<?php
/**
 * A PHP diff engine for phpwiki. (Taken from phpwiki-1.3.3)
 *
 * Copyright © 2000, 2001 Geoffrey T. Dairiki <dairiki@dairiki.org>
 * You may copy this code freely under the conditions of the GPL.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * Backdrop's implementation is directly adopted from
 * https://git.wikimedia.org/raw/mediawiki%2fcore.git/HEAD/includes%2fdiff%2fDairikiDiff.php
 * Code has been reformatted to match Backdrop standards, and MediaWiki-specific
 * profiling code has been removed.
 *
 * @file
 * @ingroup DifferenceEngine
 * @defgroup DifferenceEngine DifferenceEngine
 */

/**
 * @private
 * @ingroup DifferenceEngine
 */
abstract class DiffOp {
  public $type;
  public $orig;
  public $closing;

  abstract public function reverse();

  /**
   * @return int
   */
  function norig() {
    return $this->orig ? count($this->orig) : 0;
  }

  /**
   * @return int
   */
  function nclosing() {
    return $this->closing ? count($this->closing) : 0;
  }
}

/**
 * @private
 * @ingroup DifferenceEngine
 */
class DiffOpCopy extends DiffOp {
  public $type = 'copy';

  function __construct($orig, $closing = FALSE) {
    if (!is_array($closing)) {
      $closing = $orig;
    }
    $this->orig = $orig;
    $this->closing = $closing;
  }

  /**
   * @return DiffOpCopy
   */
  function reverse() {
    return new DiffOpCopy($this->closing, $this->orig);
  }
}

/**
 * @private
 * @ingroup DifferenceEngine
 */
class DiffOpDelete extends DiffOp {
  public $type = 'delete';

  function __construct($lines) {
    $this->orig = $lines;
    $this->closing = FALSE;
  }

  /**
   * @return DiffOpAdd
   */
  function reverse() {
    return new DiffOpAdd($this->orig);
  }
}

/**
 * @private
 * @ingroup DifferenceEngine
 */
class DiffOpAdd extends DiffOp {
  public $type = 'add';

  function __construct($lines) {
    $this->closing = $lines;
    $this->orig = FALSE;
  }

  /**
   * @return DiffOpDelete
   */
  function reverse() {
    return new DiffOpDelete($this->closing);
  }
}

/**
 * @private
 * @ingroup DifferenceEngine
 */
class DiffOpChange extends DiffOp {
  public $type = 'change';

  function __construct($orig, $closing) {
    $this->orig = $orig;
    $this->closing = $closing;
  }

  /**
   * @return DiffOpChange
   */
  function reverse() {
    return new DiffOpChange($this->closing, $this->orig);
  }
}

/**
 * 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
 * Alternative implementation added by Guy Van den Broeck, 2008-07-30
 *
 * @author Geoffrey T. Dairiki, Tim Starling, Guy Van den Broeck
 * @private
 * @ingroup DifferenceEngine
 */
class DiffEngine {
  const MAX_XREF_LENGTH = 10000;

  protected $xchanged, $ychanged;

  protected $xv = array(), $yv = array();
  protected $xind = array(), $yind = array();

  protected $seq = array(), $in_seq = array();

  protected $lcs = 0;

  /**
   * @param $from_lines
   * @param $to_lines
   *
   * @return array
   */
  function diff($from_lines, $to_lines) {
    // Diff and store locally
    $this->diffLocal($from_lines, $to_lines);

    // Merge edits when possible
    $this->shiftBoundaries($from_lines, $this->xchanged, $this->ychanged);
    $this->shiftBoundaries($to_lines, $this->ychanged, $this->xchanged);

    // Compute the edit operations.
    $n_from = count($from_lines);
    $n_to = count($to_lines);

    $edits = array();
    $xi = $yi = 0;
    while ($xi < $n_from || $yi < $n_to) {
      assert($yi < $n_to || $this->xchanged[$xi]);
      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;
  }

  /**
   * @param $from_lines
   * @param $to_lines
   */
  private function diffLocal($from_lines, $to_lines) {
    $n_from = count($from_lines);
    $n_to = count($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->lineHash($from_lines[$xi])] = 1;
    }

    for ($yi = $skip; $yi < $n_to - $endskip; $yi++) {
      $line = $to_lines[$yi];
      if (($this->ychanged[$yi] = empty($xhash[$this->lineHash($line)]))) {
        continue;
      }
      $yhash[$this->lineHash($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->lineHash($line)]))) {
        continue;
      }
      $this->xv[] = $line;
      $this->xind[] = $xi;
    }

    // Find the LCS.
    $this->compareSeq(0, count($this->xv), 0, count($this->yv));
  }

  /**
   * Returns the whole line if it's small enough, or the MD5 hash otherwise
   *
   * @param $line string
   *
   * @return string
   */
  private function lineHash($line) {
    if (strlen($line) > self::MAX_XREF_LENGTH) {
      return md5($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.
   *
   * @param $xoff
   * @param $xlim
   * @param $yoff
   * @param $ylim
   * @param $nchunks
   *
   * @return array
   */
  private function diag($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;
        }

        $k = 0;
        $matches = $ymatches[$line];
        reset($matches);
        while ($y = current($matches)) {
          next($matches);
          if (empty($this->in_seq[$y])) {
            $k = $this->lcsPos($y);
            assert($k > 0);
            $ymids[$k] = $ymids[$k - 1];
            break;
          }
        }

        while ($y = current($matches)) {
          next($matches);
          if ($y > $this->seq[$k - 1]) {
            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 = $this->lcsPos($y);
            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);
  }

  /**
   * @param $ypos
   *
   * @return int
   */
  private function lcsPos($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;
      }
    }

    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.
   *
   * @param $xoff
   * @param $xlim
   * @param $yoff
   * @param $ylim
   */
  private function compareSeq($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).
   */
  private function shiftBoundaries($lines, &$changed, $other_changed) {
    $i = 0;
    $j = 0;

    assert(count($lines) == count($changed));
    $len = count($lines);
    $other_len = count($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[$i]) {
        assert($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]) {
            $start--;
          }
          assert($j > 0);
          while ($other_changed[--$j]) {
            continue;
          }
          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++;
          }

          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;
        assert($j > 0);
        while ($other_changed[--$j]) {
          continue;
        }
        assert($j >= 0 && !$other_changed[$j]);
      }
    }
  }
}

/**
 * Class representing a 'diff' between two sequences of strings.
 * @private
 * @ingroup DifferenceEngine
 */
class Diff {
  public $edits;

  /**
   * Constructor.
   * Computes diff between sequences of strings.
   *
   * @param $from_lines array An array of strings.
   *   Typically these are lines from a file.
   * @param $to_lines array An array of strings.
   */
  function __construct($from_lines, $to_lines) {
    $eng = new DiffEngine;
    $this->edits = $eng->diff($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.
   */
  function reverse() {
    $rev = $this;
    $rev->edits = array();
    /** @var DiffOp $edit */
    foreach ($this->edits as $edit) {
      $rev->edits[] = $edit->reverse();
    }

    return $rev;
  }

  /**
   * Check for empty diff.
   *
   * @return bool True if two sequences were identical.
   */
  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.
   */
  function lcs() {
    $lcs = 0;
    foreach ($this->edits as $edit) {
      if ($edit->type == 'copy') {
        $lcs += count($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.
   */
  function orig() {
    $lines = array();

    foreach ($this->edits as $edit) {
      if ($edit->orig) {
        array_splice($lines, count($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.
   */
  function closing() {
    $lines = array();

    foreach ($this->edits as $edit) {
      if ($edit->closing) {
        array_splice($lines, count($lines), 0, $edit->closing);
      }
    }

    return $lines;
  }
}

/**
 * @private
 * @ingroup DifferenceEngine
 */
class MappedDiff extends Diff {
  /**
   * Constructor.
   *
   * Computes diff between sequences of strings.
   *
   * This can be used to compute things like
   * case-insensitve 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 $to_lines.
   */
  function __construct($from_lines, $to_lines, $mapped_from_lines, $mapped_to_lines) {
    assert(count($from_lines) == count($mapped_from_lines));
    assert(count($to_lines) == count($mapped_to_lines));

    parent::__construct($mapped_from_lines, $mapped_to_lines);

    $xi = $yi = 0;
    $editCount = count($this->edits);
    for ($i = 0; $i < $editCount; $i++) {
      $orig = & $this->edits[$i]->orig;
      if (is_array($orig)) {
        $orig = array_slice($from_lines, $xi, count($orig));
        $xi += count($orig);
      }

      $closing = & $this->edits[$i]->closing;
      if (is_array($closing)) {
        $closing = array_slice($to_lines, $yi, count($closing));
        $yi += count($closing);
      }
    }
  }
}

/**
 * Additions by Axel Boldt follow, partly taken from diff.php, phpwiki-1.3.3
 */

/**
 * @private
 * @ingroup DifferenceEngine
 */
class HWLDFWordAccumulator {
  private $lines = array();
  private $line = '';
  private $group = '';
  private $tag = '';

  /**
   * @param $new_tag
   */
  private function flushGroup($new_tag) {
    if ($this->group !== '') {
      if ($this->tag == 'ins') {
        $this->line .= '<ins class="diffchange diffchange-inline">' . htmlspecialchars($this->group) . '</ins>';
      }
      elseif ($this->tag == 'del') {
        $this->line .= '<del class="diffchange diffchange-inline">' . htmlspecialchars($this->group) . '</del>';
      }
      else {
        $this->line .= htmlspecialchars($this->group);
      }
    }
    $this->group = '';
    $this->tag = $new_tag;
  }

  /**
   * @param $new_tag
   */
  private function flushLine($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, '&#160;');
    }
    $this->line = '';
  }

  /**
   * @param $words
   * @param $tag string
   */
  public function addWords($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);
        $word = substr($word, 1);
      }
      assert(!strstr($word, "\n"));
      $this->group .= $word;
    }
  }

  /**
   * @return array
   */
  public function getLines() {
    $this->flushLine('~done');

    return $this->lines;
  }
}

/**
 * @private
 * @ingroup DifferenceEngine
 */
class WordLevelDiff extends MappedDiff {
  const MAX_LINE_LENGTH = 10000;

  /**
   * @param $orig_lines
   * @param $closing_lines
   */
  function __construct($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);
  }

  /**
   * @param $lines
   *
   * @return array
   */
  private function split($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";
        $stripped[] = "\n";
      }
      if (strlen($line) > self::MAX_LINE_LENGTH) {
        $words[] = $line;
        $stripped[] = $line;
      }
      else {
        $m = array();
        if (preg_match_all('/ ( [^\S\n]+ | [0-9_A-Za-z\x80-\xff]+ | . ) (?: (?!< \n) [^\S\n])? /xs',
          $line, $m)
        ) {
          foreach ($m[0] as $word) {
            $words[] = $word;
          }
          foreach ($m[1] as $stripped_word) {
            $stripped[] = $stripped_word;
          }
        }
      }
    }
    return array($words, $stripped);
  }

  /**
   * @return array
   */
  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, 'del');
      }
    }
    $lines = $orig->getLines();
    return $lines;
  }

  /**
   * @return array
   */
  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, 'ins');
      }
    }
    $lines = $closing->getLines();
    return $lines;
  }
}

/**
 * Base class for diff formatters
 *
 * This class formats the diff in classic diff format.
 * It is intended that this class be customized via inheritance,
 * to obtain fancier outputs.
 *
 * @ingroup DifferenceEngine
 */
abstract class DiffFormatter {
  /** @var int 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.
   */
  protected $leadingContextLines = 2;

  /** @var int 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.
   */
  protected $trailingContextLines = 2;

  /**
   * Format a diff.
   *
   * @param $diff Diff A Diff object.
   *
   * @return string The formatted output.
   */
  public function format($diff) {
    $xi = $yi = 1;
    $block = FALSE;
    $context = array();

    $nlead = $this->leadingContextLines;
    $ntrail = $this->trailingContextLines;

    $this->startDiff();

    // Initialize $x0 and $y0 to prevent IDEs from getting confused.
    $x0 = $y0 = 0;
    foreach ($diff->edits as $edit) {
      if ($edit->type == 'copy') {
        if (is_array($block)) {
          if (count($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, count($context) - $nlead);
          $x0 = $xi - count($context);
          $y0 = $yi - count($context);
          $block = array();
          if ($context) {
            $block[] = new DiffOpCopy($context);
          }
        }
        $block[] = $edit;
      }

      if ($edit->orig) {
        $xi += count($edit->orig);
      }
      if ($edit->closing) {
        $yi += count($edit->closing);
      }
    }

    if (is_array($block)) {
      $this->block($x0, $xi - $x0,
        $y0, $yi - $y0,
        $block);
    }

    $end = $this->endDiff();

    return $end;
  }

  /**
   * @param int $xbeg
   * @param int $xlen
   * @param int $ybeg
   * @param int $ylen
   * @param $edits
   *
   * @throws Exception
   */
  protected function block($xbeg, $xlen, $ybeg, $ylen, &$edits) {
    $this->startBlock($this->blockHeader($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, $edit->closing);
      }
      else {
        throw new Exception("Unknown edit type: {$edit->type}");
      }
    }
    $this->endBlock();
  }

  protected function startDiff() {
    ob_start();
  }

  /**
   * @return string
   */
  protected function endDiff() {
    $val = ob_get_contents();
    ob_end_clean();

    return $val;
  }

  /**
   * @param $xbeg
   * @param $xlen
   * @param $ybeg
   * @param $ylen
   *
   * @return string
   */
  protected function blockHeader($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 startBlock($header) {
    echo $header . "\n";
  }

  protected function endBlock() {
  }

  /**
   * @param $lines
   * @param $prefix string
   */
  protected function lines($lines, $prefix = ' ') {
    foreach ($lines as $line) {
      echo "$prefix $line\n";
    }
  }

  /**
   * @param $lines
   */
  protected function context($lines) {
    $this->lines($lines);
  }

  /**
   * @param $lines
   */
  protected function added($lines) {
    $this->lines($lines, '>');
  }

  /**
   * @param $lines
   */
  protected function deleted($lines) {
    $this->lines($lines, '<');
  }

  /**
   * @param $orig
   * @param $closing
   */
  protected function changed($orig, $closing) {
    $this->deleted($orig);
    echo "---\n";
    $this->added($closing);
  }
}

/**
 * Backdrop diff formatter which returns diffs as an array.
 *
 * @ingroup DifferenceEngine
 */
class BackdropDiffFormatter extends DiffFormatter {

  private $rows;
  private $lineStats = array(
    'counter' => array('x' => 0, 'y' => 0),
    'offset' => array('x' => 0, 'y' => 0),
  );
  private $showHeader = 0;

  protected function startDiff() {
    $this->rows = array();
  }

  protected function endDiff() {
    return $this->rows;
  }

  protected function blockHeader($xbeg, $xlen, $ybeg, $ylen) {
    return array(
      array(
        'data' => $xbeg + $this->lineStats['offset']['x'],
        'colspan' => 2,
      ),
      array(
        'data' => $ybeg + $this->lineStats['offset']['y'],
        'colspan' => 2,
      )
    );
  }

  protected function startBlock($header) {
    if ($this->showHeader) {
      $this->rows[] = $header;
    }
  }

  protected function endBlock() {
  }

  protected function lines($lines, $prefix = ' ', $color = 'white') {
  }

  protected function addedLine($line) {
    return array(
      array(
        'data' => '+',
        'class' => 'diff-marker diff-addedline',
      ),
      array(
        'data' => $line,
        'class' => 'diff-context diff-addedline',
      )
    );
  }

  protected function deletedLine($line) {
    return array(
      array(
        'data' => '-',
        'class' => 'diff-marker diff-deletedline',
      ),
      array(
        'data' => $line,
        'class' => 'diff-context diff-deletedline',
      )
    );
  }

  protected function contextLine($line) {
    return array(
      '&nbsp;',
      array(
        'data' => $line,
        'class' => 'diff-context',
      )
    );
  }

  protected function emptyLine() {
    return array(
      '&nbsp;',
      '&nbsp;',
    );
  }

  protected function added($lines) {
    foreach ($lines as $line) {
      $this->rows[] = array_merge($this->emptyLine(), $this->addedLine(check_plain($line)));
    }
  }

  protected function deleted($lines) {
    foreach ($lines as $line) {
      $this->rows[] = array_merge($this->deletedLine(check_plain($line)), $this->emptyLine());
    }
  }

  protected function context($lines) {
    foreach ($lines as $line) {
      $this->rows[] = array_merge($this->contextLine(check_plain($line)), $this->contextLine(check_plain($line)));
    }
  }

  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));
    }
  }
}
