Issue #1848266 by alexpott: Convert Diff into a proper, PSR-0-compatible PHP component.

core/core.services.yml

@@ -114,6 +114,9 @@ services:
   cron:
     class: Drupal\Core\Cron
     arguments: ['@module_handler', '@lock', '@queue', '@state', '@current_user', '@session_manager']
+  diff.formatter:
+    class: Drupal\Core\Diff\DiffFormatter
+    arguments: ['@config.factory']
   database:
     class: Drupal\Core\Database\Connection
     factory_class: Drupal\Core\Database\Database

core/lib/Drupal/Component/Diff/Diff.php

@@ -0,0 +1,172 @@
+<?php
+
+namespace Drupal\Component\Diff;
+
+use Drupal\Component\Diff\Engine\DiffEngine;
+
+/**
+ * Class representing a 'diff' between two sequences of strings.
+ * @todo document
+ * @subpackage DifferenceEngine
+ *
+ * Copied from https://drupal.org/project/diff which was based PHP diff engine
+ * for phpwiki. (Taken from phpwiki-1.3.3) The original code in phpwiki was
+ * copyright (C) 2000, 2001 Geoffrey T. Dairiki <dairiki@dairiki.org> and
+ * licensed under GPL.
+ */
+class Diff {
+
+  /**
+   * The list of differences as an array of diff operations.
+   *
+   * @var \Drupal\Component\Diff\Engine\DiffOp[]
+   */
+  protected $edits;
+
+  /**
+   * Constructor.
+   * Computes diff between sequences of strings.
+   *
+   * @param $from_lines array An array of strings.
+   *      (Typically these are lines from a file.)
+   * @param $to_lines array An array of strings.
+   */
+  public function __construct($from_lines, $to_lines) {
+    $eng = new DiffEngine();
+    $this->edits = $eng->diff($from_lines, $to_lines);
+    //$this->_check($from_lines, $to_lines);
+  }
+
+  /**
+   * Compute reversed Diff.
+   *
+   * SYNOPSIS:
+   *
+   *  $diff = new Diff($lines1, $lines2);
+   *  $rev = $diff->reverse();
+   * @return object A Diff object representing the inverse of the
+   *          original diff.
+   */
+  public function reverse() {
+    $rev = $this;
+    $rev->edits = array();
+    foreach ($this->edits as $edit) {
+      $rev->edits[] = $edit->reverse();
+    }
+    return $rev;
+  }
+
+  /**
+   * Check for empty diff.
+   *
+   * @return bool True iff two sequences were identical.
+   */
+  public function isEmpty() {
+    foreach ($this->edits as $edit) {
+      if ($edit->type != 'copy') {
+        return FALSE;
+      }
+    }
+    return TRUE;
+  }
+
+  /**
+   * Compute the length of the Longest Common Subsequence (LCS).
+   *
+   * This is mostly for diagnostic purposed.
+   *
+   * @return int The length of the LCS.
+   */
+  public function lcs() {
+    $lcs = 0;
+    foreach ($this->edits as $edit) {
+      if ($edit->type == 'copy') {
+        $lcs += sizeof($edit->orig);
+      }
+    }
+    return $lcs;
+  }
+
+  /**
+   * Get the original set of lines.
+   *
+   * This reconstructs the $from_lines parameter passed to the
+   * constructor.
+   *
+   * @return array The original sequence of strings.
+   */
+  public function orig() {
+    $lines = array();
+
+    foreach ($this->edits as $edit) {
+      if ($edit->orig) {
+        array_splice($lines, sizeof($lines), 0, $edit->orig);
+      }
+    }
+    return $lines;
+  }
+
+  /**
+   * Get the closing set of lines.
+   *
+   * This reconstructs the $to_lines parameter passed to the
+   * constructor.
+   *
+   * @return array The sequence of strings.
+   */
+  public function closing() {
+    $lines = array();
+
+    foreach ($this->edits as $edit) {
+      if ($edit->closing) {
+        array_splice($lines, sizeof($lines), 0, $edit->closing);
+      }
+    }
+    return $lines;
+  }
+
+  /**
+   * Check a Diff for validity.
+   *
+   * This is here only for debugging purposes.
+   */
+  public function check($from_lines, $to_lines) {
+    if (serialize($from_lines) != serialize($this->orig())) {
+      trigger_error("Reconstructed original doesn't match", E_USER_ERROR);
+    }
+    if (serialize($to_lines) != serialize($this->closing())) {
+      trigger_error("Reconstructed closing doesn't match", E_USER_ERROR);
+    }
+
+    $rev = $this->reverse();
+    if (serialize($to_lines) != serialize($rev->orig())) {
+      trigger_error("Reversed original doesn't match", E_USER_ERROR);
+    }
+    if (serialize($from_lines) != serialize($rev->closing())) {
+      trigger_error("Reversed closing doesn't match", E_USER_ERROR);
+    }
+
+
+    $prevtype = 'none';
+    foreach ($this->edits as $edit) {
+      if ( $prevtype == $edit->type ) {
+        trigger_error("Edit sequence is non-optimal", E_USER_ERROR);
+      }
+      $prevtype = $edit->type;
+    }
+
+    $lcs = $this->lcs();
+    trigger_error('Diff okay: LCS = ' . $lcs, E_USER_NOTICE);
+  }
+
+  /**
+   * Gets the list of differences as an array of diff operations.
+   *
+   * @return \Drupal\Component\Diff\Engine\DiffOp[]
+   *   The list of differences as an array of diff operations.
+   */
+  public function getEdits() {
+    return $this->edits;
+  }
+}
+

core/lib/Drupal/Component/Diff/DiffFormatter.php

@@ -0,0 +1,187 @@
+<?php
+
+namespace Drupal\Component\Diff;
+
+use Drupal\Component\Diff\Engine\DiffOpCopy;
+
+/**
+ * A class to format Diffs
+ *
+ * This class formats the diff in classic diff format.
+ * It is intended that this class be customized via inheritance,
+ * to obtain fancier outputs.
+ * @todo document
+ * @private
+ * @subpackage DifferenceEngine
+ */
+class DiffFormatter {
+  /**
+   * Should a block header be shown?
+   */
+  var $show_header = TRUE;
+
+  /**
+   * Number of leading context "lines" to preserve.
+   *
+   * This should be left at zero for this class, but subclasses
+   * may want to set this to other values.
+   */
+  var $leading_context_lines = 0;
+
+  /**
+   * Number of trailing context "lines" to preserve.
+   *
+   * This should be left at zero for this class, but subclasses
+   * may want to set this to other values.
+   */
+  var $trailing_context_lines = 0;
+
+  /**
+   * Format a diff.
+   *
+   * @param \Drupal\Component\Diff\Diff $diff
+   *   A Diff object.
+   *
+   * @return string
+   *   The formatted output.
+   */
+  public function format(Diff $diff) {
+    $xi = $yi = 1;
+    $block = FALSE;
+    $context = array();
+
+    $nlead = $this->leading_context_lines;
+    $ntrail = $this->trailing_context_lines;
+
+    $this->_start_diff();
+
+    foreach ($diff->getEdits() as $edit) {
+      if ($edit->type == 'copy') {
+        if (is_array($block)) {
+          if (sizeof($edit->orig) <= $nlead + $ntrail) {
+            $block[] = $edit;
+          }
+          else {
+            if ($ntrail) {
+              $context = array_slice($edit->orig, 0, $ntrail);
+              $block[] = new DiffOpCopy($context);
+            }
+            $this->_block($x0, $ntrail + $xi - $x0, $y0, $ntrail + $yi - $y0, $block);
+            $block = FALSE;
+          }
+        }
+        $context = $edit->orig;
+      }
+      else {
+        if (! is_array($block)) {
+          $context = array_slice($context, sizeof($context) - $nlead);
+          $x0 = $xi - sizeof($context);
+          $y0 = $yi - sizeof($context);
+          $block = array();
+          if ($context) {
+            $block[] = new DiffOpCopy($context);
+          }
+        }
+        $block[] = $edit;
+      }
+
+      if ($edit->orig) {
+        $xi += sizeof($edit->orig);
+      }
+      if ($edit->closing) {
+        $yi += sizeof($edit->closing);
+      }
+    }
+
+    if (is_array($block)) {
+      $this->_block($x0, $xi - $x0, $y0, $yi - $y0, $block);
+    }
+    $end = $this->_end_diff();
+
+    if (!empty($xi)) {
+      $this->line_stats['counter']['x'] += $xi;
+    }
+    if (!empty($yi)) {
+      $this->line_stats['counter']['y'] += $yi;
+    }
+
+    return $end;
+  }
+
+  protected function _block($xbeg, $xlen, $ybeg, $ylen, &$edits) {
+    $this->_start_block($this->_block_header($xbeg, $xlen, $ybeg, $ylen));
+    foreach ($edits as $edit) {
+      if ($edit->type == 'copy') {
+        $this->_context($edit->orig);
+      }
+      elseif ($edit->type == 'add') {
+        $this->_added($edit->closing);
+      }
+      elseif ($edit->type == 'delete') {
+        $this->_deleted($edit->orig);
+      }
+      elseif ($edit->type == 'change') {
+        $this->_changed($edit->orig, $edit->closing);
+      }
+      else {
+        trigger_error('Unknown edit type', E_USER_ERROR);
+      }
+    }
+    $this->_end_block();
+  }
+
+  protected function _start_diff() {
+    ob_start();
+  }
+
+  protected function _end_diff() {
+    $val = ob_get_contents();
+    ob_end_clean();
+    return $val;
+  }
+
+  protected function _block_header($xbeg, $xlen, $ybeg, $ylen) {
+    if ($xlen > 1) {
+      $xbeg .= "," . ($xbeg + $xlen - 1);
+    }
+    if ($ylen > 1) {
+      $ybeg .= "," . ($ybeg + $ylen - 1);
+    }
+
+    return $xbeg . ($xlen ? ($ylen ? 'c' : 'd') : 'a') . $ybeg;
+  }
+
+  protected function _start_block($header) {
+    if ($this->show_header) {
+      echo $header . "\n";
+    }
+  }
+
+  protected function _end_block() {
+  }
+
+  protected function _lines($lines, $prefix = ' ') {
+    foreach ($lines as $line) {
+      echo "$prefix $line\n";
+    }
+  }
+
+  protected function _context($lines) {
+    $this->_lines($lines);
+  }
+
+  protected function _added($lines) {
+    $this->_lines($lines, '>');
+  }
+
+  protected function _deleted($lines) {
+    $this->_lines($lines, '<');
+  }
+
+  protected function _changed($orig, $closing) {
+    $this->_deleted($orig);
+    echo "---\n";
+    $this->_added($closing);
+  }
+
+}

core/lib/Drupal/Component/Diff/Engine/DiffEngine.php

@@ -0,0 +1,456 @@
+<?php
+
+namespace Drupal\Component\Diff\Engine;
+
+use Drupal\Component\Utility\Unicode;
+
+/**
+ * Class used internally by Diff to actually compute the diffs.
+ *
+ * The algorithm used here is mostly lifted from the perl module
+ * Algorithm::Diff (version 1.06) by Ned Konz, which is available at:
+ *   http://www.perl.com/CPAN/authors/id/N/NE/NEDKONZ/Algorithm-Diff-1.06.zip
+ *
+ * More ideas are taken from:
+ *   http://www.ics.uci.edu/~eppstein/161/960229.html
+ *
+ * Some ideas are (and a bit of code) are from from analyze.c, from GNU
+ * diffutils-2.7, which can be found at:
+ *   ftp://gnudist.gnu.org/pub/gnu/diffutils/diffutils-2.7.tar.gz
+ *
+ * closingly, some ideas (subdivision by NCHUNKS > 2, and some optimizations)
+ * are my own.
+ *
+ * Line length limits for robustness added by Tim Starling, 2005-08-31
+ *
+ * @author Geoffrey T. Dairiki, Tim Starling
+ * @private
+ * @subpackage DifferenceEngine
+ */
+class DiffEngine {
+
+  const USE_ASSERTS = FALSE;
+
+  const MAX_XREF_LENGTH = 10000;
+
+  public function diff($from_lines, $to_lines) {
+
+    $n_from = sizeof($from_lines);
+    $n_to = sizeof($to_lines);
+
+    $this->xchanged = $this->ychanged = array();
+    $this->xv = $this->yv = array();
+    $this->xind = $this->yind = array();
+    unset($this->seq);
+    unset($this->in_seq);
+    unset($this->lcs);
+
+    // Skip leading common lines.
+    for ($skip = 0; $skip < $n_from && $skip < $n_to; $skip++) {
+      if ($from_lines[$skip] !== $to_lines[$skip]) {
+        break;
+      }
+      $this->xchanged[$skip] = $this->ychanged[$skip] = FALSE;
+    }
+    // Skip trailing common lines.
+    $xi = $n_from;
+    $yi = $n_to;
+    for ($endskip = 0; --$xi > $skip && --$yi > $skip; $endskip++) {
+      if ($from_lines[$xi] !== $to_lines[$yi]) {
+        break;
+      }
+      $this->xchanged[$xi] = $this->ychanged[$yi] = FALSE;
+    }
+
+    // Ignore lines which do not exist in both files.
+    for ($xi = $skip; $xi < $n_from - $endskip; $xi++) {
+      $xhash[$this->_line_hash($from_lines[$xi])] = 1;
+    }
+
+    for ($yi = $skip; $yi < $n_to - $endskip; $yi++) {
+      $line = $to_lines[$yi];
+      if ($this->ychanged[$yi] = empty($xhash[$this->_line_hash($line)])) {
+        continue;
+      }
+      $yhash[$this->_line_hash($line)] = 1;
+      $this->yv[] = $line;
+      $this->yind[] = $yi;
+    }
+    for ($xi = $skip; $xi < $n_from - $endskip; $xi++) {
+      $line = $from_lines[$xi];
+      if ($this->xchanged[$xi] = empty($yhash[$this->_line_hash($line)])) {
+        continue;
+      }
+      $this->xv[] = $line;
+      $this->xind[] = $xi;
+    }
+
+    // Find the LCS.
+    $this->_compareseq(0, sizeof($this->xv), 0, sizeof($this->yv));
+
+    // Merge edits when possible
+    $this->_shift_boundaries($from_lines, $this->xchanged, $this->ychanged);
+    $this->_shift_boundaries($to_lines, $this->ychanged, $this->xchanged);
+
+    // Compute the edit operations.
+    $edits = array();
+    $xi = $yi = 0;
+    while ($xi < $n_from || $yi < $n_to) {
+      $this::USE_ASSERTS && assert($yi < $n_to || $this->xchanged[$xi]);
+      $this::USE_ASSERTS && assert($xi < $n_from || $this->ychanged[$yi]);
+
+      // Skip matching "snake".
+      $copy = array();
+      while ( $xi < $n_from && $yi < $n_to && !$this->xchanged[$xi] && !$this->ychanged[$yi]) {
+        $copy[] = $from_lines[$xi++];
+        ++$yi;
+      }
+      if ($copy) {
+        $edits[] = new DiffOpCopy($copy);
+      }
+      // Find deletes & adds.
+      $delete = array();
+      while ($xi < $n_from && $this->xchanged[$xi]) {
+        $delete[] = $from_lines[$xi++];
+      }
+      $add = array();
+      while ($yi < $n_to && $this->ychanged[$yi]) {
+        $add[] = $to_lines[$yi++];
+      }
+      if ($delete && $add) {
+        $edits[] = new DiffOpChange($delete, $add);
+      }
+      elseif ($delete) {
+        $edits[] = new DiffOpDelete($delete);
+      }
+      elseif ($add) {
+        $edits[] = new DiffOpAdd($add);
+      }
+    }
+    return $edits;
+  }
+
+  /**
+   * Returns the whole line if it's small enough, or the MD5 hash otherwise.
+   */
+  protected function _line_hash($line) {
+    if (Unicode::strlen($line) > $this::MAX_XREF_LENGTH) {
+      return md5($line);
+    }
+    else {
+      return $line;
+    }
+  }
+
+
+  /**
+   * Divide the Largest Common Subsequence (LCS) of the sequences
+   * [XOFF, XLIM) and [YOFF, YLIM) into NCHUNKS approximately equally
+   * sized segments.
+   *
+   * Returns (LCS, PTS).  LCS is the length of the LCS. PTS is an
+   * array of NCHUNKS+1 (X, Y) indexes giving the diving points between
+   * sub sequences.  The first sub-sequence is contained in [X0, X1),
+   * [Y0, Y1), the second in [X1, X2), [Y1, Y2) and so on.  Note
+   * that (X0, Y0) == (XOFF, YOFF) and
+   * (X[NCHUNKS], Y[NCHUNKS]) == (XLIM, YLIM).
+   *
+   * This function assumes that the first lines of the specified portions
+   * of the two files do not match, and likewise that the last lines do not
+   * match.  The caller must trim matching lines from the beginning and end
+   * of the portions it is going to specify.
+   */
+  protected function _diag($xoff, $xlim, $yoff, $ylim, $nchunks) {
+    $flip = FALSE;
+
+    if ($xlim - $xoff > $ylim - $yoff) {
+      // Things seems faster (I'm not sure I understand why)
+      // when the shortest sequence in X.
+      $flip = TRUE;
+      list($xoff, $xlim, $yoff, $ylim) = array($yoff, $ylim, $xoff, $xlim);
+    }
+
+    if ($flip) {
+      for ($i = $ylim - 1; $i >= $yoff; $i--) {
+        $ymatches[$this->xv[$i]][] = $i;
+      }
+    }
+    else {
+      for ($i = $ylim - 1; $i >= $yoff; $i--) {
+        $ymatches[$this->yv[$i]][] = $i;
+      }
+    }
+    $this->lcs = 0;
+    $this->seq[0]= $yoff - 1;
+    $this->in_seq = array();
+    $ymids[0] = array();
+
+    $numer = $xlim - $xoff + $nchunks - 1;
+    $x = $xoff;
+    for ($chunk = 0; $chunk < $nchunks; $chunk++) {
+      if ($chunk > 0) {
+        for ($i = 0; $i <= $this->lcs; $i++) {
+          $ymids[$i][$chunk-1] = $this->seq[$i];
+        }
+      }
+
+      $x1 = $xoff + (int)(($numer + ($xlim-$xoff)*$chunk) / $nchunks);
+      for ( ; $x < $x1; $x++) {
+        $line = $flip ? $this->yv[$x] : $this->xv[$x];
+        if (empty($ymatches[$line])) {
+          continue;
+        }
+        $matches = $ymatches[$line];
+        reset($matches);
+        while (list ($junk, $y) = each($matches)) {
+          if (empty($this->in_seq[$y])) {
+            $k = $this->_lcs_pos($y);
+            $this::USE_ASSERTS && assert($k > 0);
+            $ymids[$k] = $ymids[$k-1];
+            break;
+          }
+        }
+        while (list ($junk, $y) = each($matches)) {
+          if ($y > $this->seq[$k-1]) {
+            $this::USE_ASSERTS && assert($y < $this->seq[$k]);
+            // Optimization: this is a common case:
+            // next match is just replacing previous match.
+            $this->in_seq[$this->seq[$k]] = FALSE;
+            $this->seq[$k] = $y;
+            $this->in_seq[$y] = 1;
+          }
+          elseif (empty($this->in_seq[$y])) {
+            $k = $this->_lcs_pos($y);
+            $this::USE_ASSERTS && assert($k > 0);
+            $ymids[$k] = $ymids[$k-1];
+          }
+        }
+      }
+    }
+
+    $seps[] = $flip ? array($yoff, $xoff) : array($xoff, $yoff);
+    $ymid = $ymids[$this->lcs];
+    for ($n = 0; $n < $nchunks - 1; $n++) {
+      $x1 = $xoff + (int)(($numer + ($xlim - $xoff) * $n) / $nchunks);
+      $y1 = $ymid[$n] + 1;
+      $seps[] = $flip ? array($y1, $x1) : array($x1, $y1);
+    }
+    $seps[] = $flip ? array($ylim, $xlim) : array($xlim, $ylim);
+
+    return array($this->lcs, $seps);
+  }
+
+  protected function _lcs_pos($ypos) {
+
+    $end = $this->lcs;
+    if ($end == 0 || $ypos > $this->seq[$end]) {
+      $this->seq[++$this->lcs] = $ypos;
+      $this->in_seq[$ypos] = 1;
+      return $this->lcs;
+    }
+
+    $beg = 1;
+    while ($beg < $end) {
+      $mid = (int)(($beg + $end) / 2);
+      if ($ypos > $this->seq[$mid]) {
+        $beg = $mid + 1;
+      }
+      else {
+        $end = $mid;
+      }
+    }
+
+    $this::USE_ASSERTS && assert($ypos != $this->seq[$end]);
+
+    $this->in_seq[$this->seq[$end]] = FALSE;
+    $this->seq[$end] = $ypos;
+    $this->in_seq[$ypos] = 1;
+    return $end;
+  }
+
+  /**
+   * Find LCS of two sequences.
+   *
+   * The results are recorded in the vectors $this->{x,y}changed[], by
+   * storing a 1 in the element for each line that is an insertion
+   * or deletion (ie. is not in the LCS).
+   *
+   * The subsequence of file 0 is [XOFF, XLIM) and likewise for file 1.
+   *
+   * Note that XLIM, YLIM are exclusive bounds.
+   * All line numbers are origin-0 and discarded lines are not counted.
+   */
+  protected function _compareseq($xoff, $xlim, $yoff, $ylim) {
+
+    // Slide down the bottom initial diagonal.
+    while ($xoff < $xlim && $yoff < $ylim && $this->xv[$xoff] == $this->yv[$yoff]) {
+      ++$xoff;
+      ++$yoff;
+    }
+
+    // Slide up the top initial diagonal.
+    while ($xlim > $xoff && $ylim > $yoff && $this->xv[$xlim - 1] == $this->yv[$ylim - 1]) {
+      --$xlim;
+      --$ylim;
+    }
+
+    if ($xoff == $xlim || $yoff == $ylim) {
+      $lcs = 0;
+    }
+    else {
+      // This is ad hoc but seems to work well.
+      //$nchunks = sqrt(min($xlim - $xoff, $ylim - $yoff) / 2.5);
+      //$nchunks = max(2, min(8, (int)$nchunks));
+      $nchunks = min(7, $xlim - $xoff, $ylim - $yoff) + 1;
+      list($lcs, $seps)
+      = $this->_diag($xoff, $xlim, $yoff, $ylim, $nchunks);
+    }
+
+    if ($lcs == 0) {
+      // X and Y sequences have no common subsequence:
+      // mark all changed.
+      while ($yoff < $ylim) {
+        $this->ychanged[$this->yind[$yoff++]] = 1;
+      }
+      while ($xoff < $xlim) {
+        $this->xchanged[$this->xind[$xoff++]] = 1;
+      }
+    }
+    else {
+      // Use the partitions to split this problem into subproblems.
+      reset($seps);
+      $pt1 = $seps[0];
+      while ($pt2 = next($seps)) {
+        $this->_compareseq ($pt1[0], $pt2[0], $pt1[1], $pt2[1]);
+        $pt1 = $pt2;
+      }
+    }
+  }
+
+  /**
+   * Adjust inserts/deletes of identical lines to join changes
+   * as much as possible.
+   *
+   * We do something when a run of changed lines include a
+   * line at one end and has an excluded, identical line at the other.
+   * We are free to choose which identical line is included.
+   * `compareseq' usually chooses the one at the beginning,
+   * but usually it is cleaner to consider the following identical line
+   * to be the "change".
+   *
+   * This is extracted verbatim from analyze.c (GNU diffutils-2.7).
+   */
+  protected function _shift_boundaries($lines, &$changed, $other_changed) {
+    $i = 0;
+    $j = 0;
+
+    $this::USE_ASSERTS && assert('sizeof($lines) == sizeof($changed)');
+    $len = sizeof($lines);
+    $other_len = sizeof($other_changed);
+
+    while (1) {
+      /*
+       * Scan forwards to find beginning of another run of changes.
+       * Also keep track of the corresponding point in the other file.
+       *
+       * Throughout this code, $i and $j are adjusted together so that
+       * the first $i elements of $changed and the first $j elements
+       * of $other_changed both contain the same number of zeros
+       * (unchanged lines).
+       * Furthermore, $j is always kept so that $j == $other_len or
+       * $other_changed[$j] == FALSE.
+       */
+      while ($j < $other_len && $other_changed[$j]) {
+        $j++;
+      }
+      while ($i < $len && ! $changed[$i]) {
+        $this::USE_ASSERTS && assert('$j < $other_len && ! $other_changed[$j]');
+        $i++;
+        $j++;
+        while ($j < $other_len && $other_changed[$j]) {
+          $j++;
+        }
+      }
+
+      if ($i == $len) {
+        break;
+      }
+      $start = $i;
+
+      // Find the end of this run of changes.
+      while (++$i < $len && $changed[$i]) {
+        continue;
+      }
+
+      do {
+        /*
+         * Record the length of this run of changes, so that
+         * we can later determine whether the run has grown.
+         */
+        $runlength = $i - $start;
+
+        /*
+         * Move the changed region back, so long as the
+         * previous unchanged line matches the last changed one.
+         * This merges with previous changed regions.
+         */
+        while ($start > 0 && $lines[$start - 1] == $lines[$i - 1]) {
+          $changed[--$start] = 1;
+          $changed[--$i] = FALSE;
+          while ($start > 0 && $changed[$start - 1]) {
+            $start--;
+          }
+          $this::USE_ASSERTS && assert('$j > 0');
+          while ($other_changed[--$j]) {
+            continue;
+          }
+          $this::USE_ASSERTS && assert('$j >= 0 && !$other_changed[$j]');
+        }
+
+        /*
+         * Set CORRESPONDING to the end of the changed run, at the last
+         * point where it corresponds to a changed run in the other file.
+         * CORRESPONDING == LEN means no such point has been found.
+         */
+        $corresponding = $j < $other_len ? $i : $len;
+
+        /*
+         * Move the changed region forward, so long as the
+         * first changed line matches the following unchanged one.
+         * This merges with following changed regions.
+         * Do this second, so that if there are no merges,
+         * the changed region is moved forward as far as possible.
+         */
+        while ($i < $len && $lines[$start] == $lines[$i]) {
+          $changed[$start++] = FALSE;
+          $changed[$i++] = 1;
+          while ($i < $len && $changed[$i]) {
+            $i++;
+          }
+          $this::USE_ASSERTS && assert('$j < $other_len && ! $other_changed[$j]');
+          $j++;
+          if ($j < $other_len && $other_changed[$j]) {
+            $corresponding = $i;
+            while ($j < $other_len && $other_changed[$j]) {
+              $j++;
+            }
+          }
+        }
+      } while ($runlength != $i - $start);
+
+      /*
+       * If possible, move the fully-merged run of changes
+       * back to a corresponding run in the other file.
+       */
+      while ($corresponding < $i) {
+        $changed[--$start] = 1;
+        $changed[--$i] = 0;
+        $this::USE_ASSERTS && assert('$j > 0');
+        while ($other_changed[--$j]) {
+          continue;
+        }
+        $this::USE_ASSERTS && assert('$j >= 0 && !$other_changed[$j]');
+      }
+    }
+  }
+}

core/lib/Drupal/Component/Diff/Engine/DiffOp.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace Drupal\Component\Diff\Engine;
+
+/**
+ * @todo document
+ * @private
+ * @subpackage DifferenceEngine
+ */
+class DiffOp {
+  var $type;
+  var $orig;
+  var $closing;
+
+  public function reverse() {
+    trigger_error('pure virtual', E_USER_ERROR);
+  }
+
+  public function norig() {
+    return $this->orig ? sizeof($this->orig) : 0;
+  }
+
+  public function nclosing() {
+    return $this->closing ? sizeof($this->closing) : 0;
+  }
+}

core/lib/Drupal/Component/Diff/Engine/DiffOpAdd.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Drupal\Component\Diff\Engine;
+
+/**
+ * @todo document
+ * @private
+ * @subpackage DifferenceEngine
+ */
+class DiffOpAdd extends DiffOp {
+  var $type = 'add';
+
+  public function __construct($lines) {
+    $this->closing = $lines;
+    $this->orig = FALSE;
+  }
+
+  public function reverse() {
+    return new DiffOpDelete($this->closing);
+  }
+}

core/lib/Drupal/Component/Diff/Engine/DiffOpChange.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Drupal\Component\Diff\Engine;
+
+/**
+ * @todo document
+ * @private
+ * @subpackage DifferenceEngine
+ */
+class DiffOpChange extends DiffOp {
+  var $type = 'change';
+
+  public function __construct($orig, $closing) {
+    $this->orig = $orig;
+    $this->closing = $closing;
+  }
+
+  public function reverse() {
+    return new DiffOpChange($this->closing, $this->orig);
+  }
+}

core/lib/Drupal/Component/Diff/Engine/DiffOpCopy.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace Drupal\Component\Diff\Engine;
+
+/**
+ * @todo document
+ * @private
+ * @subpackage DifferenceEngine
+ */
+class DiffOpCopy extends DiffOp {
+  var $type = 'copy';
+
+  public function __construct($orig, $closing = FALSE) {
+    if (!is_array($closing)) {
+      $closing = $orig;
+    }
+    $this->orig = $orig;
+    $this->closing = $closing;
+  }
+
+  public function reverse() {
+    return new DiffOpCopy($this->closing, $this->orig);
+  }
+}

core/lib/Drupal/Component/Diff/Engine/DiffOpDelete.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Drupal\Component\Diff\Engine;
+
+/**
+ * @todo document
+ * @private
+ * @subpackage DifferenceEngine
+ */
+class DiffOpDelete extends DiffOp {
+  var $type = 'delete';
+
+  public function __construct($lines) {
+    $this->orig = $lines;
+    $this->closing = FALSE;
+  }
+
+  public function reverse() {
+    return new DiffOpAdd($this->orig);
+  }
+}

core/lib/Drupal/Component/Diff/Engine/HWLDFWordAccumulator.php

@@ -0,0 +1,80 @@
+<?php
+
+namespace Drupal\Component\Diff\Engine;
+
+use Drupal\Component\Utility\String;
+use Drupal\Component\Utility\Unicode;
+
+/**
+ *  Additions by Axel Boldt follow, partly taken from diff.php, phpwiki-1.3.3
+ *
+ */
+
+/**
+ * @todo document
+ * @private
+ * @subpackage DifferenceEngine
+ */
+class HWLDFWordAccumulator {
+
+  /**
+   * An iso-8859-x non-breaking space.
+   */
+  const NBSP = '&#160;';
+
+  protected $lines = array();
+
+  protected $line = '';
+
+  protected $group = '';
+
+  protected $tag = '';
+
+  protected function _flushGroup($new_tag) {
+    if ($this->group !== '') {
+      if ($this->tag == 'mark') {
+        $this->line .= '<span class="diffchange">' . String::checkPlain($this->group) . '</span>';
+      }
+      else {
+        $this->line .= String::checkPlain($this->group);
+      }
+    }
+    $this->group = '';
+    $this->tag = $new_tag;
+  }
+
+  protected function _flushLine($new_tag) {
+    $this->_flushGroup($new_tag);
+    if ($this->line != '') {
+      array_push($this->lines, $this->line);
+    }
+    else {
+      // make empty lines visible by inserting an NBSP
+      array_push($this->lines, $this::NBSP);
+    }
+    $this->line = '';
+  }
+
+  public function addWords($words, $tag = '') {
+    if ($tag != $this->tag) {
+      $this->_flushGroup($tag);
+    }
+    foreach ($words as $word) {
+      // new-line should only come as first char of word.
+      if ($word == '') {
+        continue;
+      }
+      if ($word[0] == "\n") {
+        $this->_flushLine($tag);
+        $word = Unicode::substr($word, 1);
+      }
+      assert(!strstr($word, "\n"));
+      $this->group .= $word;
+    }
+  }
+
+  public function getLines() {
+    $this->_flushLine('~done');
+    return $this->lines;
+  }
+}

core/lib/Drupal/Component/Diff/MappedDiff.php

@@ -0,0 +1,59 @@
+<?php
+
+namespace Drupal\Component\Diff;
+
+/**
+ * FIXME: bad name.
+ * @todo document
+ * @private
+ * @subpackage DifferenceEngine
+ */
+class MappedDiff extends Diff {
+
+  /**
+   * Constructor.
+   *
+   * Computes diff between sequences of strings.
+   *
+   * This can be used to compute things like
+   * case-insensitive diffs, or diffs which ignore
+   * changes in white-space.
+   *
+   * @param $from_lines array An array of strings.
+   *  (Typically these are lines from a file.)
+   *
+   * @param $to_lines array An array of strings.
+   *
+   * @param $mapped_from_lines array This array should
+   *  have the same size number of elements as $from_lines.
+   *  The elements in $mapped_from_lines and
+   *  $mapped_to_lines are what is actually compared
+   *  when computing the diff.
+   *
+   * @param $mapped_to_lines array This array should
+   *  have the same number of elements as $to_lines.
+   */
+  public function __construct($from_lines, $to_lines, $mapped_from_lines, $mapped_to_lines) {
+
+    assert(sizeof($from_lines) == sizeof($mapped_from_lines));
+    assert(sizeof($to_lines) == sizeof($mapped_to_lines));
+
+    parent::__construct($mapped_from_lines, $mapped_to_lines);
+
+    $xi = $yi = 0;
+    for ($i = 0; $i < sizeof($this->edits); $i++) {
+      $orig = &$this->edits[$i]->orig;
+      if (is_array($orig)) {
+        $orig = array_slice($from_lines, $xi, sizeof($orig));
+        $xi += sizeof($orig);
+      }
+
+      $closing = &$this->edits[$i]->closing;
+      if (is_array($closing)) {
+        $closing = array_slice($to_lines, $yi, sizeof($closing));
+        $yi += sizeof($closing);
+      }
+    }
+  }
+}
+

core/lib/Drupal/Component/Diff/WordLevelDiff.php

@@ -0,0 +1,81 @@
+<?php
+
+namespace Drupal\Component\Diff;
+
+use Drupal\Component\Diff\Engine\HWLDFWordAccumulator;
+use Drupal\Component\Utility\Unicode;
+
+/**
+ * @todo document
+ * @private
+ * @subpackage DifferenceEngine
+ */
+class WordLevelDiff extends MappedDiff {
+
+  const MAX_LINE_LENGTH = 10000;
+
+  public function __construct($orig_lines, $closing_lines) {
+    list($orig_words, $orig_stripped) = $this->_split($orig_lines);
+    list($closing_words, $closing_stripped) = $this->_split($closing_lines);
+
+    parent::__construct($orig_words, $closing_words, $orig_stripped, $closing_stripped);
+  }
+
+  protected function _split($lines) {
+    $words = array();
+    $stripped = array();
+    $first = TRUE;
+    foreach ($lines as $line) {
+      // If the line is too long, just pretend the entire line is one big word
+      // This prevents resource exhaustion problems
+      if ( $first ) {
+        $first = FALSE;
+      }
+      else {
+        $words[] = "\n";
+        $stripped[] = "\n";
+      }
+      if (Unicode::strlen($line) > $this::MAX_LINE_LENGTH) {
+        $words[] = $line;
+        $stripped[] = $line;
+      }
+      else {
+        if (preg_match_all('/ ( [^\S\n]+ | [0-9_A-Za-z\x80-\xff]+ | . ) (?: (?!< \n) [^\S\n])? /xs', $line, $m)) {
+          $words = array_merge($words, $m[0]);
+          $stripped = array_merge($stripped, $m[1]);
+        }
+      }
+    }
+    return array($words, $stripped);
+  }
+
+  public function orig() {
+    $orig = new HWLDFWordAccumulator();
+
+    foreach ($this->edits as $edit) {
+      if ($edit->type == 'copy') {
+        $orig->addWords($edit->orig);
+      }
+      elseif ($edit->orig) {
+        $orig->addWords($edit->orig, 'mark');
+      }
+    }
+    $lines = $orig->getLines();
+    return $lines;
+  }
+
+  public function closing() {
+    $closing = new HWLDFWordAccumulator();
+
+    foreach ($this->edits as $edit) {
+      if ($edit->type == 'copy') {
+        $closing->addWords($edit->closing);
+      }
+      elseif ($edit->closing) {
+        $closing->addWords($edit->closing, 'mark');
+      }
+    }
+    $lines = $closing->getLines();
+    return $lines;
+  }
+}

core/lib/Drupal/Core/Config/ConfigManager.php

@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\Config;
 
+use Drupal\Component\Diff\Diff;
 use Drupal\Component\Serialization\Yaml;
 use Drupal\Core\Config\Entity\ConfigDependencyManager;
 use Drupal\Core\Entity\EntityManagerInterface;
@@ -130,10 +131,6 @@ class ConfigManager implements ConfigManagerInterface {
     if (!isset($target_name)) {
       $target_name = $source_name;
     }
-    // @todo Replace with code that can be autoloaded.
-    //   https://drupal.org/node/1848266
-    require_once __DIR__ . '/../../Component/Diff/DiffEngine.php';
-
     // The output should show configuration object differences formatted as YAML.
     // But the configuration is not necessarily stored in files. Therefore, they
     // need to be read and parsed, and lastly, dumped into YAML strings.
@@ -150,7 +147,7 @@ class ConfigManager implements ConfigManagerInterface {
       $target_data = array($this->t('File removed'));
     }
 
-    return new \Diff($source_data, $target_data);
+    return new Diff($source_data, $target_data);
   }
 
   /**

core/lib/Drupal/Core/Diff/DiffFormatter.php

@@ -0,0 +1,217 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Diff\DiffFormatter.
+ */
+
+namespace Drupal\Core\Diff;
+
+use Drupal\Component\Diff\DiffFormatter as DiffFormatterBase;
+use Drupal\Component\Diff\WordLevelDiff;
+use Drupal\Component\Utility\String;
+use Drupal\Core\Config\ConfigFactoryInterface;
+
+/**
+ * Diff formatter which uses returns output that can be rendered to a table.
+ */
+class DiffFormatter extends DiffFormatterBase {
+
+  /**
+   * The diff represented as an array of rows.
+   *
+   * @var array
+   */
+  protected $rows = array();
+
+  /**
+   * The line stats.
+   *
+   * @var array
+   */
+  protected $line_stats = array(
+    'counter' => array('x' => 0, 'y' => 0),
+    'offset' => array('x' => 0, 'y' => 0),
+  );
+
+  /**
+   * Creates a DiffFormatter to render diffs in a table.
+   *
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The config factory.
+   */
+  public function __construct(ConfigFactoryInterface $config_factory) {
+    $config = $config_factory->get('system.diff');
+    $this->leading_context_lines = $config->get('context.lines_leading');
+    $this->trailing_context_lines = $config->get('context.lines_trailing');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function _start_diff() {
+    $this->rows = array();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function _end_diff() {
+    return $this->rows;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function _block_header($xbeg, $xlen, $ybeg, $ylen) {
+    return array(
+      array(
+        'data' => $xbeg + $this->line_stats['offset']['x'],
+        'colspan' => 2,
+      ),
+      array(
+        'data' => $ybeg + $this->line_stats['offset']['y'],
+        'colspan' => 2,
+      )
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function _start_block($header) {
+    if ($this->show_header) {
+      $this->rows[] = $header;
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function _lines($lines, $prefix=' ', $color='white') {
+  }
+
+  /**
+   * Creates an added line.
+   *
+   * @param string $line
+   *   An HTML-escaped line.
+   *
+   * @return array
+   *   An array representing a table row.
+   */
+  protected function addedLine($line) {
+    return array(
+      array(
+        'data' => '+',
+        'class' => 'diff-marker',
+      ),
+      array(
+        'data' => $line,
+        'class' => 'diff-context diff-addedline',
+      )
+    );
+  }
+
+  /**
+   * Creates a deleted line.
+   *
+   * @param string $line
+   *   An HTML-escaped line.
+   *
+   * @return array
+   *   An array representing a table row.
+   */
+  protected function deletedLine($line) {
+    return array(
+      array(
+        'data' => '-',
+        'class' => 'diff-marker',
+      ),
+      array(
+        'data' => $line,
+        'class' => 'diff-context diff-deletedline',
+      )
+    );
+  }
+
+  /**
+   * Creates a context line.
+   *
+   * @param string $line
+   *   An HTML-escaped line.
+   *
+   * @return array
+   *   An array representing a table row.
+   */
+  protected function contextLine($line) {
+    return array(
+      '&nbsp;',
+      array(
+        'data' => $line,
+        'class' => 'diff-context',
+      )
+    );
+  }
+
+  /**
+   * Creates an empty line.
+   *
+   * @return array
+   *   An array representing a table row.
+   */
+  protected function emptyLine() {
+    return array(
+      '&nbsp;',
+      '&nbsp;',
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function _added($lines) {
+    foreach ($lines as $line) {
+      $this->rows[] = array_merge($this->emptyLine(), $this->addedLine(String::checkPlain($line)));
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function _deleted($lines) {
+    foreach ($lines as $line) {
+      $this->rows[] = array_merge($this->deletedLine(String::checkPlain($line)), $this->emptyLine());
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function _context($lines) {
+    foreach ($lines as $line) {
+      $this->rows[] = array_merge($this->contextLine(String::checkPlain($line)), $this->contextLine(String::checkPlain($line)));
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function _changed($orig, $closing) {
+    $diff = new WordLevelDiff($orig, $closing);
+    $del = $diff->orig();
+    $add = $diff->closing();
+
+    // Notice that WordLevelDiff returns HTML-escaped output. Hence, we will be
+    // calling addedLine/deletedLine without HTML-escaping.
+    while ($line = array_shift($del)) {
+      $aline = array_shift( $add );
+      $this->rows[] = array_merge($this->deletedLine($line), isset($aline) ? $this->addedLine($aline) : $this->emptyLine());
+    }
+
+    // If any leftovers.
+    foreach ($add as $line) {
+      $this->rows[] = array_merge($this->emptyLine(), $this->addedLine($line));
+    }
+  }
+}

core/modules/config/src/Controller/ConfigController.php

@@ -12,6 +12,7 @@ use Drupal\Component\Serialization\Yaml;
 use Drupal\Core\Config\ConfigManagerInterface;
 use Drupal\Core\Config\StorageInterface;
 use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\Core\Diff\DiffFormatter;
 use Drupal\system\FileDownloadController;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\HttpFoundation\Request;
@@ -49,6 +50,13 @@ class ConfigController implements ContainerInjectionInterface {
    */
   protected $fileDownloadController;
 
+  /**
+   * The diff formatter.
+   *
+   * @var \Drupal\Core\Diff\DiffFormatter
+   */
+  protected $diffFormatter;
+
   /**
    * {@inheritdoc}
    */
@@ -57,7 +65,8 @@ class ConfigController implements ContainerInjectionInterface {
       $container->get('config.storage'),
       $container->get('config.storage.staging'),
       $container->get('config.manager'),
-      new FileDownloadController()
+      new FileDownloadController(),
+      $container->get('diff.formatter')
     );
   }
 
@@ -71,11 +80,12 @@ class ConfigController implements ContainerInjectionInterface {
    * @param \Drupal\system\FileDownloadController $file_download_controller
    *   The file download controller.
    */
-  public function __construct(StorageInterface $target_storage, StorageInterface $source_storage, ConfigManagerInterface $config_manager, FileDownloadController $file_download_controller) {
+  public function __construct(StorageInterface $target_storage, StorageInterface $source_storage, ConfigManagerInterface $config_manager, FileDownloadController $file_download_controller, DiffFormatter $diff_formatter) {
     $this->targetStorage = $target_storage;
     $this->sourceStorage = $source_storage;
     $this->configManager = $config_manager;
     $this->fileDownloadController = $file_download_controller;
+    $this->diffFormatter = $diff_formatter;
   }
 
   /**
@@ -102,7 +112,7 @@ class ConfigController implements ContainerInjectionInterface {
   }
 
   /**
-   * Shows diff of specificed configuration file.
+   * Shows diff of specified configuration file.
    *
    * @param string $source_name
    *   The name of the configuration file.
@@ -121,8 +131,7 @@ class ConfigController implements ContainerInjectionInterface {
       $collection = StorageInterface::DEFAULT_COLLECTION;
     }
     $diff = $this->configManager->diff($this->targetStorage, $this->sourceStorage, $source_name, $target_name, $collection);
-    $formatter = new \DrupalDiffFormatter();
-    $formatter->show_header = FALSE;
+    $this->diffFormatter->show_header = FALSE;
 
     $build = array();
 
@@ -136,7 +145,7 @@ class ConfigController implements ContainerInjectionInterface {
         array('data' => t('Old'), 'colspan' => '2'),
         array('data' => t('New'), 'colspan' => '2'),
       ),
-      '#rows' => $formatter->format($diff),
+      '#rows' => $this->diffFormatter->format($diff),
     );
 
     $build['back'] = array(

core/modules/config/src/Tests/ConfigDiffTest.php

@@ -57,9 +57,10 @@ class ConfigDiffTest extends DrupalUnitTestBase {
 
     // Verify that the diff reflects a change.
     $diff = \Drupal::service('config.manager')->diff($active, $staging, $config_name);
-    $this->assertEqual($diff->edits[0]->type, 'change', 'The first item in the diff is a change.');
-    $this->assertEqual($diff->edits[0]->orig[0], $change_key . ': ' . $original_data[$change_key], format_string("The active value for key '%change_key' is '%original_data'.", array('%change_key' => $change_key, '%original_data' => $original_data[$change_key])));
-    $this->assertEqual($diff->edits[0]->closing[0], $change_key . ': ' . $change_data, format_string("The staging value for key '%change_key' is '%change_data'.", array('%change_key' => $change_key, '%change_data' => $change_data)));
+    $edits = $diff->getEdits();
+    $this->assertEqual($edits[0]->type, 'change', 'The first item in the diff is a change.');
+    $this->assertEqual($edits[0]->orig[0], $change_key . ': ' . $original_data[$change_key], format_string("The active value for key '%change_key' is '%original_data'.", array('%change_key' => $change_key, '%original_data' => $original_data[$change_key])));
+    $this->assertEqual($edits[0]->closing[0], $change_key . ': ' . $change_data, format_string("The staging value for key '%change_key' is '%change_data'.", array('%change_key' => $change_key, '%change_data' => $change_data)));
 
     // Reset data back to original, and remove a key
     $staging_data = $original_data;
@@ -68,10 +69,11 @@ class ConfigDiffTest extends DrupalUnitTestBase {
 
     // Verify that the diff reflects a removed key.
     $diff = \Drupal::service('config.manager')->diff($active, $staging, $config_name);
-    $this->assertEqual($diff->edits[0]->type, 'copy', 'The first item in the diff is a copy.');
-    $this->assertEqual($diff->edits[1]->type, 'delete', 'The second item in the diff is a delete.');
-    $this->assertEqual($diff->edits[1]->orig[0], $remove_key . ': ' . $original_data[$remove_key], format_string("The active value for key '%remove_key' is '%original_data'.", array('%remove_key' => $remove_key, '%original_data' => $original_data[$remove_key])));
-    $this->assertFalse($diff->edits[1]->closing, format_string("The key '%remove_key' does not exist in staging.", array('%remove_key' => $remove_key)));
+    $edits = $diff->getEdits();
+    $this->assertEqual($edits[0]->type, 'copy', 'The first item in the diff is a copy.');
+    $this->assertEqual($edits[1]->type, 'delete', 'The second item in the diff is a delete.');
+    $this->assertEqual($edits[1]->orig[0], $remove_key . ': ' . $original_data[$remove_key], format_string("The active value for key '%remove_key' is '%original_data'.", array('%remove_key' => $remove_key, '%original_data' => $original_data[$remove_key])));
+    $this->assertFalse($edits[1]->closing, format_string("The key '%remove_key' does not exist in staging.", array('%remove_key' => $remove_key)));
 
     // Reset data back to original and add a key
     $staging_data = $original_data;
@@ -80,10 +82,11 @@ class ConfigDiffTest extends DrupalUnitTestBase {
 
     // Verify that the diff reflects an added key.
     $diff = \Drupal::service('config.manager')->diff($active, $staging, $config_name);
-    $this->assertEqual($diff->edits[0]->type, 'copy', 'The first item in the diff is a copy.');
-    $this->assertEqual($diff->edits[1]->type, 'add', 'The second item in the diff is an add.');
-    $this->assertFalse($diff->edits[1]->orig, format_string("The key '%add_key' does not exist in active.", array('%add_key' => $add_key)));
-    $this->assertEqual($diff->edits[1]->closing[0], $add_key . ': ' . $add_data, format_string("The staging value for key '%add_key' is '%add_data'.", array('%add_key' => $add_key, '%add_data' => $add_data)));
+    $edits = $diff->getEdits();
+    $this->assertEqual($edits[0]->type, 'copy', 'The first item in the diff is a copy.');
+    $this->assertEqual($edits[1]->type, 'add', 'The second item in the diff is an add.');
+    $this->assertFalse($edits[1]->orig, format_string("The key '%add_key' does not exist in active.", array('%add_key' => $add_key)));
+    $this->assertEqual($edits[1]->closing[0], $add_key . ': ' . $add_data, format_string("The staging value for key '%add_key' is '%add_data'.", array('%add_key' => $add_key, '%add_data' => $add_data)));
 
     // Test diffing a renamed config entity.
     $test_entity_id = $this->randomName();
@@ -97,8 +100,9 @@ class ConfigDiffTest extends DrupalUnitTestBase {
     $config_name = 'config_test.dynamic.' . $test_entity_id;
     $diff = \Drupal::service('config.manager')->diff($active, $staging, $config_name, $config_name);
     // Prove the fields match.
-    $this->assertEqual($diff->edits[0]->type, 'copy',  'The first item in the diff is a copy.');
-    $this->assertEqual(count($diff->edits), 1, 'There is one item in the diff');
+    $edits = $diff->getEdits();
+    $this->assertEqual($edits[0]->type, 'copy',  'The first item in the diff is a copy.');
+    $this->assertEqual(count($edits), 1, 'There is one item in the diff');
 
     // Rename the entity.
     $new_test_entity_id = $this->randomName();
@@ -106,11 +110,12 @@ class ConfigDiffTest extends DrupalUnitTestBase {
     $test_entity->save();
 
     $diff = \Drupal::service('config.manager')->diff($active, $staging, 'config_test.dynamic.' . $new_test_entity_id, $config_name);
-    $this->assertEqual($diff->edits[0]->type, 'change',  'The second item in the diff is a copy.');
-    $this->assertEqual($diff->edits[0]->orig, array('id: ' . $new_test_entity_id));
-    $this->assertEqual($diff->edits[0]->closing, array('id: ' . $test_entity_id));
-    $this->assertEqual($diff->edits[1]->type, 'copy',  'The second item in the diff is a copy.');
-    $this->assertEqual(count($diff->edits), 2, 'There are two items in the diff.');
+    $edits = $diff->getEdits();
+    $this->assertEqual($edits[0]->type, 'change',  'The second item in the diff is a copy.');
+    $this->assertEqual($edits[0]->orig, array('id: ' . $new_test_entity_id));
+    $this->assertEqual($edits[0]->closing, array('id: ' . $test_entity_id));
+    $this->assertEqual($edits[1]->type, 'copy',  'The second item in the diff is a copy.');
+    $this->assertEqual(count($edits), 2, 'There are two items in the diff.');
   }
 
   /**
@@ -134,15 +139,17 @@ class ConfigDiffTest extends DrupalUnitTestBase {
 
     // Test the fields match in the default collection diff.
     $diff = \Drupal::service('config.manager')->diff($active, $staging, $config_name);
-    $this->assertEqual($diff->edits[0]->type, 'copy',  'The first item in the diff is a copy.');
-    $this->assertEqual(count($diff->edits), 1, 'There is one item in the diff');
+    $edits = $diff->getEdits();
+    $this->assertEqual($edits[0]->type, 'copy',  'The first item in the diff is a copy.');
+    $this->assertEqual(count($edits), 1, 'There is one item in the diff');
 
     // Test that the differences are detected when diffing the collection.
     $diff = \Drupal::service('config.manager')->diff($active, $staging, $config_name, NULL, 'test');
-    $this->assertEqual($diff->edits[0]->type, 'change',  'The second item in the diff is a copy.');
-    $this->assertEqual($diff->edits[0]->orig, array('foo: bar'));
-    $this->assertEqual($diff->edits[0]->closing, array('foo: baz'));
-    $this->assertEqual($diff->edits[1]->type, 'copy',  'The second item in the diff is a copy.');
+    $edits = $diff->getEdits();
+    $this->assertEqual($edits[0]->type, 'change',  'The second item in the diff is a copy.');
+    $this->assertEqual($edits[0]->orig, array('foo: bar'));
+    $this->assertEqual($edits[0]->closing, array('foo: baz'));
+    $this->assertEqual($edits[1]->type, 'copy',  'The second item in the diff is a copy.');
   }
 
 }

core/modules/system/config/install/system.diff.yml

@@ -0,0 +1,3 @@
+context:
+  lines_leading: 2
+  lines_trailing: 2

core/modules/system/config/schema/system.schema.yml

@@ -128,6 +128,21 @@ system.date_format.*:
       type: string
       label: 'Default language'
 
+system.diff:
+  type: mapping
+  label: 'Diff settings'
+  mapping:
+    context:
+      type: mapping
+      label: 'Context'
+      mapping:
+        lines_leading:
+          type: integer
+          label: 'Number of leading lines in a diff'
+        lines_trailing:
+          type: integer
+          label: 'Number of trailing lines in a diff'
+
 system.filter:
   type: mapping
   label: 'Filter settings'