<?php

/**
 * @ingroup database
 * @{
 */

/**
 * Interface for a conditional clause in a query.
 */
interface QueryConditionInterface {

  /**
   * Helper function to build most common conditional clauses.
   *
   * This method can take a variable number of parameters.  If called with two
   * parameters, they are taken as $field and $value with $operator having a value
   * of =.
   *
   * @param $field
   *   The name of the field to check.
   * @param $value
   *   The value to test the field against.  In most cases, this is a scalar. For more
   *   complex options, it is an array.  The meaning of each element in the array is
   *   dependent on the $operator.
   * @param $operator
   *   The comparison operator, such as =, <, or >=.  It also accepts more complex
   *   options such as IN, LIKE, or BETWEEN.
   * @param $num_args
   *   For internal use only.  This argument is used to track the recursive calls when
   *   processing complex conditions.
   * @return
   *   The called object.
   */
  public function condition($field, $value = NULL, $operator = NULL);

  /**
   * Add an arbitrary WHERE clause to the query.
   *
   * @param $snippet
   *   A portion of a WHERE clause as a prepared statement.  It must use named placeholders,
   *   not ? placeholders.
   * @param $args
   *   An associative array of arguments.
   * @return
   *   The called object.
   */
  public function where($snippet, $args = array());

  /**
   * Gets a complete list of all conditions in this conditional clause.
   *
   * This method returns by reference.  That allows alter hooks to access the
   * data structure directly and manipulate it before it gets compiled.
   *
   * The data structure that is returned is an indexed array of entries, where
   * each entry looks like the following:
   *
   * array(
   *   'field' => $field,
   *   'value' => $value,
   *   'operator' => $operator,
   * );
   *
   * In the special case that $operator is NULL, the $field is taken as a raw
   * SQL snippet (possibly containing a function) and $value is an associative
   * array of placeholders for the snippet.
   *
   * There will also be a single array entry of #conjunction, which is the
   * conjunction that will be applied to the array, such as AND.
   */
  public function &conditions();

  /**
   * Gets a complete list of all values to insert into the prepared statement.
   *
   * @returns
   *   An associative array of placeholders and values.
   */
  public function arguments();

  /**
   * Compiles the saved conditions for later retrieval.
   *
   * This method does not return anything, but simply prepares data to be
   * retrieved via __toString() and arguments().
   *
   * @param $connection
   *   The database connection for which to compile the conditionals.
   */
  public function compile(DatabaseConnection $connection);
}


/**
 * Interface for a query that can be manipulated via an alter hook.
 */
interface QueryAlterableInterface {

  /**
   * Adds a tag to a query.
   *
   * Tags are strings that identify a query.  A query may have any number of
   * tags.  Tags are used to mark a query so that alter hooks may decide if they
   * wish to take action.  Tags should be all lower-case and contain only letters,
   * numbers, and underscore, and start with a letter.  That is, they should
   * follow the same rules as PHP identifiers in general.
   *
   * @param $tag
   *   The tag to add.
   */
  public function addTag($tag);

  /**
   * Determines if a given query has a given tag.
   *
   * @param $tag
   *   The tag to check.
   * @return
   *   TRUE if this query has been marked with this tag, FALSE otherwise.
   */
  public function hasTag($tag);

  /**
   * Determines if a given query has all specified tags.
   *
   * @param $tags
   *   A variable number of arguments, one for each tag to check.
   * @return
   *   TRUE if this query has been marked with all specified tags, FALSE otherwise.
   */
  public function hasAllTags();

  /**
   * Determines if a given query has any specified tag.
   *
   * @param $tags
   *   A variable number of arguments, one for each tag to check.
   * @return
   *   TRUE if this query has been marked with at least one of the specified
   *   tags, FALSE otherwise.
   */
  public function hasAnyTag();

  /**
   * Adds additional metadata to the query.
   *
   * Often, a query may need to provide additional contextual data to alter
   * hooks.  Alter hooks may then use that information to decide if and how
   * to take action.
   *
   * @param $key
   *   The unique identifier for this piece of metadata.  Must be a string that
   *   follows the same rules as any other PHP identifier.
   * @param $object
   *   The additional data to add to the query.  May be any valid PHP variable.
   *
   */
  public function addMetaData($key, $object);

  /**
   * Retrieves a given piece of metadata.
   *
   * @param $key
   *   The unique identifier for the piece of metadata to retrieve.
   * @return
   *   The previously attached metadata object, or NULL if one doesn't exist.
   */
  public function getMetaData($key);
}

/**
 * Base class for the query builders.
 *
 * All query builders inherit from a common base class.
 */
abstract class Query {

  /**
   * The connection object on which to run this query.
   *
   * @var DatabaseConnection
   */
  protected $connection;

  /**
   * The query options to pass on to the connection object.
   *
   * @var array
   */
  protected $queryOptions;

  public function __construct(DatabaseConnection $connection, $options) {
    $this->connection = $connection;
    $this->queryOptions = $options;
  }

  /**
   * Run the query against the database.
   */
  abstract protected function execute();

  /**
   * Returns the query as a prepared statement string.
   */
  abstract public function __toString();
}

/**
 * General class for an abstracted INSERT operation.
 */
class InsertQuery extends Query {

  /**
   * The table on which to insert.
   *
   * @var string
   */
  protected $table;

  /**
   * Whether or not this query is "delay-safe".  Different database drivers
   * may or may not implement this feature in their own ways.
   *
   * @var boolean
   */
  protected $delay;

  /**
   * An array of fields on which to insert.
   *
   * @var array
   */
  protected $insertFields = array();

  /**
   * An array of fields which should be set to their database-defined defaults.
   *
   * @var array
   */
  protected $defaultFields = array();

  /**
   * A nested array of values to insert.
   *
   * $insertValues itself is an array of arrays.  Each sub-array is an array of
   * field names to values to insert.  Whether multiple insert sets
   * will be run in a single query or multiple queries is left to individual drivers
   * to implement in whatever manner is most efficient.  The order of values in each
   * sub-array must match the order of fields in $insertFields.
   *
   * @var string
   */
  protected $insertValues = array();

  public function __construct($connection, $table, Array $options = array()) {
    $options['return'] = Database::RETURN_INSERT_ID;
    $options += array('delay' => FALSE);
    parent::__construct($connection, $options);
    $this->table = $table;
  }

  /**
   * Add a set of field->value pairs to be inserted.
   *
   * This method may only be called once.  Calling it a second time will be
   * ignored.  To queue up multiple sets of values to be inserted at once,
   * use the values() method.
   *
   * @param $fields
   *   An array of fields on which to insert.  This array may be indexed or
   *   associative.  If indexed, the array is taken to be the list of fields.
   *   If associative, the keys of the array are taken to be the fields and
   *   the values are taken to be corresponding values to insert.  If a
   *   $values argument is provided, $fields must be indexed.
   * @param $values
   *   An array of fields to insert into the database.  The values must be
   *   specified in the same order as the $fields array.
   * @return
   *   The called object.
   */
  public function fields(Array $fields, Array $values = array()) {
    if (empty($this->insertFields)) {
      if (empty($values)) {
        if (!is_numeric(key($fields))) {
          $values = array_values($fields);
          $fields = array_keys($fields);
        }
      }
      $this->insertFields = $fields;
      if (!empty($values)) {
        $this->insertValues[] = $values;
      }
    }

    return $this;
  }

  /**
   * Add another set of values to the query to be inserted.
   *
   * If $values is a numeric array, it will be assumed to be in the same
   * order as the original fields() call.  If it is associative, it may be
   * in any order as long as the keys of the array match the names of the
   * fields.
   *
   * @param $values
   *   An array of values to add to the query.
   * @return
   *   The called object.
   */
  public function values(Array $values) {
    if (is_numeric(key($values))) {
      $this->insertValues[] = $values;
    }
    else {
      // Reorder the submitted values to match the fields array.
      foreach ($this->insertFields as $key) {
        $insert_values[$key] = $values[$key];
      }
      // For consistency, the values array is always numerically indexed.
      $this->insertValues[] = array_values($insert_values);
    }
    return $this;
  }

  /**
   * Specify fields for which the database-defaults should be used.
   *
   * If you want to force a given field to use the database-defined default,
   * not NULL or undefined, use this method to instruct the database to use
   * default values explicitly.  In most cases this will not be necessary
   * unless you are inserting a row that is all default values, as you cannot
   * specify no values in an INSERT query.
   *
   * Specifying a field both in fields() and in useDefaults() is an error
   * and will not execute.
   *
   * @param $fields
   *   An array of values for which to use the default values
   *   specified in the table definition.
   * @return
   *   The called object.
   */
  public function useDefaults(Array $fields) {
    $this->defaultFields = $fields;
    return $this;
  }

  /**
   * Flag this query as being delay-safe or not.
   *
   * If this method is never called, it is assumed that the query must be
   * executed immediately.  If delay is set to TRUE, then the query will be
   * flagged to run "delayed" or "low priority" on databases that support such
   * capabilities.  In that case, the database will return immediately and the
   * query will be run at some point in the future.  That makes it useful for
   * logging-style queries.
   *
   * If the database does not support delayed INSERT queries, this method
   * has no effect.
   *
   * Note that for a delayed query there is no serial ID returned, as it won't
   * be created until later when the query runs.  It should therefore not be
   * used if the value of the ID is known.
   *
   * @param $delay
   *   If TRUE, this query is delay-safe and will run delayed on supported databases.
   * @return
   *   The called object.
   */
  public function delay($delay = TRUE) {
    $this->delay = $delay;
    return $this;
  }

  /**
   * Executes the insert query.
   *
   * @return
   *   The last insert ID of the query, if one exists.  If the query
   *   was given multiple sets of values to insert, the return value is
   *   undefined.  If the query is flagged "delayed", then the insert ID
   *   won't be created until later when the query actually runs so the
   *   return value is also undefined. If no fields are specified, this
   *   method will do nothing and return NULL. That makes it safe to use
   *   in multi-insert loops.
   */
  public function execute() {

    $last_insert_id = 0;

    // Confirm that the user did not try to specify an identical
    //  field and default field.
    if (array_intersect($this->insertFields, $this->defaultFields)) {
      throw new PDOException('You may not specify the same field to have a value and a schema-default value.');
    }

    if (count($this->insertFields) + count($this->defaultFields) == 0) {
      return NULL;
    }

    // Each insert happens in its own query in the degenerate case.  However,
    // we wrap it in a transaction so that it is atomic where possible.  On many
    // databases, such as SQLite, this is also a notable performance boost.
    $transaction = $this->connection->startTransaction();
    $sql = (string)$this;
    foreach ($this->insertValues as $insert_values) {
      $last_insert_id = $this->connection->query($sql, $insert_values, $this->queryOptions);
    }
    $transaction->commit();

    // Re-initialize the values array so that we can re-use this query.
    $this->insertValues = array();

    return $last_insert_id;
  }

  public function __toString() {

    // Default fields are always placed first for consistency.
    $insert_fields = array_merge($this->defaultFields, $this->insertFields);

    // For simplicity, we will use the $placeholders array to inject
    // default keywords even though they are not, strictly speaking,
    // placeholders for prepared statements.
    $placeholders = array();
    $placeholders = array_pad($placeholders, count($this->defaultFields), 'default');
    $placeholders = array_pad($placeholders, count($this->insertFields), '?');

    return 'INSERT INTO {'. $this->table .'} ('. implode(', ', $insert_fields) .') VALUES ('. implode(', ', $placeholders) .')';
  }
}

/**
 * General class for an abstracted MERGE operation.
 */
class MergeQuery extends Query {

  /**
   * The table on which to insert.
   *
   * @var string
   */
  protected $table;

  /**
   * An array of fields on which to insert.
   *
   * @var array
   */
  protected $insertFields = array();

  /**
   * An array of fields to update instead of the values specified in
   * $insertFields;
   *
   * @var array
   */
  protected $updateFields = array();

  /**
   * An array of key fields for this query.
   *
   * @var array
   */
  protected $keyFields = array();

  /**
   * An array of fields to not update in case of a duplicate record.
   *
   * @var array
   */
  protected $excludeFields = array();

  /**
   * An array of fields to update to an expression in case of a duplicate record.
   *
   * This variable is a nested array in the following format:
   * <some field> => array(
   *  'condition' => <condition to execute, as a string>
   *  'arguments' => <array of arguments for condition, or NULL for none>
   * );
   *
   * @var array
   */
  protected $expressionFields = array();

  public function __construct($connection, $table, Array $options = array()) {
    $options['return'] = Database::RETURN_AFFECTED;
    parent::__construct($connection, $options);
    $this->table = $table;
  }

  /**
   * Set the field->value pairs to be merged into the table.
   *
   * This method should only be called once.  It may be called either
   * with a single associative array or two indexed arrays.  If called
   * with an associative array, the keys are taken to be the fields
   * and the values are taken to be the corresponding values to set.
   * If called with two arrays, the first array is taken as the fields
   * and the second array is taken as the corresponding values.
   *
   * @param $fields
   *   An array of fields to set.
   * @param $values
   *   An array of fields to set into the database.  The values must be
   *   specified in the same order as the $fields array.
   * @return
   *   The called object.
   */
  public function fields(Array $fields, Array $values = array()) {
    if (count($values) > 0) {
      $fields = array_combine($fields, $values);
    }
    $this->insertFields = $fields;

    return $this;
  }

  /**
   * Set the key field(s) to be used to insert or update into the table.
   *
   * This method should only be called once.  It may be called either
   * with a single associative array or two indexed arrays.  If called
   * with an associative array, the keys are taken to be the fields
   * and the values are taken to be the corresponding values to set.
   * If called with two arrays, the first array is taken as the fields
   * and the second array is taken as the corresponding values.
   *
   * These fields are the "pivot" fields of the query.  Typically they
   * will be the fields of the primary key.  If the record does not
   * yet exist, they will be inserted into the table along with the
   * values set in the fields() method.  If the record does exist,
   * these fields will be used in the WHERE clause to select the
   * record to update.
   *
   * @param $fields
   *   An array of fields to set.
   * @param $values
   *   An array of fields to set into the database.  The values must be
   *   specified in the same order as the $fields array.
   * @return
   *   The called object.
   */
  public function key(Array $fields, Array $values = array()) {
    if ($values) {
      $fields = array_combine($fields, $values);
    }
    $this->keyFields = $fields;

    return $this;
  }

  /**
   * Specify fields to update in case of a duplicate record.
   *
   * If a record with the values in keys() already exists, the fields and values
   * specified here will be updated in that record.  If this method is not called,
   * it defaults to the same values as were passed to the fields() method.
   *
   * @param $fields
   *   An array of fields to set.
   * @param $values
   *   An array of fields to set into the database.  The values must be
   *   specified in the same order as the $fields array.
   * @return
   *   The called object.
   */
  public function update(Array $fields, Array $values = array()) {
   if ($values) {
      $fields = array_combine($fields, $values);
    }
    $this->updateFields = $fields;

    return $this;
  }

  /**
   * Specify fields that should not be updated in case of a duplicate record.
   *
   * If this method is called and a record with the values in keys() already
   * exists, Drupal will instead update the record with the values passed
   * in the fields() method except for the fields specified in this method. That
   * is, calling this method is equivalent to calling update() with identical
   * parameters as fields() minus the keys specified here.
   *
   * The update() method takes precedent over this method.  If update() is called,
   * this method has no effect.
   *
   * @param $exclude_fields
   *   An array of fields in the query that should not be updated to match those
   *   specified by the fields() method.
   *   Alternatively, the fields may be specified as a variable number of string
   *   parameters.
   * @return
   *   The called object.
   */
  public function updateExcept($exclude_fields) {
    if (!is_array($exclude_fields)) {
      $exclude_fields = func_get_args();
    }
    $this->excludeFields = $exclude_fields;

    return $this;
  }

  /**
   * Specify fields to be updated as an expression.
   *
   * Expression fields are cases such as counter=counter+1.  This method only
   * applies if a duplicate key is detected.  This method takes precedent over
   * both update() and updateExcept().
   *
   * @param $field
   *   The field to set.
   * @param $expression
   *   The field will be set to the value of this expression.  This parameter
   *   may include named placeholders.
   * @param $arguments
   *   If specified, this is an array of key/value pairs for named placeholders
   *   corresponding to the expression.
   * @return
   *   The called object.
   */
  public function expression($field, $expression, Array $arguments = NULL) {
    $this->expressionFields[$field] = array(
      'expression' => $expression,
      'arguments' => $arguments,
    );

    return $this;
  }

  public function execute() {

    // In the degenerate case of this query type, we have to run multiple
    // queries as there is no universal single-query mechanism that will work.
    // Our degenerate case is not designed for performance efficiency but
    // for comprehensibility.  Any practical database driver will override
    // this method with database-specific logic, so this function serves only
    // as a fallback to aid developers of new drivers.

    // Wrap multiple queries in a transaction, if the database supports it.
    $transaction = $this->connection->startTransaction();

    // Manually check if the record already exists.
    $select = $this->connection->select($this->table);
    foreach ($this->keyFields as $field => $value) {
      $select->condition($field, $value);
    }

    $select = $select->countQuery();
    $sql = (string)$select;
    $arguments = $select->getArguments();
    $num_existing = db_query($sql, $arguments)->fetchField();


    if ($num_existing) {
      // If there is already an existing record, run an update query.

      if ($this->updateFields) {
        $update_fields = $this->updateFields;
      }
      else {
        $update_fields = $this->insertFields;
        // If there are no exclude fields, this is a no-op.
        foreach ($this->excludeFields as $exclude_field) {
          unset($update_fields[$exclude_field]);
        }
      }
      $update = $this->connection->update($this->table, $this->queryOptions)->fields($update_fields);
      foreach ($this->keyFields as $field => $value) {
        $update->condition($field, $value);
      }
      foreach ($this->expressionFields as $field => $expression) {
        $update->expression($field, $expression['expression'], $expression['arguments']);
      }
      $update->execute();
    }
    else {
      // If there is no existing record, run an insert query.
      $insert_fields = $this->insertFields + $this->keyFields;
      $this->connection->insert($this->table, $this->queryOptions)->fields($insert_fields)->execute();
    }

    // Commit the transaction.
    $transaction->commit();
  }

  public function __toString() {
    // In the degenerate case, there is no string-able query as this operation
    // is potentially two queries.
    return '';
  }
}


/**
 * General class for an abstracted DELETE operation.
 *
 * The conditional WHERE handling of this class is all inherited from Query.
 */
class DeleteQuery extends Query implements QueryConditionInterface {

  /**
   * The table from which to delete.
   *
   * @var string
   */
  protected $table;

  /**
   * The condition object for this query.  Condition handling is handled via
   * composition.
   *
   * @var DatabaseCondition
   */
  protected $condition;

  public function __construct(DatabaseConnection $connection, $table, Array $options = array()) {
    $options['return'] = Database::RETURN_AFFECTED;
    parent::__construct($connection, $options);
    $this->table = $table;

    $this->condition = new DatabaseCondition('AND');
  }

  public function condition($field, $value = NULL, $operator = '=') {
    if (!isset($num_args)) {
      $num_args = func_num_args();
    }
    $this->condition->condition($field, $value, $operator, $num_args);
    return $this;
  }

  public function &conditions() {
    return $this->condition->conditions();
  }

  public function arguments() {
    return $this->condition->arguments();
  }

  public function where($snippet, $args = array()) {
    $this->condition->where($snippet, $args);
    return $this;
  }

  public function compile(DatabaseConnection $connection) {
    return $this->condition->compile($connection);
  }

  public function execute() {
    $values = array();
    if (count($this->condition)) {
      $this->condition->compile($this->connection);
      $values = $this->condition->arguments();
    }

    return $this->connection->query((string)$this, $values, $this->queryOptions);
  }

  public function __toString() {
    $query = 'DELETE FROM {' . $this->connection->escapeTable($this->table) . '} ';

    if (count($this->condition)) {
      $this->condition->compile($this->connection);
      $query .= "\nWHERE " . $this->condition;
    }

    return $query;
  }
}

/**
 * General class for an abstracted UPDATE operation.
 *
 * The conditional WHERE handling of this class is all inherited from Query.
 */
class UpdateQuery extends Query implements QueryConditionInterface {

  /**
   * The table to update.
   *
   * @var string
   */
  protected $table;

  /**
   * An array of fields that will be updated.
   *
   * @var array
   */
  protected $fields;

  /**
   * An array of values to update to.
   *
   * @var array
   */
  protected $arguments = array();

  /**
   * The condition object for this query.  Condition handling is handled via
   * composition.
   *
   * @var DatabaseCondition
   */
  protected $condition;

  /**
   * An array of fields to update to an expression in case of a duplicate record.
   *
   * This variable is a nested array in the following format:
   * <some field> => array(
   *  'condition' => <condition to execute, as a string>
   *  'arguments' => <array of arguments for condition, or NULL for none>
   * );
   *
   * @var array
   */
  protected $expressionFields = array();


  public function __construct(DatabaseConnection $connection, $table, Array $options = array()) {
    $options['return'] = Database::RETURN_AFFECTED;
    parent::__construct($connection, $options);
    $this->table = $table;

    $this->condition = new DatabaseCondition('AND');
  }

  public function condition($field, $value = NULL, $operator = '=') {
    if (!isset($num_args)) {
      $num_args = func_num_args();
    }
    $this->condition->condition($field, $value, $operator, $num_args);
    return $this;
  }

  public function &conditions() {
    return $this->condition->conditions();
  }

  public function arguments() {
    return $this->condition->arguments();
  }

  public function where($snippet, $args = array()) {
    $this->condition->where($snippet, $args);
    return $this;
  }

  public function compile(DatabaseConnection $connection) {
    return $this->condition->compile($connection);
  }

  /**
   * Add a set of field->value pairs to be updated.
   *
   * @param $fields
   *   An associative array of fields to write into the database.  The array keys
   *   are the field names while the values are the values to which to set them.
   * @return
   *   The called object.
   */
  public function fields(Array $fields) {
    $this->fields = $fields;
    return $this;
  }

  /**
   * Specify fields to be updated as an expression.
   *
   * Expression fields are cases such as counter=counter+1.  This method takes
   * precedence over fields().
   *
   * @param $field
   *   The field to set.
   * @param $expression
   *   The field will be set to the value of this expression.  This parameter
   *   may include named placeholders.
   * @param $arguments
   *   If specified, this is an array of key/value pairs for named placeholders
   *   corresponding to the expression.
   * @return
   *   The called object.
   */
  public function expression($field, $expression, Array $arguments = NULL) {
    $this->expressionFields[$field] = array(
      'expression' => $expression,
      'arguments' => $arguments,
    );

    return $this;
  }

  public function execute() {

    // Expressions take priority over literal fields, so we process those first
    // and remove any literal fields that conflict.
    $fields = $this->fields;
    $update_values = array();
    foreach ($this->expressionFields as $field => $data) {
      if (!empty($data['arguments'])) {
        $update_values += $data['arguments'];
      }
      unset($fields[$field]);
    }

    // Because we filter $fields the same way here and in __toString(), the
    // placeholders will all match up properly.
    $max_placeholder = 0;
    foreach ($fields as $field => $value) {
      $update_values[':db_update_placeholder_' . ($max_placeholder++)] = $value;
    }

    if (count($this->condition)) {
      $this->condition->compile($this->connection);
      $update_values = array_merge($update_values, $this->condition->arguments());
    }

    return $this->connection->query((string)$this, $update_values, $this->queryOptions);
  }

  public function __toString() {
    // Expressions take priority over literal fields, so we process those first
    // and remove any literal fields that conflict.
    $fields = $this->fields;
    $update_fields = array();
    foreach ($this->expressionFields as $field => $data) {
      $update_fields[] = $field . '=' . $data['expression'];
      unset($fields[$field]);
    }

    $max_placeholder = 0;
    foreach ($fields as $field => $value) {
      $update_fields[] = $field . '=:db_update_placeholder_' . ($max_placeholder++);
    }

    $query = 'UPDATE {' . $this->connection->escapeTable($this->table) . '} SET ' . implode(', ', $update_fields);

    if (count($this->condition)) {
      $this->condition->compile($this->connection);
      // There is an implicit string cast on $this->condition.
      $query .= "\nWHERE " . $this->condition;
    }

    return $query;
  }

}

/**
 * Generic class for a series of conditions in a query.
 */
class DatabaseCondition implements QueryConditionInterface, Countable {

  protected $conditions = array();
  protected $arguments = array();

  protected $changed = TRUE;

  public function __construct($conjunction) {
    $this->conditions['#conjunction'] = $conjunction;
  }

  /**
   * Return the size of this conditional.  This is part of the Countable interface.
   *
   * The size of the conditional is the size of its conditional array minus
   * one, because one element is the the conjunction.
   */
  public function count() {
    return count($this->conditions) - 1;
  }

  public function condition($field, $value = NULL, $operator = '=') {
    $this->conditions[] = array(
      'field' => $field,
      'value' => $value,
      'operator' => $operator,
    );

    $this->changed = TRUE;

    return $this;
  }

  public function where($snippet, $args = array()) {
    $this->conditions[] = array(
      'field' => $snippet,
      'value' => $args,
      'operator' => NULL,
    );
    $this->changed = TRUE;

    return $this;
  }

  public function &conditions() {
    return $this->conditions;
  }

  public function arguments() {
    // If the caller forgot to call compile() first, refuse to run.
    if ($this->changed) {
      return NULL;
    }
    return $this->arguments;
  }

  public function compile(DatabaseConnection $connection) {
    // This value is static, so it will increment across the entire request
    // rather than just this query.  That is OK, because we only need definitive
    // placeholder names if we're going to use them for _alter hooks, which we
    // are not.  The alter hook would intervene before compilation.
    static $next_placeholder = 1;

    if ($this->changed) {

      $condition_fragments = array();
      $arguments = array();

      $conditions = $this->conditions;
      $conjunction = $conditions['#conjunction'];
      unset($conditions['#conjunction']);
      foreach ($conditions as $condition) {
        if (empty($condition['operator'])) {
          // This condition is a literal string, so let it through as is.
          $condition_fragments[] = ' (' . $condition['field'] . ') ';
          $arguments += $condition['value'];
        }
        else {
          // It's a structured condition, so parse it out accordingly.
          if ($condition['field'] instanceof QueryConditionInterface) {
            // Compile the sub-condition recursively and add it to the list.
            $condition['field']->compile($connection);
            $condition_fragments[] = '(' . (string)$condition['field'] . ')';
            $arguments += $condition['field']->arguments();
          }
          else {
            // For simplicity, we treat all operators as the same data structure.
            // In the typical degenerate case, this won't get changed.
            $operator_defaults = array(
              'prefix' => '',
              'postfix' => '',
              'delimiter' => '',
              'operator' => $condition['operator'],
            );
            $operator = $connection->mapConditionOperator($condition['operator']);
            if (!isset($operator)) {
              $operator = $this->mapConditionOperator($condition['operator']);
            }
            $operator += $operator_defaults;

            if ($condition['value'] instanceof SelectQuery) {
              $placeholders[] = (string)$condition['value'];
              $arguments += $condition['value']->arguments();
            }
            // We assume that if there is a delimiter, then the value is an
            // array.  If not, it is a scalar.  For simplicity, we first convert
            // up to an array so that we can build the placeholders in the same way.
            elseif (!$operator['delimiter']) {
              $condition['value'] = array($condition['value']);
            }
            $placeholders = array();
            foreach ($condition['value'] as $value) {
              $placeholder = ':db_condition_placeholder_' . $next_placeholder++;
              $arguments[$placeholder] = $value;
              $placeholders[] = $placeholder;
            }
            $condition_fragments[] = ' (' . $condition['field'] . ' ' . $operator['operator'] . ' ' . $operator['prefix'] . implode($operator['delimiter'], $placeholders) . $operator['postfix'] . ') ';

          }
        }
      }

      $this->changed = FALSE;
      $this->stringVersion = implode($conjunction, $condition_fragments);
      $this->arguments = $arguments;
    }
  }

  public function __toString() {
    // If the caller forgot to call compile() first, refuse to run.
    if ($this->changed) {
      return NULL;
    }
    return $this->stringVersion;
  }

  /**
   * Gets any special processing requirements for the condition operator.
   *
   * Some condition types require special processing, such as IN, because
   * the value data they pass in is not a simple value.  This is a simple
   * overridable lookup function.
   *
   * @param $operator
   *   The condition operator, such as "IN", "BETWEEN", etc.  Case-sensitive.
   * @return
   *   The extra handling directives for the specified operator, or NULL.
   */
  protected function mapConditionOperator($operator) {
    static $specials = array(
      'BETWEEN' => array('delimiter' => ' AND '),
      'IN' => array('delimiter' => ', ', 'prefix' => ' (', 'postfix' => ')'),
      'NOT IN' => array('delimiter' => ', ', 'prefix' => ' (', 'postfix' => ')'),
      'LIKE' => array('operator' => 'LIKE'),
    );

    $return = isset($specials[$operator]) ? $specials[$operator] : array();
    $return += array('operator' => $operator);

    return $return;
  }

}

/**
 * Returns a new DatabaseCondition, set to "OR" all conditions together.
 */
function db_or() {
  return new DatabaseCondition('OR');
}

/**
 * Returns a new DatabaseCondition, set to "AND" all conditions together.
 */
function db_and() {
  return new DatabaseCondition('AND');
}

/**
 * Returns a new DatabaseCondition, set to "XOR" all conditions together.
 */
function db_xor() {
  return new DatabaseCondition('XOR');
}

/**
 * Returns a new DatabaseCondition, set to the specified conjunction.
 *
 * @param
 *   The conjunction (AND, OR, XOR, etc.) to use on conditions.
 */
function db_condition($conjunction) {
  return new DatabaseCondition($conjunction);
}

/**
 * @} End of "ingroup database".
 */