such
    // page state API exists, so instead, ajax.js adds all of the in-use HTML
    // IDs to the POST data of Ajax submissions. Direct use of $_POST is
    // normally not recommended as it could open up security risks, but because
    // the raw POST data is cast to a number before being returned by this
    // function, this usage is safe.
    if (empty($_POST['ajax_html_ids'])) {
      $seen_ids_init = array();
    }
    else {
      // This function ensures uniqueness by appending a counter to the base id
      // requested by the calling function after the first occurrence of that
      // requested id. $_POST['ajax_html_ids'] contains the ids as they were
      // returned by this function, potentially with the appended counter, so
      // we parse that to reconstruct the $seen_ids array.
      $ajax_html_ids = explode(' ', $_POST['ajax_html_ids']);
      foreach ($ajax_html_ids as $seen_id) {
        // We rely on '--' being used solely for separating a base id from the
        // counter, which this function ensures when returning an id.
        $parts = explode('--', $seen_id, 2);
        if (!empty($parts[1]) && is_numeric($parts[1])) {
          list($seen_id, $i) = $parts;
        }
        else {
          $i = 1;
        }
        if (!isset($seen_ids_init[$seen_id]) || ($i > $seen_ids_init[$seen_id])) {
          $seen_ids_init[$seen_id] = $i;
        }
      }
    }
  }
  $seen_ids = &drupal_static(__FUNCTION__, $seen_ids_init);

  $id = strtr(drupal_strtolower($id), array(' ' => '-', '_' => '-', '[' => '-', ']' => ''));

  // As defined in http://www.w3.org/TR/html4/types.html#type-name, HTML IDs can
  // only contain letters, digits ([0-9]), hyphens ("-"), underscores ("_"),
  // colons (":"), and periods ("."). We strip out any character not in that
  // list. Note that the CSS spec doesn't allow colons or periods in identifiers
  // (http://www.w3.org/TR/CSS21/syndata.html#characters), so we strip those two
  // characters as well.
  $id = preg_replace('/[^A-Za-z0-9\-_]/', '', $id);

  // Removing multiple consecutive hyphens.
  $id = preg_replace('/\-+/', '-', $id);
  // Ensure IDs are unique by appending a counter after the first occurrence.
  // The counter needs to be appended with a delimiter that does not exist in
  // the base ID. Requiring a unique delimiter helps ensure that we really do
  // return unique IDs and also helps us re-create the $seen_ids array during
  // Ajax requests.
  if (isset($seen_ids[$id])) {
    $id = $id . '--' . ++$seen_ids[$id];
  }
  else {
    $seen_ids[$id] = 1;
  }

  return $id;
}

/**
 * Adds a JavaScript file, setting, or inline code to the page.
 *
 * The behavior of this function depends on the parameters it is called with.
 * Generally, it handles the addition of JavaScript to the page, either as
 * reference to an existing file or as inline code. The following actions can be
 * performed using this function:
 * - Add a file ('file'): Adds a reference to a JavaScript file to the page.
 * - Add inline JavaScript code ('inline'): Executes a piece of JavaScript code
 *   on the current page by placing the code directly in the page (for example,
 *   to tell the user that a new message arrived, by opening a pop up, alert
 *   box, etc.). This should only be used for JavaScript that cannot be executed
 *   from a file. When adding inline code, make sure that you are not relying on
 *   $() being the jQuery function. Wrap your code in
 *   @code (function ($) {... })(jQuery); @endcode
 *   or use jQuery() instead of $().
 * - Add external JavaScript ('external'): Allows the inclusion of external
 *   JavaScript files that are not hosted on the local server. Note that these
 *   external JavaScript references do not get aggregated when preprocessing is
 *   on.
 * - Add settings ('setting'): Adds settings to Drupal's global storage of
 *   JavaScript settings. Per-page settings are required by some modules to
 *   function properly. All settings will be accessible at Drupal.settings.
 *
 * Examples:
 * @code
 *   drupal_add_js('core/misc/collapse.js');
 *   drupal_add_js('core/misc/collapse.js', 'file');
 *   drupal_add_js('jQuery(document).ready(function () { alert("Hello!"); });', 'inline');
 *   drupal_add_js('jQuery(document).ready(function () { alert("Hello!"); });',
 *     array('type' => 'inline', 'scope' => 'footer', 'weight' => 5)
 *   );
 *   drupal_add_js('http://example.com/example.js', 'external');
 *   drupal_add_js(array('myModule' => array('key' => 'value')), 'setting');
 * @endcode
 *
 * Calling drupal_static_reset('drupal_add_js') will clear all JavaScript added
 * so far.
 *
 * If JavaScript aggregation is enabled, all JavaScript files added with
 * $options['preprocess'] set to TRUE will be merged into one aggregate file.
 * Preprocessed inline JavaScript will not be aggregated into this single file.
 * Externally hosted JavaScripts are never aggregated.
 *
 * The reason for aggregating the files is outlined quite thoroughly here:
 * http://www.die.net/musings/page_load_time/ "Load fewer external objects. Due
 * to request overhead, one bigger file just loads faster than two smaller ones
 * half its size."
 *
 * $options['preprocess'] should be only set to TRUE when a file is required for
 * all typical visitors and most pages of a site. It is critical that all
 * preprocessed files are added unconditionally on every page, even if the
 * files are not needed on a page. This is normally done by calling
 * drupal_add_js() in a hook_page_build() implementation.
 *
 * Non-preprocessed files should only be added to the page when they are
 * actually needed.
 *
 * @param $data
 *   (optional) If given, the value depends on the $options parameter, or
 *   $options['type'] if $options is passed as an associative array:
 *   - 'file': Path to the file relative to base_path().
 *   - 'inline': The JavaScript code that should be placed in the given scope.
 *   - 'external': The absolute path to an external JavaScript file that is not
 *     hosted on the local server. These files will not be aggregated if
 *     JavaScript aggregation is enabled.
 *   - 'setting': An associative array with configuration options. The array is
 *     merged directly into Drupal.settings. All modules should wrap their
 *     actual configuration settings in another variable to prevent conflicts in
 *     the Drupal.settings namespace. Items added with a string key will replace
 *     existing settings with that key; items with numeric array keys will be
 *     added to the existing settings array.
 * @param $options
 *   (optional) A string defining the type of JavaScript that is being added in
 *   the $data parameter ('file'/'setting'/'inline'/'external'), or an
 *   associative array. JavaScript settings should always pass the string
 *   'setting' only. Other types can have the following elements in the array:
 *   - type: The type of JavaScript that is to be added to the page. Allowed
 *     values are 'file', 'inline', 'external' or 'setting'. Defaults
 *     to 'file'.
 *   - scope: The location in which you want to place the script. Possible
 *     values are 'header' or 'footer'. If your theme implements different
 *     regions, you can also use these. Defaults to 'header'.
 *   - group: A number identifying the group in which to add the JavaScript.
 *     Available constants are:
 *     - JS_LIBRARY: Any libraries, settings, or jQuery plugins.
 *     - JS_DEFAULT: Any module-layer JavaScript.
 *     - JS_THEME: Any theme-layer JavaScript.
 *     The group number serves as a weight: JavaScript within a lower weight
 *     group is presented on the page before JavaScript within a higher weight
 *     group.
 *   - every_page: For optimal front-end performance when aggregation is
 *     enabled, this should be set to TRUE if the JavaScript is present on every
 *     page of the website for users for whom it is present at all. This
 *     defaults to FALSE. It is set to TRUE for JavaScript files that are added
 *     via module and theme .info.yml files. Modules that add JavaScript within
 *     hook_page_build() implementations, or from other code that ensures that
 *     the JavaScript is added to all website pages, should also set this flag
 *     to TRUE. All JavaScript files within the same group and that have the
 *     'every_page' flag set to TRUE and do not have 'preprocess' set to FALSE
 *     are aggregated together into a single aggregate file, and that aggregate
 *     file can be reused across a user's entire site visit, leading to faster
 *     navigation between pages. However, JavaScript that is only needed on
 *     pages less frequently visited, can be added by code that only runs for
 *     those particular pages, and that code should not set the 'every_page'
 *     flag. This minimizes the size of the aggregate file that the user needs
 *     to download when first visiting the website. JavaScript without the
 *     'every_page' flag is aggregated into a separate aggregate file. This
 *     other aggregate file is likely to change from page to page, and each new
 *     aggregate file needs to be downloaded when first encountered, so it
 *     should be kept relatively small by ensuring that most commonly needed
 *     JavaScript is added to every page.
 *   - weight: A number defining the order in which the JavaScript is added to
 *     the page relative to other JavaScript with the same 'scope', 'group',
 *     and 'every_page' value. In some cases, the order in which the JavaScript
 *     is presented on the page is very important. jQuery, for example, must be
 *     added to the page before any jQuery code is run, so jquery.js uses the
 *     JS_LIBRARY group and a weight of -20, jquery.once.js (a library drupal.js
 *     depends on) uses the JS_LIBRARY group and a weight of -19, drupal.js uses
 *     the JS_LIBRARY group and a weight of -1, other libraries use the
 *     JS_LIBRARY group and a weight of 0 or higher, and all other scripts use
 *     one of the other group constants. The exact ordering of JavaScript is as
 *     follows:
 *     - First by scope, with 'header' first, 'footer' last, and any other
 *       scopes provided by a custom theme coming in between, as determined by
 *       the theme.
 *     - Then by group.
 *     - Then by the 'every_page' flag, with TRUE coming before FALSE.
 *     - Then by weight.
 *     - Then by the order in which the JavaScript was added. For example, all
 *       else being the same, JavaScript added by a call to drupal_add_js() that
 *       happened later in the page request gets added to the page after one for
 *       which drupal_add_js() happened earlier in the page request.
 *   - cache: If set to FALSE, the JavaScript file is loaded anew on every page
 *     call; in other words, it is not cached. Used only when 'type' references
 *     a JavaScript file. Defaults to TRUE.
 *   - preprocess: If TRUE and JavaScript aggregation is enabled, the script
 *     file will be aggregated. Defaults to TRUE.
 *   - attributes: An associative array of attributes for the <script> tag. This
 *     may be used to add 'defer', 'async', or custom attributes. Note that
 *     setting any attributes will disable preprocessing as though the
 *     'preprocess' option was set to FALSE.
 *   - browsers: An array containing information specifying which browsers
 *     should load the JavaScript item. See
 *     drupal_pre_render_conditional_comments() for details.
 *
 * @return
 *   The current array of JavaScript files, settings, and in-line code,
 *   including Drupal defaults, anything previously added with calls to
 *   drupal_add_js(), and this function call's additions.
 *
 * @deprecated as of Drupal 8.0. Use the #attached key in render arrays instead.
 *
 * @see drupal_get_js()
 */
function drupal_add_js($data = NULL, $options = NULL) {
  $javascript = &drupal_static(__FUNCTION__, array());

  // Construct the options, taking the defaults into consideration.
  if (isset($options)) {
    if (!is_array($options)) {
      $options = array('type' => $options);
    }
  }
  else {
    $options = array();
  }
  $options += drupal_js_defaults($data);

  // Preprocess can only be set if caching is enabled and no attributes are set.
  $options['preprocess'] = $options['cache'] && empty($options['attributes']) ? $options['preprocess'] : FALSE;

  // Tweak the weight so that files of the same weight are included in the
  // order of the calls to drupal_add_js().
  $options['weight'] += count($javascript) / 1000;
  if (isset($data)) {
    switch ($options['type']) {
      case 'setting':
        // If the setting array doesn't exist, add defaults values.
        if (!isset($javascript['settings'])) {
          $javascript['settings'] = array(
            'type' => 'setting',
            'scope' => 'header',
            'group' => JS_SETTING,
            'every_page' => TRUE,
            'weight' => 0,
            'browsers' => array(),
          );
          // url() generates the script and prefix using hook_url_outbound_alter().
          // Instead of running the hook_url_outbound_alter() again here, extract
          // them from url().
          // @todo Make this less hacky: http://drupal.org/node/1547376.
          $scriptPath = $GLOBALS['script_path'];
          $pathPrefix = '';
          url('', array('script' => &$scriptPath, 'prefix' => &$pathPrefix));
          $javascript['settings']['data'][] = array(
            'basePath' => base_path(),
            'scriptPath' => $scriptPath,
            'pathPrefix' => $pathPrefix,
            'currentPath' => current_path(),
          );
        }
        // All JavaScript settings are placed in the header of the page with
        // the library weight so that inline scripts appear afterwards.
        $javascript['settings']['data'][] = $data;
        break;

      case 'inline':
        $javascript[] = $options;
        break;

      default: // 'file' and 'external'
        // Local and external files must keep their name as the associative key
        // so the same JavaScript file is not added twice.
        $javascript[$options['data']] = $options;
    }
  }
  return $javascript;
}

/**
 * Constructs an array of the defaults that are used for JavaScript items.
 *
 * @param $data
 *   (optional) The default data parameter for the JavaScript item array.
 *
 * @see drupal_get_js()
 * @see drupal_add_js()
 */
function drupal_js_defaults($data = NULL) {
  return array(
    'type' => 'file',
    'group' => JS_DEFAULT,
    'every_page' => FALSE,
    'weight' => 0,
    'scope' => 'header',
    'cache' => TRUE,
    'preprocess' => TRUE,
    'attributes' => array(),
    'version' => NULL,
    'data' => $data,
    'browsers' => array(),
  );
}

/**
 * Returns a themed presentation of all JavaScript code for the current page.
 *
 * References to JavaScript files are placed in a certain order: first, all
 * 'core' files, then all 'module' and finally all 'theme' JavaScript files
 * are added to the page. Then, all settings are output, followed by 'inline'
 * JavaScript code. If running update.php, all preprocessing is disabled.
 *
 * Note that hook_js_alter(&$javascript) is called during this function call
 * to allow alterations of the JavaScript during its presentation. Calls to
 * drupal_add_js() from hook_js_alter() will not be added to the output
 * presentation. The correct way to add JavaScript during hook_js_alter()
 * is to add another element to the $javascript array, deriving from
 * drupal_js_defaults(). See locale_js_alter() for an example of this.
 *
 * @param $scope
 *   (optional) The scope for which the JavaScript rules should be returned.
 *   Defaults to 'header'.
 * @param $javascript
 *   (optional) An array with all JavaScript code. Defaults to the default
 *   JavaScript array for the given scope.
 * @param $skip_alter
 *   (optional) If set to TRUE, this function skips calling drupal_alter() on
 *   $javascript, useful when the calling function passes a $javascript array
 *   that has already been altered.
 *
 * @return
 *   All JavaScript code segments and includes for the scope as HTML tags.
 *
 * @see drupal_add_js()
 * @see locale_js_alter()
 * @see drupal_js_defaults()
 */
function drupal_get_js($scope = 'header', $javascript = NULL, $skip_alter = FALSE) {
  if (!isset($javascript)) {
    $javascript = drupal_add_js();
  }
  if (empty($javascript)) {
    return '';
  }

  // Allow modules to alter the JavaScript.
  if (!$skip_alter) {
    drupal_alter('js', $javascript);
  }

  // Filter out elements of the given scope.
  $items = array();
  foreach ($javascript as $key => $item) {
    if ($item['scope'] == $scope) {
      $items[$key] = $item;
    }
  }

  if (!empty($items)) {
    // Sort the JavaScript files so that they appear in the correct order.
    uasort($items, 'drupal_sort_css_js');
    // Don't add settings if there is no other JavaScript on the page, unless
    // this is an AJAX request.
    // @todo Clean up container call.
    $container = drupal_container();
    if ($container->has('content_negotiation') && $container->isScopeActive('request')) {
      $type = $container->get('content_negotiation')->getContentType($container->get('request'));
    }
    if (!empty($items['settings']) || (!empty($type) && $type == 'ajax')) {
      global $theme_key;
      // Provide the page with information about the theme that's used, so that
      // a later AJAX request can be rendered using the same theme.
      // @see ajax_base_page_theme()
      $setting['ajaxPageState']['theme'] = $theme_key;
      // Checks that the DB is available before filling theme_token.
      if (!defined('MAINTENANCE_MODE')) {
        $setting['ajaxPageState']['theme_token'] = drupal_get_token($theme_key);
      }

      // Provide the page with information about the individual JavaScript files
      // used, information not otherwise available when aggregation is enabled.
      $setting['ajaxPageState']['js'] = array_fill_keys(array_keys($javascript), 1);
      unset($setting['ajaxPageState']['js']['settings']);

      // Provide the page with information about the individual CSS files used,
      // information not otherwise available when CSS aggregation is enabled.
      // The setting is attached later in this function, but is set here, so
      // that CSS files removed in drupal_process_attached() are still
      // considered "used" and prevented from being added in a later AJAX
      // request.
      // Skip if no files were added to the page otherwise jQuery.extend() will
      // overwrite the Drupal.settings.ajaxPageState.css object with an empty
      // array.
      $css = drupal_add_css();
      if (!empty($css)) {
        // Cast the array to an object to be on the safe side even if not empty.
        $setting['ajaxPageState']['css'] = (object) array_fill_keys(array_keys($css), 1);
      }

      drupal_add_js($setting, 'setting');

      // If we're outputting the header scope, then this might be the final time
      // that drupal_get_js() is running, so add the settings to this output as well
      // as to the drupal_add_js() cache. If $items['settings'] doesn't exist, it's
      // because drupal_get_js() was intentionally passed a $javascript argument
      // stripped of settings, potentially in order to override how settings get
      // output, so in this case, do not add the setting to this output.
      if ($scope == 'header' && isset($items['settings'])) {
        $items['settings']['data'][] = $setting;
      }
    }
  }

  // Render the HTML needed to load the JavaScript.
  $elements = array(
    '#type' => 'scripts',
    '#items' => $items,
  );

  return drupal_render($elements);
}

/**
 * Merges an array of settings arrays into a single settings array.
 *
 * This function merges the items in the same way that
 *
 * @code
 *   jQuery.extend(true, {}, $settings_items[0], $settings_items[1], ...)
 * @endcode
 *
 * would. This means integer indices are preserved just like string indices are,
 * rather than re-indexed as is common in PHP array merging.
 *
 * Example:
 * @code
 * function module1_page_build(&$page) {
 *   $page['#attached']['js'][] = array(
 *     'type' => 'setting',
 *     'data' => array('foo' => array('a', 'b', 'c')),
 *   );
 * }
 * function module2_page_build(&$page) {
 *   $page['#attached']['js'][] = array(
 *     'type' => 'setting',
 *     'data' => array('foo' => array('d')),
 *   );
 * }
 * // When the page is rendered after the above code, and the browser runs the
 * // resulting <SCRIPT> tags, the value of drupalSettings.foo is
 * // ['d', 'b', 'c'], not ['a', 'b', 'c', 'd'].
 * @endcode
 *
 * By following jQuery.extend() merge logic rather than common PHP array merge
 * logic, the following are ensured:
 * - drupal_add_js() is idempotent: calling it twice with the same parameters
 *   does not change the output sent to the browser.
 * - If pieces of the page are rendered in separate PHP requests and the
 *   returned settings are merged by JavaScript, the resulting settings are the
 *   same as if rendered in one PHP request and merged by PHP.
 *
 * @param $settings_items
 *   An array of settings arrays, as returned by:
 *   @code
 *     $js = drupal_add_js();
 *     $settings_items = $js['settings']['data'];
 *   @endcode
 *
 * @return
 *   A merged $settings array, suitable for JSON encoding and returning to the
 *   browser.
 *
 * @see drupal_add_js()
 * @see drupal_pre_render_scripts()
 */
function drupal_merge_js_settings($settings_items) {
  return NestedArray::mergeDeepArray($settings_items, TRUE);
}

/**
 * #pre_render callback to add the elements needed for JavaScript tags to be rendered.
 *
 * This function evaluates the aggregation enabled/disabled condition on a group
 * by group basis by testing whether an aggregate file has been made for the
 * group rather than by testing the site-wide aggregation setting. This allows
 * this function to work correctly even if modules have implemented custom
 * logic for grouping and aggregating files.
 *
 * @param $element
 *   A render array containing:
 *   - #items: The JavaScript items as returned by drupal_add_js() and
 *     altered by drupal_get_js().
 *   - #group_callback: A function to call to group #items. Following
 *     this function, #aggregate_callback is called to aggregate items within
 *     the same group into a single file.
 *   - #aggregate_callback: A function to call to aggregate the items within
 *     the groups arranged by the #group_callback function.
 *
 * @return
 *   A render array that will render to a string of JavaScript tags.
 *
 * @see drupal_get_js()
 */
function drupal_pre_render_scripts($elements) {
  // Group and aggregate the items.
  if (isset($elements['#group_callback'])) {
    $elements['#groups'] = $elements['#group_callback']($elements['#items']);
  }
  if (isset($elements['#aggregate_callback'])) {
    $elements['#aggregate_callback']($elements['#groups']);
  }

  // A dummy query-string is added to filenames, to gain control over
  // browser-caching. The string changes on every update or full cache
  // flush, forcing browsers to load a new copy of the files, as the
  // URL changed. Files that should not be cached (see drupal_add_js())
  // get REQUEST_TIME as query-string instead, to enforce reload on every
  // page request.
  $default_query_string = variable_get('css_js_query_string', '0');

  // For inline JavaScript to validate as XHTML, all JavaScript containing
  // XHTML needs to be wrapped in CDATA. To make that backwards compatible
  // with HTML 4, we need to comment out the CDATA-tag.
  $embed_prefix = "\n<!--//--><![CDATA[//><!--\n";
  $embed_suffix = "\n//--><!]]>\n";

  // Since JavaScript may look for arguments in the URL and act on them, some
  // third-party code might require the use of a different query string.
  $js_version_string = variable_get('drupal_js_version_query_string', 'v=');

  // Defaults for each SCRIPT element.
  $element_defaults = array(
    '#type' => 'html_tag',
    '#tag' => 'script',
    '#value' => '',
  );

  // Loop through each group.
  foreach ($elements['#groups'] as $group) {
    // If a group of files has been aggregated into a single file,
    // $group['data'] contains the URI of the aggregate file. Add a single
    // script element for this file.
    if ($group['type'] == 'file' && isset($group['data'])) {
      $element = $element_defaults;
      $element['#attributes']['src'] = file_create_url($group['data']);
      $element['#browsers'] = $group['browsers'];
      $elements[] = $element;
    }
    // For non-file types, and non-aggregated files, add a script element per
    // item.
    else {
      foreach ($group['items'] as $item) {
        // Element properties that do not depend on item type.
        $element = $element_defaults;
        $element['#browsers'] = $item['browsers'];

        // Element properties that depend on item type.
        switch ($item['type']) {
          case 'setting':
            $element['#value_prefix'] = $embed_prefix;
            $element['#value'] = 'var drupalSettings = ' . drupal_json_encode(drupal_merge_js_settings($item['data'])) . ";";
            $element['#value_suffix'] = $embed_suffix;
            break;

          case 'inline':
            $element['#value_prefix'] = $embed_prefix;
            $element['#value'] = $item['data'];
            $element['#value_suffix'] = $embed_suffix;
            break;

          case 'file':
            $query_string = empty($item['version']) ? $default_query_string : $js_version_string . $item['version'];
            $query_string_separator = (strpos($item['data'], '?') !== FALSE) ? '&' : '?';
            $element['#attributes']['src'] = file_create_url($item['data']) . $query_string_separator . ($item['cache'] ? $query_string : REQUEST_TIME);
            break;

          case 'external':
            $element['#attributes']['src'] = $item['data'];
            break;
        }

        // Attributes may only be set if this script is output independently.
        if (!empty($element['#attributes']['src']) && !empty($item['attributes'])) {
          $element['#attributes'] += $item['attributes'];
        }

        $elements[] = $element;
      }
    }
  }

  return $elements;
}

/**
 * Default callback to group JavaScript items.
 *
 * This function arranges the JavaScript items that are in the #items property
 * of the scripts element into groups. When aggregation is enabled, files within
 * a group are aggregated into a single file, significantly improving page
 * loading performance by minimizing network traffic overhead.
 *
 * This function puts multiple items into the same group if they are groupable
 * and if they are for the same browsers. Items of the 'file' type are groupable
 * if their 'preprocess' flag is TRUE. Items of the 'inline', 'settings', or
 * 'external' type are not groupable.
 *
 * This function also ensures that the process of grouping items does not change
 * their relative order. This requirement may result in multiple groups for the
 * same type and browsers, if needed to accommodate other items in
 * between.
 *
 * @param $javascript
 *   An array of JavaScript items, as returned by drupal_add_js(), but after
 *   alteration performed by drupal_get_js().
 *
 * @return
 *   An array of JavaScript groups. Each group contains the same keys (e.g.,
 *   'data', etc.) as a JavaScript item from the $javascript parameter, with the
 *   value of each key applying to the group as a whole. Each group also
 *   contains an 'items' key, which is the subset of items from $javascript that
 *   are in the group.
 *
 * @see drupal_pre_render_scripts()
 */
function drupal_group_js($javascript) {
  $groups = array();
  // If a group can contain multiple items, we track the information that must
  // be the same for each item in the group, so that when we iterate the next
  // item, we can determine if it can be put into the current group, or if a
  // new group needs to be made for it.
  $current_group_keys = NULL;
  $index = -1;
  foreach ($javascript as $item) {
    // The browsers for which the JavaScript item needs to be loaded is part of
    // the information that determines when a new group is needed, but the order
    // of keys in the array doesn't matter, and we don't want a new group if all
    // that's different is that order.
    ksort($item['browsers']);

    switch ($item['type']) {
      case 'file':
        // Group file items if their 'preprocess' flag is TRUE.
        // Help ensure maximum reuse of aggregate files by only grouping
        // together items that share the same 'group' value and 'every_page'
        // flag. See drupal_add_js() for details about that.
        $group_keys = $item['preprocess'] ? array($item['type'], $item['group'], $item['every_page'], $item['browsers']) : FALSE;
        break;

      case 'external':
      case 'setting':
      case 'inline':
        // Do not group external, settings, and inline items.
        $group_keys = FALSE;
        break;
    }

    // If the group keys don't match the most recent group we're working with,
    // then a new group must be made.
    if ($group_keys !== $current_group_keys) {
      $index++;
      // Initialize the new group with the same properties as the first item
      // being placed into it. The item's 'data' and 'weight' properties are
      // unique to the item and should not be carried over to the group.
      $groups[$index] = $item;
      unset($groups[$index]['data'], $groups[$index]['weight']);
      $groups[$index]['items'] = array();
      $current_group_keys = $group_keys ? $group_keys : NULL;
    }

    // Add the item to the current group.
    $groups[$index]['items'][] = $item;
  }

  return $groups;
}

/**
 * Default callback to aggregate JavaScript files.
 *
 * Having the browser load fewer JavaScript files results in much faster page
 * loads than when it loads many small files. This function aggregates files
 * within the same group into a single file unless the site-wide setting to do
 * so is disabled (commonly the case during site development). To optimize
 * download, it also compresses the aggregate files by removing comments,
 * whitespace, and other unnecessary content.
 *
 * @param $js_groups
 *   An array of JavaScript groups as returned by drupal_group_js(). For each
 *   group that is aggregated, this function sets the value of the group's
 *   'data' key to the URI of the aggregate file.
 *
 * @see drupal_group_js()
 * @see drupal_pre_render_scripts()
 */
function drupal_aggregate_js(&$js_groups) {
  // Only aggregate during normal site operation.
  if (defined('MAINTENANCE_MODE')) {
    $preprocess_js = FALSE;
  }
  else {
    $config = config('system.performance');
    $preprocess_js = $config->get('js.preprocess');
  }

  if ($preprocess_js) {
    foreach ($js_groups as $key => $group) {
      if ($group['type'] == 'file' && $group['preprocess']) {
        $js_groups[$key]['data'] = drupal_build_js_cache($group['items']);
      }
    }
  }
}

/**
 * Adds attachments to a render() structure.
 *
 * Libraries, JavaScript, CSS and other types of custom structures are attached
 * to elements using the #attached property. The #attached property is an
 * associative array, where the keys are the the attachment types and the values
 * are the attached data. For example:
 * @code
 * $build['#attached'] = array(
 *   'library' => array(array('taxonomy', 'taxonomy')),
 *   'css' => array(drupal_get_path('module', 'taxonomy') . '/css/taxonomy.module.css'),
 * );
 * @endcode
 *
 * 'js', 'css', and 'library' are types that get special handling. For any
 * other kind of attached data, the array key must be the full name of the
 * callback function and each value an array of arguments. For example:
 * @code
 * $build['#attached']['drupal_add_http_header'] = array(
 *   array('Content-Type', 'application/rss+xml; charset=utf-8'),
 * );
 * @endcode
 *
 * External 'js' and 'css' files can also be loaded. For example:
 * @code
 * $build['#attached']['js'] = array(
 *   'http://code.jquery.com/jquery-1.4.2.min.js' => array(
 *     'type' => 'external',
 *   ),
 * );
 * @endcode
 *
 * @param $elements
 *   The structured array describing the data being rendered.
 * @param $dependency_check
 *   When TRUE, will exit if a given library's dependencies are missing. When
 *   set to FALSE, will continue to add the libraries, even though one or more
 *   dependencies are missing. Defaults to FALSE.
 *
 * @return
 *   FALSE if there were any missing library dependencies; TRUE if all library
 *   dependencies were met.
 *
 * @see drupal_add_library()
 * @see drupal_add_js()
 * @see drupal_add_css()
 * @see drupal_render()
 */
function drupal_process_attached($elements, $dependency_check = FALSE) {
  // Add defaults to the special attached structures that should be processed differently.
  $elements['#attached'] += array(
    'library' => array(),
    'js' => array(),
    'css' => array(),
  );

  // Add the libraries first.
  $success = TRUE;
  foreach ($elements['#attached']['library'] as $library) {
    if (drupal_add_library($library[0], $library[1]) === FALSE) {
      $success = FALSE;
      // Exit if the dependency is missing.
      if ($dependency_check) {
        return $success;
      }
    }
  }
  unset($elements['#attached']['library']);

  // Add both the JavaScript and the CSS.
  // The parameters for drupal_add_js() and drupal_add_css() require special
  // handling.
  foreach (array('js', 'css') as $type) {
    foreach ($elements['#attached'][$type] as $data => $options) {
      // If the value is not an array, it's a filename and passed as first
      // (and only) argument.
      if (!is_array($options)) {
        $data = $options;
        $options = NULL;
      }
      // In some cases, the first parameter ($data) is an array. Arrays can't be
      // passed as keys in PHP, so we have to get $data from the value array.
      if (is_numeric($data)) {
        $data = $options['data'];
        unset($options['data']);
      }
      call_user_func('drupal_add_' . $type, $data, $options);
    }
    unset($elements['#attached'][$type]);
  }

  // Add additional types of attachments specified in the render() structure.
  // Libraries, JavaScript and CSS have been added already, as they require
  // special handling.
  foreach ($elements['#attached'] as $callback => $options) {
    foreach ($elements['#attached'][$callback] as $args) {
      call_user_func_array($callback, $args);
    }
  }

  return $success;
}

/**
 * Adds JavaScript to change the state of an element based on another element.
 *
 * A "state" means a certain property on a DOM element, such as "visible" or
 * "checked". A state can be applied to an element, depending on the state of
 * another element on the page. In general, states depend on HTML attributes and
 * DOM element properties, which change due to user interaction.
 *
 * Since states are driven by JavaScript only, it is important to understand
 * that all states are applied on presentation only, none of the states force
 * any server-side logic, and that they will not be applied for site visitors
 * without JavaScript support. All modules implementing states have to make
 * sure that the intended logic also works without JavaScript being enabled.
 *
 * #states is an associative array in the form of:
 * @code
 * array(
 *   STATE1 => CONDITIONS_ARRAY1,
 *   STATE2 => CONDITIONS_ARRAY2,
 *   ...
 * )
 * @endcode
 * Each key is the name of a state to apply to the element, such as 'visible'.
 * Each value is a list of conditions that denote when the state should be
 * applied.
 *
 * Multiple different states may be specified to act on complex conditions:
 * @code
 * array(
 *   'visible' => CONDITIONS,
 *   'checked' => OTHER_CONDITIONS,
 * )
 * @endcode
 *
 * Every condition is a key/value pair, whose key is a jQuery selector that
 * denotes another element on the page, and whose value is an array of
 * conditions, which must bet met on that element:
 * @code
 * array(
 *   'visible' => array(
 *     JQUERY_SELECTOR => REMOTE_CONDITIONS,
 *     JQUERY_SELECTOR => REMOTE_CONDITIONS,
 *     ...
 *   ),
 * )
 * @endcode
 * All conditions must be met for the state to be applied.
 *
 * Each remote condition is a key/value pair specifying conditions on the other
 * element that need to be met to apply the state to the element:
 * @code
 * array(
 *   'visible' => array(
 *     ':input[name="remote_checkbox"]' => array('checked' => TRUE),
 *   ),
 * )
 * @endcode
 *
 * For example, to show a textfield only when a checkbox is checked:
 * @code
 * $form['toggle_me'] = array(
 *   '#type' => 'checkbox',
 *   '#title' => t('Tick this box to type'),
 * );
 * $form['settings'] = array(
 *   '#type' => 'textfield',
 *   '#states' => array(
 *     // Only show this field when the 'toggle_me' checkbox is enabled.
 *     'visible' => array(
 *       ':input[name="toggle_me"]' => array('checked' => TRUE),
 *     ),
 *   ),
 * );
 * @endcode
 *
 * The following states may be applied to an element:
 * - enabled
 * - disabled
 * - required
 * - optional
 * - visible
 * - invisible
 * - checked
 * - unchecked
 * - expanded
 * - collapsed
 *
 * The following states may be used in remote conditions:
 * - empty
 * - filled
 * - checked
 * - unchecked
 * - expanded
 * - collapsed
 * - value
 *
 * The following states exist for both elements and remote conditions, but are
 * not fully implemented and may not change anything on the element:
 * - relevant
 * - irrelevant
 * - valid
 * - invalid
 * - touched
 * - untouched
 * - readwrite
 * - readonly
 *
 * When referencing select lists and radio buttons in remote conditions, a
 * 'value' condition must be used:
 * @code
 *   '#states' => array(
 *     // Show the settings if 'bar' has been selected for 'foo'.
 *     'visible' => array(
 *       ':input[name="foo"]' => array('value' => 'bar'),
 *     ),
 *   ),
 * @endcode
 *
 * @param $elements
 *   A renderable array element having a #states property as described above.
 *
 * @see form_example_states_form()
 */
function drupal_process_states(&$elements) {
  $elements['#attached']['library'][] = array('system', 'drupal.states');
  $elements['#attached']['js'][] = array(
    'type' => 'setting',
    'data' => array('states' => array('#' . $elements['#id'] => $elements['#states'])),
  );
}

/**
 * Adds multiple JavaScript or CSS files at the same time.
 *
 * A library defines a set of JavaScript and/or CSS files, optionally using
 * settings, and optionally requiring another library. For example, a library
 * can be a jQuery plugin, a JavaScript framework, or a CSS framework. This
 * function allows modules to load a library defined/shipped by itself or a
 * depending module, without having to add all files of the library separately.
 * Each library is only loaded once.
 *
 * @param $module
 *   The name of the module that registered the library.
 * @param $name
 *   The name of the library to add.
 * @param $every_page
 *   Set to TRUE if this library is added to every page on the site.
 *
 * @return
 *   TRUE if the library was successfully added; FALSE if the library or one of
 *   its dependencies could not be added.
 *
 * @see drupal_get_library()
 * @see hook_library_info()
 * @see hook_library_info_alter()
 */
function drupal_add_library($module, $name, $every_page = NULL) {
  $added = &drupal_static(__FUNCTION__, array());

  // Only process the library if it exists and it was not added already.
  if (!isset($added[$module][$name])) {
    if ($library = drupal_get_library($module, $name)) {
      // Add all components within the library.
      $elements['#attached'] = array(
        'library' => $library['dependencies'],
        'js' => $library['js'],
        'css' => $library['css'],
      );
      foreach (array('js', 'css') as $type) {
        foreach ($elements['#attached'][$type] as $data => $options) {
          // Apply the JS_LIBRARY group if it isn't explicitly given.
          if ($type == 'js' && !isset($options['group'])) {
            $elements['#attached']['js'][$data]['group'] = JS_LIBRARY;
          }
          // Set the every_page flag if one was passed.
          if (isset($every_page)) {
            $elements['#attached'][$type][$data]['every_page'] = $every_page;
          }
        }
      }

      $added[$module][$name] = drupal_process_attached($elements, TRUE);
    }
    else {
      // Requested library does not exist.
      $added[$module][$name] = FALSE;
    }
  }

  return $added[$module][$name];
}

/**
 * Retrieves information for a JavaScript/CSS library.
 *
 * Library information is statically cached. Libraries are keyed by module for
 * several reasons:
 * - Libraries are not unique. Multiple modules might ship with the same library
 *   in a different version or variant. This registry cannot (and does not
 *   attempt to) prevent library conflicts.
 * - Modules implementing and thereby depending on a library that is registered
 *   by another module can only rely on that module's library.
 * - Two (or more) modules can still register the same library and use it
 *   without conflicts in case the libraries are loaded on certain pages only.
 *
 * @param $module
 *   The name of a module that registered a library.
 * @param $name
 *   (optional) The name of a registered library to retrieve. By default, all
 *   libraries registered by $module are returned.
 *
 * @return
 *   The definition of the requested library, if $name was passed and it exists,
 *   or FALSE if it does not exist. If no $name was passed, an associative array
 *   of libraries registered by $module is returned (which may be empty).
 *
 * @see drupal_add_library()
 * @see hook_library_info()
 * @see hook_library_info_alter()
 *
 * @todo The purpose of drupal_get_*() is completely different to other page
 *   requisite API functions; find and use a different name.
 */
function drupal_get_library($module, $name = NULL) {
  $libraries = &drupal_static(__FUNCTION__, array());

  if (!isset($libraries[$module])) {
    // Retrieve all libraries associated with the module.
    $module_libraries = module_invoke($module, 'library_info');
    if (empty($module_libraries)) {
      $module_libraries = array();
    }
    // Allow modules to alter the module's registered libraries.
    drupal_alter('library_info', $module_libraries, $module);

    foreach ($module_libraries as $key => $data) {
      if (is_array($data)) {
        // Add default elements to allow for easier processing.
        $module_libraries[$key] += array('dependencies' => array(), 'js' => array(), 'css' => array());
        foreach ($module_libraries[$key]['js'] as $file => $options) {
          if (is_scalar($options)) {
            // The JavaScript or CSS file has been specified in shorthand
            // format, without an array of options. In this case $options is the
            // filename.
            $file = $options;
            $options = array();
          }
          $module_libraries[$key]['js'][$file]['version'] = $module_libraries[$key]['version'];
        }
      }
    }
    $libraries[$module] = $module_libraries;
  }
  if (isset($name)) {
    if (!isset($libraries[$module][$name])) {
      $libraries[$module][$name] = FALSE;
    }
    return $libraries[$module][$name];
  }
  return $libraries[$module];
}

/**
 * Assists in adding the tableDrag JavaScript behavior to a themed table.
 *
 * Draggable tables should be used wherever an outline or list of sortable items
 * needs to be arranged by an end-user. Draggable tables are very flexible and
 * can manipulate the value of form elements placed within individual columns.
 *
 * To set up a table to use drag and drop in place of weight select-lists or in
 * place of a form that contains parent relationships, the form must be themed
 * into a table. The table must have an ID attribute set. If using
 * theme_table(), the ID may be set as follows:
 * @code
 * $output = theme('table', array('header' => $header, 'rows' => $rows, 'attributes' => array('id' => 'my-module-table')));
 * return $output;
 * @endcode
 *
 * In the theme function for the form, a special class must be added to each
 * form element within the same column, "grouping" them together.
 *
 * In a situation where a single weight column is being sorted in the table, the
 * classes could be added like this (in the theme function):
 * @code
 * $form['my_elements'][$delta]['weight']['#attributes']['class'] = array('my-elements-weight');
 * @endcode
 *
 * Each row of the table must also have a class of "draggable" in order to
 * enable the drag handles:
 * @code
 * $row = array(...);
 * $rows[] = array(
 *   'data' => $row,
 *   'class' => array('draggable'),
 * );
 * @endcode
 *
 * When tree relationships are present, the two additional classes
 * 'tabledrag-leaf' and 'tabledrag-root' can be used to refine the behavior:
 * - Rows with the 'tabledrag-leaf' class cannot have child rows.
 * - Rows with the 'tabledrag-root' class cannot be nested under a parent row.
 *
 * Calling drupal_add_tabledrag() would then be written as such:
 * @code
 * drupal_add_tabledrag('my-module-table', 'order', 'sibling', 'my-elements-weight');
 * @endcode
 *
 * In a more complex case where there are several groups in one column (such as
 * the block regions on the admin/structure/block page), a separate subgroup
 * class must also be added to differentiate the groups.
 * @code
 * $form['my_elements'][$region][$delta]['weight']['#attributes']['class'] = array('my-elements-weight', 'my-elements-weight-' . $region);
 * @endcode
 *
 * $group is still 'my-element-weight', and the additional $subgroup variable
 * will be passed in as 'my-elements-weight-' . $region. This also means that
 * you'll need to call drupal_add_tabledrag() once for every region added.
 *
 * @code
 * foreach ($regions as $region) {
 *   drupal_add_tabledrag('my-module-table', 'order', 'sibling', 'my-elements-weight', 'my-elements-weight-' . $region);
 * }
 * @endcode
 *
 * In a situation where tree relationships are present, adding multiple
 * subgroups is not necessary, because the table will contain indentations that
 * provide enough information about the sibling and parent relationships. See
 * theme_menu_overview_form() for an example creating a table containing parent
 * relationships.
 *
 * Note that this function should be called from the theme layer, such as in a
 * .tpl.php file, theme_ function, or in a template_preprocess function, not in
 * a form declaration. Though the same JavaScript could be added to the page
 * using drupal_add_js() directly, this function helps keep template files
 * clean and readable. It also prevents tabledrag.js from being added twice
 * accidentally.
 *
 * @param $table_id
 *   String containing the target table's id attribute. If the table does not
 *   have an id, one will need to be set, such as <table id="my-module-table">.
 * @param $action
 *   String describing the action to be done on the form item. Either 'match'
 *   'depth', or 'order'. Match is typically used for parent relationships.
 *   Order is typically used to set weights on other form elements with the same
 *   group. Depth updates the target element with the current indentation.
 * @param $relationship
 *   String describing where the $action variable should be performed. Either
 *   'parent', 'sibling', 'group', or 'self'. Parent will only look for fields
 *   up the tree. Sibling will look for fields in the same group in rows above
 *   and below it. Self affects the dragged row itself. Group affects the
 *   dragged row, plus any children below it (the entire dragged group).
 * @param $group
 *   A class name applied on all related form elements for this action.
 * @param $subgroup
 *   (optional) If the group has several subgroups within it, this string should
 *   contain the class name identifying fields in the same subgroup.
 * @param $source
 *   (optional) If the $action is 'match', this string should contain the class
 *   name identifying what field will be used as the source value when matching
 *   the value in $subgroup.
 * @param $hidden
 *   (optional) The column containing the field elements may be entirely hidden
 *   from view dynamically when the JavaScript is loaded. Set to FALSE if the
 *   column should not be hidden.
 * @param $limit
 *   (optional) Limit the maximum amount of parenting in this table.
 * @see block-admin-display-form.tpl.php
 * @see theme_menu_overview_form()
 */
function drupal_add_tabledrag($table_id, $action, $relationship, $group, $subgroup = NULL, $source = NULL, $hidden = TRUE, $limit = 0) {
  $js_added = &drupal_static(__FUNCTION__, FALSE);
  $tabledrag_id = &drupal_static(__FUNCTION__  . '_setting', FALSE);
  $tabledrag_id = (!isset($tabledrag_id)) ? 0 : $tabledrag_id + 1;

  if (!$js_added) {
    // Add the table drag JavaScript to the page before the module JavaScript
    // to ensure that table drag behaviors are registered before any module
    // uses it.
    drupal_add_library('system', 'drupal.tabledrag');
    $js_added = TRUE;
  }

  // If a subgroup or source isn't set, assume it is the same as the group.
  $target = isset($subgroup) ? $subgroup : $group;
  $source = isset($source) ? $source : $target;
  $settings['tableDrag'][$table_id][$group][$tabledrag_id] = array(
    'target' => $target,
    'source' => $source,
    'relationship' => $relationship,
    'action' => $action,
    'hidden' => $hidden,
    'limit' => $limit,
  );
  drupal_add_js($settings, 'setting');
}

/**
 * Aggregates JavaScript files into a cache file in the files directory.
 *
 * The file name for the JavaScript cache file is generated from the hash of
 * the aggregated contents of the files in $files. This forces proxies and
 * browsers to download new JavaScript when the JavaScript changes.
 *
 * The cache file name is retrieved on a page load via a lookup variable that
 * contains an associative array. The array key is the hash of the names in
 * $files while the value is the cache file name. The cache file is generated
 * in two cases. First, if there is no file name value for the key, which will
 * happen if a new file name has been added to $files or after the lookup
 * variable is emptied to force a rebuild of the cache. Second, the cache file
 * is generated if it is missing on disk. Old cache files are not deleted
 * immediately when the lookup variable is emptied, but are deleted after a set
 * period by drupal_delete_file_if_stale(). This ensures that files referenced
 * by a cached page will still be available.
 *
 * @param $files
 *   An array of JavaScript files to aggregate and compress into one file.
 *
 * @return
 *   The URI of the cache file, or FALSE if the file could not be saved.
 */
function drupal_build_js_cache($files) {
  $contents = '';
  $uri = '';
  $map = Drupal::state()->get('system.js_cache_files') ?: array();
  // Create a new array so that only the file names are used to create the hash.
  // This prevents new aggregates from being created unnecessarily.
  $js_data = array();
  foreach ($files as $file) {
    $js_data[] = $file['data'];
  }
  $key = hash('sha256', serialize($js_data));
  if (isset($map[$key])) {
    $uri = $map[$key];
  }

  if (empty($uri) || !file_exists($uri)) {
    // Build aggregate JS file.
    foreach ($files as $info) {
      if ($info['preprocess']) {
        // Append a ';' and a newline after each JS file to prevent them from running together.
        $contents .= file_get_contents($info['data']) . ";\n";
      }
    }
    // Prefix filename to prevent blocking by firewalls which reject files
    // starting with "ad*".
    $filename = 'js_' . Crypt::hashBase64($contents) . '.js';
    // Create the js/ within the files folder.
    $jspath = 'public://js';
    $uri = $jspath . '/' . $filename;
    // Create the JS file.
    file_prepare_directory($jspath, FILE_CREATE_DIRECTORY);
    if (!file_exists($uri) && !file_unmanaged_save_data($contents, $uri, FILE_EXISTS_REPLACE)) {
      return FALSE;
    }
    // If JS gzip compression is enabled and the zlib extension is available
    // then create a gzipped version of this file. This file is served
    // conditionally to browsers that accept gzip using .htaccess rules.
    // It's possible that the rewrite rules in .htaccess aren't working on this
    // server, but there's no harm (other than the time spent generating the
    // file) in generating the file anyway. Sites on servers where rewrite rules
    // aren't working can set js.gzip to FALSE in order to skip
    // generating a file that won't be used.
    if (config('system.performance')->get('js.gzip') && extension_loaded('zlib')) {
      if (!file_exists($uri . '.gz') && !file_unmanaged_save_data(gzencode($contents, 9, FORCE_GZIP), $uri . '.gz', FILE_EXISTS_REPLACE)) {
        return FALSE;
      }
    }
    $map[$key] = $uri;
    Drupal::state()->set('system.js_cache_files', $map);
  }
  return $uri;
}

/**
 * Deletes old cached JavaScript files and variables.
 */
function drupal_clear_js_cache() {
  Drupal::state()->delete('system.javascript_parsed');
  Drupal::state()->delete('system.js_cache_files');
  file_scan_directory('public://js', '/.*/', array('callback' => 'drupal_delete_file_if_stale'));
}

/**
 * Converts a PHP variable into its JavaScript equivalent.
 *
 * We use HTML-safe strings, with several characters escaped.
 *
 * @see drupal_json_decode()
 * @ingroup php_wrappers
 * @deprecated as of Drupal 8.0. Use Drupal\Component\Utility\Json::encode()
 *   directly instead.
 */
function drupal_json_encode($var) {
  return Json::encode($var);
}

/**
 * Converts an HTML-safe JSON string into its PHP equivalent.
 *
 * @see drupal_json_encode()
 * @ingroup php_wrappers
 * @deprecated as of Drupal 8.0. Use Drupal\Component\Utility\Json::decode()
 *   directly instead.
 */
function drupal_json_decode($var) {
  return Json::decode($var);
}

/**
 * Ensures the private key variable used to generate tokens is set.
 *
 * @return
 *   The private key.
 */
function drupal_get_private_key() {
  if (!($key = Drupal::state()->get('system.private_key'))) {
    $key = Crypt::randomStringHashed(55);
    Drupal::state()->set('system.private_key', $key);
  }
  return $key;
}

/**
 * Generates a token based on $value, the user session, and the private key.
 *
 * @param $value
 *   An additional value to base the token on.
 *
 * @return string
 *   A 43-character URL-safe token for validation, based on the user session ID,
 *   the hash salt provided from drupal_get_hash_salt(), and the
 *   'drupal_private_key' configuration variable.
 *
 * @see drupal_get_hash_salt()
 */
function drupal_get_token($value = '') {
  return Crypt::hmacBase64($value, session_id() . drupal_get_private_key() . drupal_get_hash_salt());
}

/**
 * Validates a token based on $value, the user session, and the private key.
 *
 * @param $token
 *   The token to be validated.
 * @param $value
 *   An additional value to base the token on.
 * @param $skip_anonymous
 *   Set to true to skip token validation for anonymous users.
 *
 * @return
 *   True for a valid token, false for an invalid token. When $skip_anonymous
 *   is true, the return value will always be true for anonymous users.
 */
function drupal_valid_token($token, $value = '', $skip_anonymous = FALSE) {
  global $user;
  return (($skip_anonymous && $user->uid == 0) || ($token == drupal_get_token($value)));
}

/**
 * Loads code for subsystems and modules, and registers stream wrappers.
 */
function _drupal_bootstrap_code() {
  require_once __DIR__ . '/../../' . settings()->get('path_inc', 'core/includes/path.inc');
  require_once __DIR__ . '/theme.inc';
  require_once __DIR__ . '/pager.inc';
  require_once __DIR__ . '/../../' . settings()->get('menu_inc', 'core/includes/menu.inc');
  require_once __DIR__ . '/tablesort.inc';
  require_once __DIR__ . '/file.inc';
  require_once __DIR__ . '/unicode.inc';
  require_once __DIR__ . '/image.inc';
  require_once __DIR__ . '/form.inc';
  require_once __DIR__ . '/mail.inc';
  require_once __DIR__ . '/ajax.inc';
  require_once __DIR__ . '/errors.inc';
  require_once __DIR__ . '/schema.inc';
  require_once __DIR__ . '/entity.inc';

  // Load all enabled modules
  Drupal::moduleHandler()->loadAll();

  // Set our bridge extension manager to Zend Feed.
  Reader::setExtensionManager(Drupal::service('feed.bridge.reader'));
  Writer::setExtensionManager(Drupal::service('feed.bridge.writer'));

  // Make sure all stream wrappers are registered.
  file_get_stream_wrappers();

  // Now that stream wrappers are registered, log fatal errors from a simpletest
  // child site to a test specific file directory.
  $test_info = &$GLOBALS['drupal_test_info'];
  if (!empty($test_info['in_child_site'])) {
    ini_set('log_errors', 1);
    ini_set('error_log', 'public://error.log');
  }

  // Set the allowed protocols once we have the config available.
  $allowed_protocols = \Drupal::config('system.filter')->get('protocols');
  if (!isset($allowed_protocols)) {
    // filter_xss_admin() is called by the installer and update.php, in which
    // case the configuration may not exist (yet). Provide a minimal default set
    // of allowed protocols for these cases.
    $allowed_protocols = array('http', 'https');
  }
  UrlValidator::setAllowedProtocols($allowed_protocols);
}

/**
 * Temporary BC function for scripts not using DrupalKernel.
 *
 * DrupalKernel skips this and replicates it via event listeners.
 *
 * @see Drupal\Core\EventSubscriber\PathSubscriber;
 * @see Drupal\Core\EventSubscriber\LegacyRequestSubscriber;
 */
function _drupal_bootstrap_full($skip = FALSE) {
  static $called = FALSE;

  if ($called || $skip) {
    $called = TRUE;
    return;
  }

  // Initialize language (which can strip path prefix) prior to initializing
  // current_path().
  drupal_language_initialize();

  // Let all modules take action before the menu system handles the request.
  // We do not want this while running update.php.
  if (!defined('MAINTENANCE_MODE') || MAINTENANCE_MODE != 'update') {
    menu_set_custom_theme();
    drupal_theme_initialize();
  }
}

/**
 * Stores the current page in the cache.
 *
 * If page_compression is enabled, a gzipped version of the page is stored in
 * the cache to avoid compressing the output on each request. The cache entry
 * is unzipped in the relatively rare event that the page is requested by a
 * client without gzip support.
 *
 * Page compression requires the PHP zlib extension
 * (http://php.net/manual/ref.zlib.php).
 *
 * @param $body
 *   The response body.
 * @return
 *   The cached object or NULL if the page cache was not set.
 *
 * @see drupal_page_header()
 */
function drupal_page_set_cache(Response $response, Request $request) {
  global $base_root;

  if (drupal_page_is_cacheable()) {
    $cache = (object) array(
      'cid' => $base_root . $request->getRequestUri(),
      'data' => array(
        'path' => $request->attributes->get('system_path'),
        'body' => $response->getContent(),
        'title' => drupal_get_title(),
        'headers' => array(),
      ),
      'tags' => array('content' => TRUE),
      'expire' => CacheBackendInterface::CACHE_PERMANENT,
      'created' => REQUEST_TIME,
    );

    $cache->data['headers'] = $response->headers->all();

    // Hack: exclude the x-drupal-cache header; it may make it in here because
    // of awkwardness in how we defer sending it over in _drupal_page_get_cache.
    if (isset($cache->data['headers']['x-drupal-cache'])) {
      unset($cache->data['headers']['x-drupal-cache']);
    }

     // Use the actual timestamp from an Expires header, if available.
    if ($date = $response->getExpires()) {
      $date = new DrupalDateTime($date);
      $cache->expire = $date->getTimestamp();
    }

    if ($cache->data['body']) {
      if (config('system.performance')->get('response.gzip') && extension_loaded('zlib')) {
        $cache->data['body'] = gzencode($cache->data['body'], 9, FORCE_GZIP);
      }
      cache('page')->set($cache->cid, $cache->data, $cache->expire, $cache->tags);
    }
    return $cache;
  }
}

/**
 * Executes a cron run when called.
 *
 * Do not call this function from a test. Use $this->cronRun() instead.
 *
 * @return
 *   TRUE if cron ran successfully.
 */
function drupal_cron_run() {
  // Allow execution to continue even if the request gets canceled.
  @ignore_user_abort(TRUE);

  // Prevent session information from being saved while cron is running.
  $original_session_saving = drupal_save_session();
  drupal_save_session(FALSE);

  // Force the current user to anonymous to ensure consistent permissions on
  // cron runs.
  $original_user = $GLOBALS['user'];
  $GLOBALS['user'] = drupal_anonymous_user();

  // Try to allocate enough time to run all the hook_cron implementations.
  drupal_set_time_limit(240);

  $return = FALSE;
  // Grab the defined cron queues.
  $queues = module_invoke_all('queue_info');
  drupal_alter('queue_info', $queues);

  // Try to acquire cron lock.
  if (!lock()->acquire('cron', 240.0)) {
    // Cron is still running normally.
    watchdog('cron', 'Attempting to re-run cron while it is already running.', array(), WATCHDOG_WARNING);
  }
  else {
    // Make sure every queue exists. There is no harm in trying to recreate an
    // existing queue.
    foreach ($queues as $queue_name => $info) {
      if (isset($info['cron'])) {
        Drupal::queue($queue_name)->createQueue();
      }
    }

    // Iterate through the modules calling their cron handlers (if any):
    foreach (module_implements('cron') as $module) {
      // Do not let an exception thrown by one module disturb another.
      try {
        module_invoke($module, 'cron');
      }
      catch (Exception $e) {
        watchdog_exception('cron', $e);
      }
    }

    // Record cron time.
    Drupal::state()->set('system.cron_last', REQUEST_TIME);
    watchdog('cron', 'Cron run completed.', array(), WATCHDOG_NOTICE);

    // Release cron lock.
    lock()->release('cron');

    // Return TRUE so other functions can check if it did run successfully
    $return = TRUE;
  }

  foreach ($queues as $queue_name => $info) {
    if (isset($info['cron'])) {
      $callback = $info['worker callback'];
      $end = time() + (isset($info['cron']['time']) ? $info['cron']['time'] : 15);
      $queue = Drupal::queue($queue_name);
      while (time() < $end && ($item = $queue->claimItem())) {
        call_user_func_array($callback, array($item->data));
        $queue->deleteItem($item);
      }
    }
  }
  // Restore the user.
  $GLOBALS['user'] = $original_user;
  drupal_save_session($original_session_saving);

  return $return;
}

/**
 * This function is kept only for backward compatibility.
 *
 * @see \Drupal\Core\SystemListing::scan().
 */
function drupal_system_listing($mask, $directory, $key = 'name', $min_depth = 1) {
  // As SystemListing is required to build a dependency injection container
  // from scratch and SystemListingInfo only extends SystemLising, this
  // class needs to be hardwired.
  $listing = new SystemListingInfo();
  return $listing->scan($mask, $directory, $key, $min_depth);
}

/**
 * Sets the main page content value for later use.
 *
 * Given the nature of the Drupal page handling, this will be called once with
 * a string or array. We store that and return it later as the block is being
 * displayed.
 *
 * @param $content
 *   A string or renderable array representing the body of the page.
 *
 * @return
 *   If called without $content, a renderable array representing the body of
 *   the page.
 */
function drupal_set_page_content($content = NULL) {
  $content_block = &drupal_static(__FUNCTION__, NULL);
  $main_content_display = &drupal_static('system_main_content_added', FALSE);

  // Filter out each empty value, though allow '0' and 0, which would be
  // filtered out by empty().
  if ($content !== NULL && $content !== '') {
    $content_block = (is_array($content) ? $content : array('main' => array('#markup' => $content)));
  }
  else {
    // Indicate that the main content has been requested. We assume that
    // the module requesting the content will be adding it to the page.
    // A module can indicate that it does not handle the content by setting
    // the static variable back to FALSE after calling this function.
    $main_content_display = TRUE;
    return $content_block;
  }
}

/**
 * Pre-render callback: Renders #browsers into #prefix and #suffix.
 *
 * @param $elements
 *   A render array with a '#browsers' property. The '#browsers' property can
 *   contain any or all of the following keys:
 *   - 'IE': If FALSE, the element is not rendered by Internet Explorer. If
 *     TRUE, the element is rendered by Internet Explorer. Can also be a string
 *     containing an expression for Internet Explorer to evaluate as part of a
 *     conditional comment. For example, this can be set to 'lt IE 7' for the
 *     element to be rendered in Internet Explorer 6, but not in Internet
 *     Explorer 7 or higher. Defaults to TRUE.
 *   - '!IE': If FALSE, the element is not rendered by browsers other than
 *     Internet Explorer. If TRUE, the element is rendered by those browsers.
 *     Defaults to TRUE.
 *   Examples:
 *   - To render an element in all browsers, '#browsers' can be left out or set
 *     to array('IE' => TRUE, '!IE' => TRUE).
 *   - To render an element in Internet Explorer only, '#browsers' can be set
 *     to array('!IE' => FALSE).
 *   - To render an element in Internet Explorer 6 only, '#browsers' can be set
 *     to array('IE' => 'lt IE 7', '!IE' => FALSE).
 *   - To render an element in Internet Explorer 8 and higher and in all other
 *     browsers, '#browsers' can be set to array('IE' => 'gte IE 8').
 *
 * @return
 *   The passed-in element with markup for conditional comments potentially
 *   added to '#prefix' and '#suffix'.
 */
function drupal_pre_render_conditional_comments($elements) {
  $browsers = isset($elements['#browsers']) ? $elements['#browsers'] : array();
  $browsers += array(
    'IE' => TRUE,
    '!IE' => TRUE,
  );

  // If rendering in all browsers, no need for conditional comments.
  if ($browsers['IE'] === TRUE && $browsers['!IE']) {
    return $elements;
  }

  // Determine the conditional comment expression for Internet Explorer to
  // evaluate.
  if ($browsers['IE'] === TRUE) {
    $expression = 'IE';
  }
  elseif ($browsers['IE'] === FALSE) {
    $expression = '!IE';
  }
  else {
    $expression = $browsers['IE'];
  }

  // Wrap the element's potentially existing #prefix and #suffix properties with
  // conditional comment markup. The conditional comment expression is evaluated
  // by Internet Explorer only. To control the rendering by other browsers,
  // either the "downlevel-hidden" or "downlevel-revealed" technique must be
  // used. See http://en.wikipedia.org/wiki/Conditional_comment for details.
  $elements += array(
    '#prefix' => '',
    '#suffix' => '',
  );
  if (!$browsers['!IE']) {
    // "downlevel-hidden".
    $elements['#prefix'] = "\n<!--[if $expression]>\n" . $elements['#prefix'];
    $elements['#suffix'] .= "<![endif]-->\n";
  }
  else {
    // "downlevel-revealed".
    $elements['#prefix'] = "\n<!--[if $expression]><!-->\n" . $elements['#prefix'];
    $elements['#suffix'] .= "<!--<![endif]-->\n";
  }

  return $elements;
}

/**
 * Pre-render callback: Renders a generic HTML tag with attributes into #markup.
 *
 * @param array $element
 *   An associative array containing:
 *   - #tag: The tag name to output. Typical tags added to the HTML HEAD:
 *     - meta: To provide meta information, such as a page refresh.
 *     - link: To refer to stylesheets and other contextual information.
 *     - script: To load JavaScript.
 *   - #attributes: (optional) An array of HTML attributes to apply to the
 *     tag.
 *   - #value: (optional) A string containing tag content, such as inline
 *     CSS.
 *   - #value_prefix: (optional) A string to prepend to #value, e.g. a CDATA
 *     wrapper prefix.
 *   - #value_suffix: (optional) A string to append to #value, e.g. a CDATA
 *     wrapper suffix.
 */
function drupal_pre_render_html_tag($element) {
  $attributes = isset($element['#attributes']) ? new Attribute($element['#attributes']) : '';
  if (!isset($element['#value'])) {
    $markup = '<' . $element['#tag'] . $attributes . " />\n";
  }
  else {
    $markup = '<' . $element['#tag'] . $attributes . '>';
    if (isset($element['#value_prefix'])) {
      $markup .= $element['#value_prefix'];
    }
    $markup .= $element['#value'];
    if (isset($element['#value_suffix'])) {
      $markup .= $element['#value_suffix'];
    }
    $markup .= '</' . $element['#tag'] . ">\n";
  }
  $element['#markup'] = $markup;
  return $element;
}

/**
 * Pre-render callback: Renders a link into #markup.
 *
 * Doing so during pre_render gives modules a chance to alter the link parts.
 *
 * @param $elements
 *   A structured array whose keys form the arguments to l():
 *   - #title: The link text to pass as argument to l().
 *   - #href: The URL path component to pass as argument to l().
 *   - #options: (optional) An array of options to pass to l().
 *
 * @return
 *   The passed-in elements containing a rendered link in '#markup'.
 */
function drupal_pre_render_link($element) {
  // By default, link options to pass to l() are normally set in #options.
  $element += array('#options' => array());
  // However, within the scope of renderable elements, #attributes is a valid
  // way to specify attributes, too. Take them into account, but do not override
  // attributes from #options.
  if (isset($element['#attributes'])) {
    $element['#options'] += array('attributes' => array());
    $element['#options']['attributes'] += $element['#attributes'];
  }

  // This #pre_render callback can be invoked from inside or outside of a Form
  // API context, and depending on that, a HTML ID may be already set in
  // different locations. #options should have precedence over Form API's #id.
  // #attributes have been taken over into #options above already.
  if (isset($element['#options']['attributes']['id'])) {
    $element['#id'] = $element['#options']['attributes']['id'];
  }
  elseif (isset($element['#id'])) {
    $element['#options']['attributes']['id'] = $element['#id'];
  }

  // Conditionally invoke ajax_pre_render_element(), if #ajax is set.
  if (isset($element['#ajax']) && !isset($element['#ajax_processed'])) {
    // If no HTML ID was found above, automatically create one.
    if (!isset($element['#id'])) {
      $element['#id'] = $element['#options']['attributes']['id'] = drupal_html_id('ajax-link');
    }
    // If #ajax['path] was not specified, use the href as Ajax request URL.
    if (!isset($element['#ajax']['path'])) {
      $element['#ajax']['path'] = $element['#href'];
      $element['#ajax']['options'] = $element['#options'];
    }
    $element = ajax_pre_render_element($element);
  }

  $element['#markup'] = l($element['#title'], $element['#href'], $element['#options']);
  return $element;
}

/**
 * Pre-render callback: Collects child links into a single array.
 *
 * This function can be added as a pre_render callback for a renderable array,
 * usually one which will be themed by theme_links(). It iterates through all
 * unrendered children of the element, collects any #links properties it finds,
 * merges them into the parent element's #links array, and prevents those
 * children from being rendered separately.
 *
 * The purpose of this is to allow links to be logically grouped into related
 * categories, so that each child group can be rendered as its own list of
 * links if drupal_render() is called on it, but calling drupal_render() on the
 * parent element will still produce a single list containing all the remaining
 * links, regardless of what group they were in.
 *
 * A typical example comes from node links, which are stored in a renderable
 * array similar to this:
 * @code
 * $node->content['links'] = array(
 *   '#theme' => 'links__node',
 *   '#pre_render' => array('drupal_pre_render_links'),
 *   'comment' => array(
 *     '#theme' => 'links__node__comment',
 *     '#links' => array(
 *       // An array of links associated with node comments, suitable for
 *       // passing in to theme_links().
 *     ),
 *   ),
 *   'statistics' => array(
 *     '#theme' => 'links__node__statistics',
 *     '#links' => array(
 *       // An array of links associated with node statistics, suitable for
 *       // passing in to theme_links().
 *     ),
 *   ),
 *   'translation' => array(
 *     '#theme' => 'links__node__translation',
 *     '#links' => array(
 *       // An array of links associated with node translation, suitable for
 *       // passing in to theme_links().
 *     ),
 *   ),
 * );
 * @endcode
 *
 * In this example, the links are grouped by functionality, which can be
 * helpful to themers who want to display certain kinds of links independently.
 * For example, adding this code to node.html.twig will result in the comment
 * links being rendered as a single list:
 * @code
 * print render($content['links']['comment']);
 * @endcode
 *
 * (where $node->content has been transformed into $content before handing
 * control to the node.html.twig template).
 *
 * The pre_render function defined here allows the above flexibility, but also
 * allows the following code to be used to render all remaining links into a
 * single list, regardless of their group:
 * @code
 * print render($content['links']);
 * @endcode
 *
 * In the above example, this will result in the statistics and translation
 * links being rendered together in a single list (but not the comment links,
 * which were rendered previously on their own).
 *
 * Because of the way this function works, the individual properties of each
 * group (for example, a group-specific #theme property such as
 * 'links__node__comment' in the example above, or any other property such as
 * #attributes or #pre_render that is attached to it) are only used when that
 * group is rendered on its own. When the group is rendered together with other
 * children, these child-specific properties are ignored, and only the overall
 * properties of the parent are used.
 */
function drupal_pre_render_links($element) {
  $element += array('#links' => array());
  foreach (element_children($element) as $key) {
    $child = &$element[$key];
    // If the child has links which have not been printed yet and the user has
    // access to it, merge its links in to the parent.
    if (isset($child['#links']) && empty($child['#printed']) && (!isset($child['#access']) || $child['#access'])) {
      $element['#links'] += $child['#links'];
      // Mark the child as having been printed already (so that its links
      // cannot be mistakenly rendered twice).
      $child['#printed'] = TRUE;
    }
  }
  return $element;
}

/**
 * Pre-render callback: Attaches the dropbutton library and required markup.
 */
function drupal_pre_render_dropbutton($element) {
  $element['#attached']['library'][] = array('system', 'drupal.dropbutton');
  $element['#attributes']['class'][] = 'dropbutton';
  if (!isset($element['#theme_wrappers'])) {
    $element['#theme_wrappers'] = array();
  }
  array_unshift($element['#theme_wrappers'], 'dropbutton_wrapper');

  // Enable targeted theming of specific dropbuttons (e.g., 'operations' or
  // 'operations__node').
  if (isset($element['#subtype'])) {
    $element['#theme'] .= '__' . $element['#subtype'];
  }

  return $element;
}

/**
 * Renders the page, including all theming.
 *
 * @param $page
 *   A string or array representing the content of a page. The array consists of
 *   the following keys:
 *   - #type: Value is always 'page'. This pushes the theming through
 *     the page template (required).
 *   - #show_messages: Suppress drupal_get_message() items. Used by Batch
 *     API (optional).
 *
 * @see hook_page_alter()
 * @see element_info()
 */
function drupal_render_page($page) {
  $main_content_display = &drupal_static('system_main_content_added', FALSE);

  // Allow menu callbacks to return strings or arbitrary arrays to render.
  // If the array returned is not of #type page directly, we need to fill
  // in the page with defaults.
  if (is_string($page) || (is_array($page) && (!isset($page['#type']) || ($page['#type'] != 'page')))) {
    drupal_set_page_content($page);
    $page = element_info('page');
  }

  // Modules can add elements to $page as needed in hook_page_build().
  foreach (module_implements('page_build') as $module) {
    $function = $module . '_page_build';
    $function($page);
  }
  // Modules alter the $page as needed. Blocks are populated into regions like
  // 'sidebar_first', 'footer', etc.
  drupal_alter('page', $page);

  // If no module has taken care of the main content, add it to the page now.
  // This allows the site to still be usable even if no modules that
  // control page regions (for example, the Block module) are enabled.
  if (!$main_content_display) {
    $page['content']['system_main'] = drupal_set_page_content();
  }

  return drupal_render($page);
}

/**
 * Renders HTML given a structured array tree.
 *
 * Recursively iterates over each of the array elements, generating HTML code.
 *
 * Renderable arrays have two kinds of key/value pairs: properties and
 * children. Properties have keys starting with '#' and their values influence
 * how the array will be rendered. Children are all elements whose keys do not
 * start with a '#'. Their values should be renderable arrays themselves,
 * which will be rendered during the rendering of the parent array. The markup
 * provided by the children is typically inserted into the markup generated by
 * the parent array.
 *
 * HTML generation for a renderable array, and the treatment of any children,
 * is controlled by two properties containing theme functions, #theme and
 * #theme_wrappers.
 *
 * #theme is the theme function called first. If it is set and the element has
 * any children, it is the responsibility of the theme function to render
 * these children. For elements that are not allowed to have any children,
 * e.g. buttons or textfields, the theme function can be used to render the
 * element itself. If #theme is not present and the element has children, each
 * child is itself rendered by a call to drupal_render(), and the results are
 * concatenated.
 *
 * The #theme_wrappers property contains an array of theme functions which will
 * be called, in order, after #theme has run. These can be used to add further
 * markup around the rendered children; e.g., details add the required markup
 * for a details element around their rendered child elements. All wrapper theme
 * functions have to include the element's #children property in their output,
 * as it contains the output of the previous theme functions and the rendered
 * children.
 *
 * For example, for the form element type, by default only the #theme_wrappers
 * property is set, which adds the form markup around the rendered child
 * elements of the form. This allows you to set the #theme property on a
 * specific form to a custom theme function, giving you complete control over
 * the placement of the form's children while not at all having to deal with
 * the form markup itself.
 *
 * drupal_render() can optionally cache the rendered output of elements to
 * improve performance. To use drupal_render() caching, set the element's #cache
 * property to an associative array with one or several of the following keys:
 * - 'keys': An array of one or more keys that identify the element. If 'keys'
 *   is set, the cache ID is created automatically from these keys. See
 *   drupal_render_cid_create().
 * - 'granularity' (optional): Define the cache granularity using binary
 *   combinations of the cache granularity constants, e.g.
 *   DRUPAL_CACHE_PER_USER to cache for each user separately or
 *   DRUPAL_CACHE_PER_PAGE | DRUPAL_CACHE_PER_ROLE to cache separately for each
 *   page and role. If not specified the element is cached globally for each
 *   theme and language.
 * - 'cid': Specify the cache ID directly. Either 'keys' or 'cid' is required.
 *   If 'cid' is set, 'keys' and 'granularity' are ignored. Use only if you
 *   have special requirements.
 * - 'expire': Set to one of the cache lifetime constants.
 * - 'bin': Specify a cache bin to cache the element in. Defaults to 'cache'.
 *
 * This function is usually called from within another function, like
 * drupal_get_form() or a theme function. Elements are sorted internally
 * using uasort(). Since this is expensive, when passing already sorted
 * elements to drupal_render(), for example from a database query, set
 * $elements['#sorted'] = TRUE to avoid sorting them a second time.
 *
 * drupal_render() flags each element with a '#printed' status to indicate that
 * the element has been rendered, which allows individual elements of a given
 * array to be rendered independently and prevents them from being rendered
 * more than once on subsequent calls to drupal_render() (e.g., as part of a
 * larger array). If the same array or array element is passed more than once
 * to drupal_render(), it simply returns an empty string.
 *
 * @param array $elements
 *   The structured array describing the data to be rendered.
 *
 * @return string
 *   The rendered HTML.
 */
function drupal_render(&$elements) {
  // Early-return nothing if user does not have access.
  if (empty($elements) || (isset($elements['#access']) && !$elements['#access'])) {
    return '';
  }

  // Do not print elements twice.
  if (!empty($elements['#printed'])) {
    return '';
  }

  // Try to fetch the element's markup from cache and return.
  if (isset($elements['#cache'])) {
    $cached_output = drupal_render_cache_get($elements);
    if ($cached_output !== FALSE) {
      return $cached_output;
    }
  }

  // If the default values for this element have not been loaded yet, populate
  // them.
  if (isset($elements['#type']) && empty($elements['#defaults_loaded'])) {
    $elements += element_info($elements['#type']);
  }

  // Make any final changes to the element before it is rendered. This means
  // that the $element or the children can be altered or corrected before the
  // element is rendered into the final text.
  if (isset($elements['#pre_render'])) {
    foreach ($elements['#pre_render'] as $callable) {
      $elements = call_user_func($callable, $elements);
    }
  }

  // Allow #pre_render to abort rendering.
  if (!empty($elements['#printed'])) {
    return '';
  }

  // Get the children of the element, sorted by weight.
  $children = element_children($elements, TRUE);

  // Initialize this element's #children, unless a #pre_render callback already
  // preset #children.
  if (!isset($elements['#children'])) {
    $elements['#children'] = '';
  }
  // Call the element's #theme function if it is set. Then any children of the
  // element have to be rendered there. If the internal #render_children
  // property is set, do not call the #theme function to prevent infinite
  // recursion.
  if (isset($elements['#theme']) && !isset($elements['#render_children'])) {
    $elements['#children'] = theme($elements['#theme'], $elements);
  }
  // If #theme was not set and the element has children, render them now.
  // This is the same process as drupal_render_children() but is inlined
  // for speed.
  if ($elements['#children'] === '') {
    foreach ($children as $key) {
      $elements['#children'] .= drupal_render($elements[$key]);
    }
  }
  // If #theme was not set, but the element has raw #markup, prepend the content
  // in #markup to #children. #children may contain the rendered content
  // supplied by #theme, or the rendered child elements, as processed above. If
  // both #theme and #markup are set, then #theme is responsible for rendering
  // the element. Eventually assigned #theme_wrappers will expect both the
  // element's #markup and the rendered content of child elements in #children.
  if (!isset($elements['#theme']) && isset($elements['#markup'])) {
    $elements['#children'] = $elements['#markup'] . $elements['#children'];
  }

  // Add any JavaScript state information associated with the element.
  if (!empty($elements['#states'])) {
    drupal_process_states($elements);
  }

  // Add additional libraries, CSS, JavaScript an other custom
  // attached data associated with this element.
  if (!empty($elements['#attached'])) {
    drupal_process_attached($elements);
  }

  // Let the theme functions in #theme_wrappers add markup around the rendered
  // children.
  // #states and #attached have to be processed before #theme_wrappers, because
  // the #type 'page' render array from drupal_render_page() would render the
  // $page and wrap it into the html.tpl.php template without the attached
  // assets otherwise.
  // If the internal #render_children property is set, do not call the
  // #theme_wrappers function(s) to prevent infinite recursion.
  if (isset($elements['#theme_wrappers']) && !isset($elements['#render_children'])) {
    foreach ($elements['#theme_wrappers'] as $theme_wrapper) {
      $elements['#children'] = theme($theme_wrapper, $elements);
    }
  }

  // Filter the outputted content and make any last changes before the
  // content is sent to the browser. The changes are made on $content
  // which allows the output'ed text to be filtered.
  if (isset($elements['#post_render'])) {
    foreach ($elements['#post_render'] as $callable) {
      $elements['#children'] = call_user_func($callable, $elements['#children'], $elements);
    }
  }

  $prefix = isset($elements['#prefix']) ? $elements['#prefix'] : '';
  $suffix = isset($elements['#suffix']) ? $elements['#suffix'] : '';
  $output = $prefix . $elements['#children'] . $suffix;

  // Cache the processed element if #cache is set.
  if (isset($elements['#cache'])) {
    drupal_render_cache_set($output, $elements);
  }

  $elements['#printed'] = TRUE;
  return $output;
}

/**
 * Renders children of an element and concatenates them.
 *
 * This renders all children of an element using drupal_render() and then
 * joins them together into a single string.
 *
 * @param $element
 *   The structured array whose children shall be rendered.
 * @param $children_keys
 *   If the keys of the element's children are already known, they can be passed
 *   in to save another run of element_children().
 */
function drupal_render_children(&$element, $children_keys = NULL) {
  if ($children_keys === NULL) {
    $children_keys = element_children($element);
  }
  $output = '';
  foreach ($children_keys as $key) {
    if (!empty($element[$key])) {
      $output .= drupal_render($element[$key]);
    }
  }
  return $output;
}

/**
 * Renders an element.
 *
 * This function renders an element using drupal_render(). The top level
 * element is shown with show() before rendering, so it will always be rendered
 * even if hide() had been previously used on it.
 *
 * @param $element
 *   The element to be rendered.
 *
 * @return
 *   The rendered element.
 *
 * @see drupal_render()
 * @see show()
 * @see hide()
 */
function render(&$element) {
  if (!$element && $element !== 0) {
    return NULL;
  }
  if (is_array($element)) {
    show($element);
    return drupal_render($element);
  }
  else {
    // Safe-guard for inappropriate use of render() on flat variables: return
    // the variable as-is.
    return $element;
  }
}

/**
 * Hides an element from later rendering.
 *
 * The first time render() or drupal_render() is called on an element tree,
 * as each element in the tree is rendered, it is marked with a #printed flag
 * and the rendered children of the element are cached. Subsequent calls to
 * render() or drupal_render() will not traverse the child tree of this element
 * again: they will just use the cached children. So if you want to hide an
 * element, be sure to call hide() on the element before its parent tree is
 * rendered for the first time, as it will have no effect on subsequent
 * renderings of the parent tree.
 *
 * @param $element
 *   The element to be hidden.
 *
 * @return
 *   The element.
 *
 * @see render()
 * @see show()
 */
function hide(&$element) {
  $element['#printed'] = TRUE;
  return $element;
}

/**
 * Shows a hidden element for later rendering.
 *
 * You can also use render($element), which shows the element while rendering
 * it.
 *
 * The first time render() or drupal_render() is called on an element tree,
 * as each element in the tree is rendered, it is marked with a #printed flag
 * and the rendered children of the element are cached. Subsequent calls to
 * render() or drupal_render() will not traverse the child tree of this element
 * again: they will just use the cached children. So if you want to show an
 * element, be sure to call show() on the element before its parent tree is
 * rendered for the first time, as it will have no effect on subsequent
 * renderings of the parent tree.
 *
 * @param $element
 *   The element to be shown.
 *
 * @return
 *   The element.
 *
 * @see render()
 * @see hide()
 */
function show(&$element) {
  $element['#printed'] = FALSE;
  return $element;
}

/**
 * Gets the rendered output of a renderable element from the cache.
 *
 * @param $elements
 *   A renderable array.
 *
 * @return
 *   A markup string containing the rendered content of the element, or FALSE
 *   if no cached copy of the element is available.
 *
 * @see drupal_render()
 * @see drupal_render_cache_set()
 */
function drupal_render_cache_get($elements) {
  if (!in_array($_SERVER['REQUEST_METHOD'], array('GET', 'HEAD')) || !$cid = drupal_render_cid_create($elements)) {
    return FALSE;
  }
  $bin = isset($elements['#cache']['bin']) ? $elements['#cache']['bin'] : 'cache';

  if (!empty($cid) && $cache = cache($bin)->get($cid)) {
    // Add additional libraries, JavaScript, CSS and other data attached
    // to this element.
    if (isset($cache->data['#attached'])) {
      drupal_process_attached($cache->data);
    }
    // Return the rendered output.
    return $cache->data['#markup'];
  }
  return FALSE;
}

/**
 * Caches the rendered output of a renderable element.
 *
 * This is called by drupal_render() if the #cache property is set on an
 * element.
 *
 * @param $markup
 *   The rendered output string of $elements.
 * @param $elements
 *   A renderable array.
 *
 * @see drupal_render_cache_get()
 */
function drupal_render_cache_set(&$markup, $elements) {
  // Create the cache ID for the element.
  if (!in_array($_SERVER['REQUEST_METHOD'], array('GET', 'HEAD')) || !$cid = drupal_render_cid_create($elements)) {
    return FALSE;
  }

  // Cache implementations are allowed to modify the markup, to support
  // replacing markup with edge-side include commands. The supporting cache
  // backend will store the markup in some other key (like
  // $data['#real-value']) and return an include command instead. When the
  // ESI command is executed by the content accelerator, the real value can
  // be retrieved and used.
  $data['#markup'] = &$markup;
  // Persist attached data associated with this element.
  $attached = drupal_render_collect_attached($elements, TRUE);
  if ($attached) {
    $data['#attached'] = $attached;
  }
  $bin = isset($elements['#cache']['bin']) ? $elements['#cache']['bin'] : 'cache';
  $expire = isset($elements['#cache']['expire']) ? $elements['#cache']['expire'] : CacheBackendInterface::CACHE_PERMANENT;
  $tags = isset($elements['#cache']['tags']) ? $elements['#cache']['tags'] : array();
  cache($bin)->set($cid, $data, $expire, $tags);
}

/**
 * Collects #attached for an element and its children into a single array.
 *
 * When caching elements, it is necessary to collect all libraries, JavaScript
 * and CSS into a single array, from both the element itself and all child
 * elements. This allows drupal_render() to add these back to the page when the
 * element is returned from cache.
 *
 * @param $elements
 *   The element to collect #attached from.
 * @param $return
 *   Whether to return the attached elements and reset the internal static.
 *
 * @return
 *   The #attached array for this element and its descendants.
 */
function drupal_render_collect_attached($elements, $return = FALSE) {
  $attached = &drupal_static(__FUNCTION__, array());

  // Collect all #attached for this element.
  if (isset($elements['#attached'])) {
    foreach ($elements['#attached'] as $key => $value) {
      if (!isset($attached[$key])) {
        $attached[$key] = array();
      }
      $attached[$key] = array_merge($attached[$key], $value);
    }
  }
  if ($children = element_children($elements)) {
    foreach ($children as $child) {
      drupal_render_collect_attached($elements[$child]);
    }
  }

  // If this was the first call to the function, return all attached elements
  // and reset the static cache.
  if ($return) {
    $return = $attached;
    $attached = array();
    return $return;
  }
}

/**
 * Prepares an element for caching based on a query.
 *
 * This smart caching strategy saves Drupal from querying and rendering to HTML
 * when the underlying query is unchanged.
 *
 * Expensive queries should use the query builder to create the query and then
 * call this function. Executing the query and formatting results should happen
 * in a #pre_render callback.
 *
 * @param $query
 *   A select query object as returned by db_select().
 * @param $function
 *   The name of the function doing this caching. A _pre_render suffix will be
 *   added to this string and is also part of the cache key in
 *   drupal_render_cache_set() and drupal_render_cache_get().
 * @param $expire
 *   The cache expire time, passed eventually to cache()->set().
 * @param $granularity
 *   One or more granularity constants passed to drupal_render_cid_parts().
 *
 * @return
 *   A renderable array with the following keys and values:
 *   - #query: The passed-in $query.
 *   - #pre_render: $function with a _pre_render suffix.
 *   - #cache: An associative array prepared for drupal_render_cache_set().
 */
function drupal_render_cache_by_query($query, $function, $expire = CacheBackendInterface::CACHE_PERMANENT, $granularity = NULL) {
  $cache_keys = array_merge(array($function), drupal_render_cid_parts($granularity));
  $query->preExecute();
  $cache_keys[] = hash('sha256', serialize(array((string) $query, $query->getArguments())));
  return array(
    '#query' => $query,
    '#pre_render' => array($function . '_pre_render'),
    '#cache' => array(
      'keys' => $cache_keys,
      'expire' => $expire,
    ),
  );
}

/**
 * Returns cache ID parts for building a cache ID.
 *
 * @param $granularity
 *   One or more cache granularity constants. For example, to cache separately
 *   for each user, use DRUPAL_CACHE_PER_USER. To cache separately for each
 *   page and role, use the expression:
 *   @code
 *   DRUPAL_CACHE_PER_PAGE | DRUPAL_CACHE_PER_ROLE
 *   @endcode
 *
 * @return
 *   An array of cache ID parts, always containing the active theme. If the
 *   locale module is enabled it also contains the active language. If
 *   $granularity was passed in, more parts are added.
 */
function drupal_render_cid_parts($granularity = NULL) {
  global $theme, $base_root, $user;

  $cid_parts[] = $theme;
  // If Locale is enabled but we have only one language we do not need it as cid
  // part.
  if (language_multilingual()) {
    foreach (language_types_get_configurable() as $language_type) {
      $cid_parts[] = language($language_type)->id;
    }
  }

  if (!empty($granularity)) {
    // 'PER_ROLE' and 'PER_USER' are mutually exclusive. 'PER_USER' can be a
    // resource drag for sites with many users, so when a module is being
    // equivocal, we favor the less expensive 'PER_ROLE' pattern.
    if ($granularity & DRUPAL_CACHE_PER_ROLE) {
      $cid_parts[] = 'r.' . implode(',', $user->roles);
    }
    elseif ($granularity & DRUPAL_CACHE_PER_USER) {
      $cid_parts[] = "u.$user->uid";
    }

    if ($granularity & DRUPAL_CACHE_PER_PAGE) {
      $cid_parts[] = $base_root . request_uri();
    }
  }

  return $cid_parts;
}

/**
 * Creates the cache ID for a renderable element.
 *
 * This creates the cache ID string, either by returning the #cache['cid']
 * property if present or by building the cache ID out of the #cache['keys']
 * and, optionally, the #cache['granularity'] properties.
 *
 * @param $elements
 *   A renderable array.
 *
 * @return
 *   The cache ID string, or FALSE if the element may not be cached.
 */
function drupal_render_cid_create($elements) {
  if (isset($elements['#cache']['cid'])) {
    return $elements['#cache']['cid'];
  }
  elseif (isset($elements['#cache']['keys'])) {
    $granularity = isset($elements['#cache']['granularity']) ? $elements['#cache']['granularity'] : NULL;
    // Merge in additional cache ID parts based provided by drupal_render_cid_parts().
    $cid_parts = array_merge($elements['#cache']['keys'], drupal_render_cid_parts($granularity));
    return implode(':', $cid_parts);
  }
  return FALSE;
}

/**
 * Sorts a structured array by '#weight' property.
 *
 * Callback for uasort() within element_children().
 *
 * @param $a
 *   First item for comparison. The compared items should be associative arrays
 *   that optionally include a '#weight' key.
 * @param $b
 *   Second item for comparison.
 */
function element_sort($a, $b) {
  $a_weight = (is_array($a) && isset($a['#weight'])) ? $a['#weight'] : 0;
  $b_weight = (is_array($b) && isset($b['#weight'])) ? $b['#weight'] : 0;
  if ($a_weight == $b_weight) {
    return 0;
  }
  return ($a_weight < $b_weight) ? -1 : 1;
}

/**
 * Sorts a structured array by '#title' property.
 *
 * Callback for uasort() within:
 * - system_modules()
 * - theme_simpletest_test_table()
 *
 * @param $a
 *   First item for comparison. The compared items should be associative arrays
 *   that optionally include a '#title' key.
 * @param $b
 *   Second item for comparison.
 */
function element_sort_by_title($a, $b) {
  $a_title = (is_array($a) && isset($a['#title'])) ? $a['#title'] : '';
  $b_title = (is_array($b) && isset($b['#title'])) ? $b['#title'] : '';
  return strnatcasecmp($a_title, $b_title);
}

/**
 * Retrieves the default properties for the defined element type.
 *
 * @param $type
 *   An element type as defined by hook_element_info().
 */
function element_info($type) {
  // Use the advanced drupal_static() pattern, since this is called very often.
  static $drupal_static_fast;
  if (!isset($drupal_static_fast)) {
    $drupal_static_fast['cache'] = &drupal_static(__FUNCTION__);
  }
  $cache = &$drupal_static_fast['cache'];

  if (!isset($cache)) {
    $cache = module_invoke_all('element_info');
    foreach ($cache as $element_type => $info) {
      $cache[$element_type]['#type'] = $element_type;
    }
    // Allow modules to alter the element type defaults.
    drupal_alter('element_info', $cache);
  }

  return isset($cache[$type]) ? $cache[$type] : array();
}

/**
 * Retrieves a single property for the defined element type.
 *
 * @param $type
 *   An element type as defined by hook_element_info().
 * @param $property_name
 *   The property within the element type that should be returned.
 * @param $default
 *   (Optional) The value to return if the element type does not specify a
 *   value for the property. Defaults to NULL.
 */
function element_info_property($type, $property_name, $default = NULL) {
  return (($info = element_info($type)) && array_key_exists($property_name, $info)) ? $info[$property_name] : $default;
}

/**
 * Sorts a structured array by the 'weight' element.
 *
 * Note that the sorting is by the 'weight' array element, not by the render
 * element property '#weight'.
 *
 * Callback for uasort() used in various functions.
 *
 * @param $a
 *   First item for comparison. The compared items should be associative arrays
 *   that optionally include a 'weight' element. For items without a 'weight'
 *   element, a default value of 0 will be used.
 * @param $b
 *   Second item for comparison.
 */
function drupal_sort_weight($a, $b) {
  $a_weight = (is_array($a) && isset($a['weight'])) ? $a['weight'] : 0;
  $b_weight = (is_array($b) && isset($b['weight'])) ? $b['weight'] : 0;
  if ($a_weight == $b_weight) {
    return 0;
  }
  return ($a_weight < $b_weight) ? -1 : 1;
}

/**
 * Sorts a structured array by 'title' key (no # prefix).
 *
 * Callback for uasort() within system_admin_index().
 *
 * @param $a
 *   First item for comparison. The compared items should be associative arrays
 *   that optionally include a 'title' key.
 * @param $b
 *   Second item for comparison.
 */
function drupal_sort_title($a, $b) {
  if (!isset($b['title'])) {
    return -1;
  }
  if (!isset($a['title'])) {
    return 1;
  }
  return strcasecmp($a['title'], $b['title']);
}

/**
 * Checks if the key is a property.
 */
function element_property($key) {
  return $key[0] == '#';
}

/**
 * Gets properties of a structured array element (keys beginning with '#').
 */
function element_properties($element) {
  return array_filter(array_keys((array) $element), 'element_property');
}

/**
 * Checks if the key is a child.
 */
function element_child($key) {
  return !isset($key[0]) || $key[0] != '#';
}

/**
 * Identifies the children of an element array, optionally sorted by weight.
 *
 * The children of a element array are those key/value pairs whose key does
 * not start with a '#'. See drupal_render() for details.
 *
 * @param $elements
 *   The element array whose children are to be identified.
 * @param $sort
 *   Boolean to indicate whether the children should be sorted by weight.
 *
 * @return
 *   The array keys of the element's children.
 */
function element_children(&$elements, $sort = FALSE) {
  // Do not attempt to sort elements which have already been sorted.
  $sort = isset($elements['#sorted']) ? !$elements['#sorted'] : $sort;

  // Filter out properties from the element, leaving only children.
  $children = array();
  $sortable = FALSE;
  foreach ($elements as $key => $value) {
    if ($key === '' || $key[0] !== '#') {
      if (is_array($value)) {
        $children[$key] = $value;
        if (isset($value['#weight'])) {
          $sortable = TRUE;
        }
      }
      // Only trigger an error if the value is not null.
      // @see http://drupal.org/node/1283892
      elseif (isset($value)) {
        trigger_error(t('"@key" is an invalid render array key', array('@key' => $key)), E_USER_ERROR);
      }
    }
  }
  // Sort the children if necessary.
  if ($sort && $sortable) {
    uasort($children, 'element_sort');
    // Put the sorted children back into $elements in the correct order, to
    // preserve sorting if the same element is passed through
    // element_children() twice.
    foreach ($children as $key => $child) {
      unset($elements[$key]);
      $elements[$key] = $child;
    }
    $elements['#sorted'] = TRUE;
  }

  return array_keys($children);
}

/**
 * Returns the visible children of an element.
 *
 * @param $elements
 *   The parent element.
 *
 * @return
 *   The array keys of the element's visible children.
 */
function element_get_visible_children(array $elements) {
  $visible_children = array();

  foreach (element_children($elements) as $key) {
    $child = $elements[$key];

    // Skip un-accessible children.
    if (isset($child['#access']) && !$child['#access']) {
      continue;
    }

    // Skip value and hidden elements, since they are not rendered.
    if (isset($child['#type']) && in_array($child['#type'], array('value', 'hidden'))) {
      continue;
    }

    $visible_children[$key] = $child;
  }

  return array_keys($visible_children);
}

/**
 * Sets HTML attributes based on element properties.
 *
 * @param $element
 *   The renderable element to process.
 * @param $map
 *   An associative array whose keys are element property names and whose values
 *   are the HTML attribute names to set for corresponding the property; e.g.,
 *   array('#propertyname' => 'attributename'). If both names are identical
 *   except for the leading '#', then an attribute name value is sufficient and
 *   no property name needs to be specified.
 */
function element_set_attributes(array &$element, array $map) {
  foreach ($map as $property => $attribute) {
    // If the key is numeric, the attribute name needs to be taken over.
    if (is_int($property)) {
      $property = '#' . $attribute;
    }
    // Do not overwrite already existing attributes.
    if (isset($element[$property]) && !isset($element['#attributes'][$attribute])) {
      $element['#attributes'][$attribute] = $element[$property];
    }
  }
}

/**
 * Parses Drupal module and theme .info.yml files.
 *
 * Info files are NOT for placing arbitrary theme and module-specific settings.
 * Use Config::get() and Config::set()->save() for that. Info files are
 * formatted as YAML. If the 'version' key is set to 'VERSION' in any info file,
 * then the value will be substituted with the current version of Drupal core.
 *
 * Information stored in a module .info.yml file:
 * - name: The real name of the module for display purposes.
 * - description: A brief description of the module.
 * - dependencies: An array of shortnames of other modules this module requires.
 * - package: The name of the package of modules this module belongs to.
 *
 * See forum.info.yml for an example of a module .info.yml file.
 *
 * Information stored in a theme .info.yml file:
 * - name: The real name of the theme for display purposes.
 * - description: Brief description.
 * - screenshot: Path to screenshot relative to the theme's .info.yml file.
 * - engine: Theme engine; typically twig.
 * - base theme: Name of a base theme, if applicable.
 * - regions: Listed regions.
 * - features: Features available.
 * - stylesheets: Theme stylesheets.
 * - scripts: Theme scripts.
 *
 * See bartik.info.yml for an example of a theme .info.yml file.
 *
 * @param string $filename
 *   The file we are parsing. Accepts file with relative or absolute path.
 *
 * @return array
 *   The info array.
 */
function drupal_parse_info_file($filename) {
  static $info = array();

  if (!isset($info[$filename])) {
    if (!file_exists($filename)) {
      $info[$filename] = array();
    }
    else {
      $parser = new Parser();
      $info[$filename] = $parser->parse(file_get_contents($filename));
      if (isset($info[$filename]['version']) && $info[$filename]['version'] === 'VERSION') {
        $info[$filename]['version'] = VERSION;
      }
    }
  }
  return $info[$filename];
}

/**
 * Returns a list of severity levels, as defined in RFC 3164.
 *
 * @return
 *   Array of the possible severity levels for log messages.
 *
 * @see http://www.ietf.org/rfc/rfc3164.txt
 * @see watchdog()
 * @ingroup logging_severity_levels
 */
function watchdog_severity_levels() {
  return array(
    WATCHDOG_EMERGENCY   => t('Emergency'),
    WATCHDOG_ALERT   => t('Alert'),
    WATCHDOG_CRITICAL    => t('Critical'),
    WATCHDOG_ERROR     => t('Error'),
    WATCHDOG_WARNING => t('Warning'),
    WATCHDOG_NOTICE  => t('Notice'),
    WATCHDOG_INFO    => t('Info'),
    WATCHDOG_DEBUG   => t('Debug'),
  );
}


/**
 * Explodes a string of tags into an array.
 *
 * @see drupal_implode_tags()
 * @see \Drupal\Component\Utility\String::explodeTags().
 *
 * @deprecated as of Drupal 8.0. Use Tags::explode() instead.
 */
function drupal_explode_tags($tags) {
  return Tags::explode($tags);
}

/**
 * Implodes an array of tags into a string.
 *
 * @see drupal_explode_tags()
 * @see \Drupal\Component\Utility\String::implodeTags().
 *
 * @deprecated as of Drupal 8.0. Use Tags::implode() instead.
 */
function drupal_implode_tags($tags) {
  return Tags::implode($tags);
}

/**
 * Flushes all persistent caches, resets all variables, and rebuilds all data structures.
 *
 * At times, it is necessary to re-initialize the entire system to account for
 * changed or new code. This function:
 * - Clears all persistent caches:
 *   - The bootstrap cache bin containing base system, module system, and theme
 *     system information.
 *   - The common 'cache' cache bin containing arbitrary caches.
 *   - The page cache.
 *   - The URL alias path cache.
 * - Resets all static variables that have been defined via drupal_static().
 * - Clears asset (JS/CSS) file caches.
 * - Updates the system with latest information about extensions (modules and
 *   themes).
 * - Updates the bootstrap flag for modules implementing bootstrap_hooks().
 * - Rebuilds the full database schema information (invoking hook_schema()).
 * - Rebuilds data structures of all modules (invoking hook_rebuild()). In
 *   core this means
 *   - blocks, node types, date formats and actions are synchronized with the
 *     database
 *   - The 'active' status of fields is refreshed.
 * - Rebuilds the menu router.
 *
 * This means the entire system is reset so all caches and static variables are
 * effectively empty. After that is guaranteed, information about the currently
 * active code is updated, and rebuild operations are successively called in
 * order to synchronize the active system according to the current information
 * defined in code.
 *
 * All modules need to ensure that all of their caches are flushed when
 * hook_cache_flush() is invoked; any previously known information must no
 * longer exist. All following hook_rebuild() operations must be based on fresh
 * and current system data. All modules must be able to rely on this contract.
 *
 * @see \Drupal\Core\Cache\CacheHelper::getBins()
 * @see hook_cache_flush()
 * @see hook_rebuild()
 *
 * This function also resets the theme, which means it is not initialized
 * anymore and all previously added JavaScript and CSS is gone. Normally, this
 * function is called as an end-of-POST-request operation that is followed by a
 * redirect, so this effect is not visible. Since the full reset is the whole
 * point of this function, callers need to take care for backing up all needed
 * variables and properly restoring or re-initializing them on their own. For
 * convenience, this function automatically re-initializes the maintenance theme
 * if it was initialized before.
 *
 * @todo Try to clear page/JS/CSS caches last, so cached pages can still be
 *   served during this possibly long-running operation. (Conflict on bootstrap
 *   cache though.)
 * @todo Add a global lock to ensure that caches are not primed in concurrent
 *   requests.
 */
function drupal_flush_all_caches() {
  $module_handler = Drupal::moduleHandler();
  // Flush all persistent caches.
  // This is executed based on old/previously known information, which is
  // sufficient, since new extensions cannot have any primed caches yet.
  $module_handler->invokeAll('cache_flush');
  foreach (Cache::getBins() as $service_id => $cache_backend) {
    if ($service_id != 'cache.menu') {
      $cache_backend->deleteAll();
    }
  }

  // Flush asset file caches.
  drupal_clear_css_cache();
  drupal_clear_js_cache();
  _drupal_flush_css_js();

  // Reset all static caches.
  drupal_static_reset();

  // Clear all non-drupal_static() static caches.
  Drupal::entityManager()->clearCachedDefinitions();

  // Wipe the PHP Storage caches.
  PhpStorageFactory::get('service_container')->deleteAll();
  PhpStorageFactory::get('twig')->deleteAll();

  // Rebuild module and theme data.
  $module_data = system_rebuild_module_data();
  system_rebuild_theme_data();

  // Rebuild and reboot a new kernel. A simple DrupalKernel reboot is not
  // sufficient, since the list of enabled modules might have been adjusted
  // above due to changed code.
  $files = array();
  foreach ($module_data as $module => $data) {
    if (isset($data->uri) && $data->status) {
      $files[$module] = $data->uri;
    }
  }
  Drupal::service('kernel')->updateModules($module_handler->getModuleList(), $files);
  // New container, new module handler.
  $module_handler = Drupal::moduleHandler();

  // Ensure that all modules that are currently supposed to be enabled are
  // actually loaded.
  $module_handler->loadAll();

  // Update the list of bootstrap modules.
  // Allows developers to get new bootstrap hooks implementations registered
  // without having to write a hook_update_N() function.
  _system_update_bootstrap_status();

  // Rebuild the schema and cache a fully-built schema based on new module data.
  // This is necessary for any invocation of index.php, because setting cache
  // table entries requires schema information and that occurs during bootstrap
  // before any modules are loaded, so if there is no cached schema,
  // drupal_get_schema() will try to generate one, but with no loaded modules,
  // it will return nothing.
  drupal_get_schema(NULL, TRUE);

  // Rebuild all information based on new module data.
  $module_handler->invokeAll('rebuild');

  // Rebuild the menu router based on all rebuilt data.
  // Important: This rebuild must happen last, so the menu router is guaranteed
  // to be based on up to date information.
  Drupal::service('router.builder')->rebuild();
  menu_router_rebuild();

  // Re-initialize the maintenance theme, if the current request attempted to
  // use it. Unlike regular usages of this function, the installer and update
  // scripts need to flush all caches during GET requests/page building.
  if (function_exists('_drupal_maintenance_theme')) {
    unset($GLOBALS['theme']);
    drupal_maintenance_theme();
  }
}

/**
 * Changes the dummy query string added to all CSS and JavaScript files.
 *
 * Changing the dummy query string appended to CSS and JavaScript files forces
 * all browsers to reload fresh files.
 */
function _drupal_flush_css_js() {
  // The timestamp is converted to base 36 in order to make it more compact.
  variable_set('css_js_query_string', base_convert(REQUEST_TIME, 10, 36));
}

/**
 * Outputs debug information.
 *
 * The debug information is passed on to trigger_error() after being converted
 * to a string using _drupal_debug_message().
 *
 * @param $data
 *   Data to be output.
 * @param $label
 *   Label to prefix the data.
 * @param $print_r
 *   Flag to switch between print_r() and var_export() for data conversion to
 *   string. Set $print_r to TRUE when dealing with a recursive data structure
 *   as var_export() will generate an error.
 */
function debug($data, $label = NULL, $print_r = FALSE) {
  // Print $data contents to string.
  $string = check_plain($print_r ? print_r($data, TRUE) : var_export($data, TRUE));

  // Display values with pre-formatting to increase readability.
  $string = '<pre>' . $string . '</pre>';

  trigger_error(trim($label ? "$label: $string" : $string));
}

/**
 * Checks whether a version is compatible with a given dependency.
 *
 * @param $v
 *   A parsed dependency structure e.g. from ModuleHandler::parseDependency().
 * @param $current_version
 *   The version to check against (like 4.2).
 *
 * @return
 *   NULL if compatible, otherwise the original dependency version string that
 *   caused the incompatibility.
 *
 * @see \Drupal\Core\Extension\ModuleHandler::parseDependency()
 */
function drupal_check_incompatibility($v, $current_version) {
  if (!empty($v['versions'])) {
    foreach ($v['versions'] as $required_version) {
      if ((isset($required_version['op']) && !version_compare($current_version, $required_version['version'], $required_version['op']))) {
        return $v['original_version'];
      }
    }
  }
}

/**
 * Returns a string of supported archive extensions.
 *
 * @return
 *   A space-separated string of extensions suitable for use by the file
 *   validation system.
 */
function archiver_get_extensions() {
  $valid_extensions = array();
  foreach (Drupal::service('plugin.manager.archiver')->getDefinitions() as $archive) {
    foreach ($archive['extensions'] as $extension) {
      foreach (explode('.', $extension) as $part) {
        if (!in_array($part, $valid_extensions)) {
          $valid_extensions[] = $part;
        }
      }
    }
  }
  return implode(' ', $valid_extensions);
}

/**
 * Creates the appropriate archiver for the specified file.
 *
 * @param $file
 *   The full path of the archive file. Note that stream wrapper paths are
 *   supported, but not remote ones.
 *
 * @return
 *   A newly created instance of the archiver class appropriate
 *   for the specified file, already bound to that file.
 *   If no appropriate archiver class was found, will return FALSE.
 */
function archiver_get_archiver($file) {
  // Archivers can only work on local paths
  $filepath = drupal_realpath($file);
  if (!is_file($filepath)) {
    throw new Exception(t('Archivers can only operate on local files: %file not supported', array('%file' => $file)));
  }
  return Drupal::service('plugin.manager.archiver')->getInstance(array('filepath' => $filepath));
}

/**
 * Assembles the Drupal Updater registry.
 *
 * An Updater is a class that knows how to update various parts of the Drupal
 * file system, for example to update modules that have newer releases, or to
 * install a new theme.
 *
 * @return array
 *   The Drupal Updater class registry.
 *
 * @see Drupal\Core\Updater\Updater
 * @see hook_updater_info()
 * @see hook_updater_info_alter()
 */
function drupal_get_updaters() {
  $updaters = &drupal_static(__FUNCTION__);
  if (!isset($updaters)) {
    $updaters = module_invoke_all('updater_info');
    drupal_alter('updater_info', $updaters);
    uasort($updaters, 'drupal_sort_weight');
  }
  return $updaters;
}

/**
 * Assembles the Drupal FileTransfer registry.
 *
 * @return
 *   The Drupal FileTransfer class registry.
 *
 * @see Drupal\Core\FileTransfer\FileTransfer
 * @see hook_filetransfer_info()
 * @see hook_filetransfer_info_alter()
 */
function drupal_get_filetransfer_info() {
  $info = &drupal_static(__FUNCTION__);
  if (!isset($info)) {
    $info = module_invoke_all('filetransfer_info');
    drupal_alter('filetransfer_info', $info);
    uasort($info, 'drupal_sort_weight');
  }
  return $info;
}

/**
 * @defgroup queue Queue operations
 * @{
 * Queue items to allow later processing.
 *
 * The queue system allows placing items in a queue and processing them later.
 * The system tries to ensure that only one consumer can process an item.
 *
 * Before a queue can be used it needs to be created by
 * Drupal\Core\Queue\QueueInterface::createQueue().
 *
 * Items can be added to the queue by passing an arbitrary data object to
 * Drupal\Core\Queue\QueueInterface::createItem().
 *
 * To process an item, call Drupal\Core\Queue\QueueInterface::claimItem() and
 * specify how long you want to have a lease for working on that item.
 * When finished processing, the item needs to be deleted by calling
 * Drupal\Core\Queue\QueueInterface::deleteItem(). If the consumer dies, the
 * item will be made available again by the Drupal\Core\Queue\QueueInterface
 * implementation once the lease expires. Another consumer will then be able to
 * receive it when calling Drupal\Core\Queue\QueueInterface::claimItem().
 * Due to this, the processing code should be aware that an item might be handed
 * over for processing more than once.
 *
 * The $item object used by the Drupal\Core\Queue\QueueInterface can contain
 * arbitrary metadata depending on the implementation. Systems using the
 * interface should only rely on the data property which will contain the
 * information passed to Drupal\Core\Queue\QueueInterface::createItem().
 * The full queue item returned by Drupal\Core\Queue\QueueInterface::claimItem()
 * needs to be passed to Drupal\Core\Queue\QueueInterface::deleteItem() once
 * processing is completed.
 *
 * There are two kinds of queue backends available: reliable, which preserves
 * the order of messages and guarantees that every item will be executed at
 * least once. The non-reliable kind only does a best effort to preserve order
 * in messages and to execute them at least once but there is a small chance
 * that some items get lost. For example, some distributed back-ends like
 * Amazon SQS will be managing jobs for a large set of producers and consumers
 * where a strict FIFO ordering will likely not be preserved. Another example
 * would be an in-memory queue backend which might lose items if it crashes.
 * However, such a backend would be able to deal with significantly more writes
 * than a reliable queue and for many tasks this is more important. See
 * aggregator_cron() for an example of how to effectively utilize a
 * non-reliable queue. Another example is doing Twitter statistics -- the small
 * possibility of losing a few items is insignificant next to power of the
 * queue being able to keep up with writes. As described in the processing
 * section, regardless of the queue being reliable or not, the processing code
 * should be aware that an item might be handed over for processing more than
 * once (because the processing code might time out before it finishes).
 */

/**
 * @} End of "defgroup queue".
 */