Websites/webkit.org/blog/wp-includes/Text/Diff/Engine/native.php - platform/external/chromium_org/third_party/WebKit - Git at Google

 <?php
 /**
  * Class used internally by Text_Diff to actually compute the diffs.
  *
  * This class is implemented using native PHP code.
  *
  * 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 (and a bit of code) are taken from analyze.c, of GNU
  * diffutils-2.7, which can be found at:
  * ftp://gnudist.gnu.org/pub/gnu/diffutils/diffutils-2.7.tar.gz
  *
  * Some ideas (subdivision by NCHUNKS > 2, and some optimizations) are from
  * Geoffrey T. Dairiki <dairiki@dairiki.org>. The original PHP version of this
  * code was written by him, and is used/adapted with his permission.
  *
  * Copyright 2004-2010 The Horde Project (http://www.horde.org/)
  *
  * See the enclosed file COPYING for license information (LGPL). If you did
  * not receive this file, see http://opensource.org/licenses/lgpl-license.php.
  *
  * @author  Geoffrey T. Dairiki <dairiki@dairiki.org>
  * @package Text_Diff
  */
 class Text_Diff_Engine_native {

     function diff($from_lines, $to_lines)
     {
         array_walk($from_lines, array('Text_Diff', 'trimNewlines'));
         array_walk($to_lines, array('Text_Diff', 'trimNewlines'));

         $n_from = count($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++) {
             $xhash[$from_lines[$xi]] = 1;
         }
         for ($yi = $skip; $yi < $n_to - $endskip; $yi++) {
             $line = $to_lines[$yi];
             if (($this->ychanged[$yi] = empty($xhash[$line]))) {
                 continue;
             }
             $yhash[$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($yhash[$line]))) {
                 continue;
             }
             $this->xv[] = $line;
             $this->xind[] = $xi;
         }

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

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

         // Compute the edit operations.
         $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) {
                 $edits[] = &new Text_Diff_Op_copy($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) {
                 $edits[] = &new Text_Diff_Op_change($delete, $add);
             } elseif ($delete) {
                 $edits[] = &new Text_Diff_Op_delete($delete);
             } elseif ($add) {
                 $edits[] = &new Text_Diff_Op_add($add);
             }
         }

         return $edits;
     }

     /**
      * Divides 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.
      */
     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 is 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(, $y) = each($matches)) {
                     if (empty($this->in_seq[$y])) {
                         $k = $this->_lcsPos($y);
                         assert($k > 0);
                         $ymids[$k] = $ymids[$k - 1];
                         break;
                     }
                 }
                 while (list(, $y) = each($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);
     }

     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;
     }

     /**
      * Finds LCS of two sequences.
      *
      * The results are recorded in the vectors $this->{x,y}changed[], by
      * storing a 1 in the element for each line that is an insertion or
      * deletion (ie. is not in the LCS).
      *
      * The subsequence of file 0 is (XOFF, XLIM) and likewise for file 1.
      *
      * Note that XLIM, YLIM are exclusive bounds.  All line numbers are
      * origin-0 and discarded lines are not counted.
      */
     function _compareseq ($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;
             }
         }
     }

     /**
      * Adjusts 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).
      */
     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 forward to find the 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]');
             }
         }
     }

 }
	<?php
	/**
	* Class used internally by Text_Diff to actually compute the diffs.
	*
	* This class is implemented using native PHP code.
	*
	* 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 (and a bit of code) are taken from analyze.c, of GNU
	* diffutils-2.7, which can be found at:
	* ftp://gnudist.gnu.org/pub/gnu/diffutils/diffutils-2.7.tar.gz
	*
	* Some ideas (subdivision by NCHUNKS > 2, and some optimizations) are from
	* Geoffrey T. Dairiki <dairiki@dairiki.org>. The original PHP version of this
	* code was written by him, and is used/adapted with his permission.
	*
	* Copyright 2004-2010 The Horde Project (http://www.horde.org/)
	*
	* See the enclosed file COPYING for license information (LGPL). If you did
	* not receive this file, see http://opensource.org/licenses/lgpl-license.php.
	*
	* @author Geoffrey T. Dairiki <dairiki@dairiki.org>
	* @package Text_Diff
	*/
	class Text_Diff_Engine_native {

	function diff($from_lines, $to_lines)
	{
	array_walk($from_lines, array('Text_Diff', 'trimNewlines'));
	array_walk($to_lines, array('Text_Diff', 'trimNewlines'));

	$n_from = count($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++) {
	$xhash[$from_lines[$xi]] = 1;
	}
	for ($yi = $skip; $yi < $n_to - $endskip; $yi++) {
	$line = $to_lines[$yi];
	if (($this->ychanged[$yi] = empty($xhash[$line]))) {
	continue;
	}
	$yhash[$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($yhash[$line]))) {
	continue;
	}
	$this->xv[] = $line;
	$this->xind[] = $xi;
	}

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

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

	// Compute the edit operations.
	$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) {
	$edits[] = &new Text_Diff_Op_copy($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) {
	$edits[] = &new Text_Diff_Op_change($delete, $add);
	} elseif ($delete) {
	$edits[] = &new Text_Diff_Op_delete($delete);
	} elseif ($add) {
	$edits[] = &new Text_Diff_Op_add($add);
	}
	}

	return $edits;
	}

	/**
	* Divides 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.
	*/
	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 is 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(, $y) = each($matches)) {
	if (empty($this->in_seq[$y])) {
	$k = $this->_lcsPos($y);
	assert($k > 0);
	$ymids[$k] = $ymids[$k - 1];
	break;
	}
	}
	while (list(, $y) = each($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);
	}

	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;
	}

	/**
	* Finds LCS of two sequences.
	*
	* The results are recorded in the vectors $this->{x,y}changed[], by
	* storing a 1 in the element for each line that is an insertion or
	* deletion (ie. is not in the LCS).
	*
	* The subsequence of file 0 is (XOFF, XLIM) and likewise for file 1.
	*
	* Note that XLIM, YLIM are exclusive bounds. All line numbers are
	* origin-0 and discarded lines are not counted.
	*/
	function _compareseq ($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;
	}
	}
	}

	/**
	* Adjusts 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).
	*/
	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 forward to find the 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]');
	}
	}
	}

	}