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/**
 * 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 &gt; 2, and some optimizations) are from
 * Geoffrey T. Dairiki &lt;dairiki@dairiki.org&gt;. 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 &lt;dairiki@dairiki.org&gt;
 * @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-&gt;xchanged = $this-&gt;ychanged = array();
        $this-&gt;xv = $this-&gt;yv = array();
        $this-&gt;xind = $this-&gt;yind = array();
        unset($this-&gt;seq);
        unset($this-&gt;in_seq);
        unset($this-&gt;lcs);

        // Skip leading common lines.
        for ($skip = 0; $skip &lt; $n_from &amp;&amp; $skip &lt; $n_to; $skip++) {
            if ($from_lines[$skip] !== $to_lines[$skip]) {
                break;
            }
            $this-&gt;xchanged[$skip] = $this-&gt;ychanged[$skip] = false;
        }

        // Skip trailing common lines.
        $xi = $n_from; $yi = $n_to;
        for ($endskip = 0; --$xi &gt; $skip &amp;&amp; --$yi &gt; $skip; $endskip++) {
            if ($from_lines[$xi] !== $to_lines[$yi]) {
                break;
            }
            $this-&gt;xchanged[$xi] = $this-&gt;ychanged[$yi] = false;
        }

        // Ignore lines which do not exist in both files.
        for ($xi = $skip; $xi &lt; $n_from - $endskip; $xi++) {
            $xhash[$from_lines[$xi]] = 1;
        }
        for ($yi = $skip; $yi &lt; $n_to - $endskip; $yi++) {
            $line = $to_lines[$yi];
            if (($this-&gt;ychanged[$yi] = empty($xhash[$line]))) {
                continue;
            }
            $yhash[$line] = 1;
            $this-&gt;yv[] = $line;
            $this-&gt;yind[] = $yi;
        }
        for ($xi = $skip; $xi &lt; $n_from - $endskip; $xi++) {
            $line = $from_lines[$xi];
            if (($this-&gt;xchanged[$xi] = empty($yhash[$line]))) {
                continue;
            }
            $this-&gt;xv[] = $line;
            $this-&gt;xind[] = $xi;
        }

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

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

        // Compute the edit operations.
        $edits = array();
        $xi = $yi = 0;
        while ($xi &lt; $n_from || $yi &lt; $n_to) {
            assert($yi &lt; $n_to || $this-&gt;xchanged[$xi]);
            assert($xi &lt; $n_from || $this-&gt;ychanged[$yi]);

            // Skip matching &quot;snake&quot;.
            $copy = array();
            while ($xi &lt; $n_from &amp;&amp; $yi &lt; $n_to
                   &amp;&amp; !$this-&gt;xchanged[$xi] &amp;&amp; !$this-&gt;ychanged[$yi]) {
                $copy[] = $from_lines[$xi++];
                ++$yi;
            }
            if ($copy) {
                $edits[] = new Text_Diff_Op_copy($copy);
            }

            // Find deletes &amp; adds.
            $delete = array();
            while ($xi &lt; $n_from &amp;&amp; $this-&gt;xchanged[$xi]) {
                $delete[] = $from_lines[$xi++];
            }

            $add = array();
            while ($yi &lt; $n_to &amp;&amp; $this-&gt;ychanged[$yi]) {
                $add[] = $to_lines[$yi++];
            }

            if ($delete &amp;&amp; $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 &gt; $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 &gt;= $yoff; $i--) {
                $ymatches[$this-&gt;xv[$i]][] = $i;
            }
        } else {
            for ($i = $ylim - 1; $i &gt;= $yoff; $i--) {
                $ymatches[$this-&gt;yv[$i]][] = $i;
            }
        }

        $this-&gt;lcs = 0;
        $this-&gt;seq[0]= $yoff - 1;
        $this-&gt;in_seq = array();
        $ymids[0] = array();

        $numer = $xlim - $xoff + $nchunks - 1;
        $x = $xoff;
        for ($chunk = 0; $chunk &lt; $nchunks; $chunk++) {
            if ($chunk &gt; 0) {
                for ($i = 0; $i &lt;= $this-&gt;lcs; $i++) {
                    $ymids[$i][$chunk - 1] = $this-&gt;seq[$i];
                }
            }

            $x1 = $xoff + (int)(($numer + ($xlim - $xoff) * $chunk) / $nchunks);
            for (; $x &lt; $x1; $x++) {
                $line = $flip ? $this-&gt;yv[$x] : $this-&gt;xv[$x];
                if (empty($ymatches[$line])) {
                    continue;
                }
                $matches = $ymatches[$line];
                reset($matches);
                while ($y = current($matches)) {
                    if (empty($this-&gt;in_seq[$y])) {
                        $k = $this-&gt;_lcsPos($y);
                        assert($k &gt; 0);
                        $ymids[$k] = $ymids[$k - 1];
                        break;
                    }
                    next($matches);
                }
                while ($y = current($matches)) {
                    if ($y &gt; $this-&gt;seq[$k - 1]) {
                        assert($y &lt;= $this-&gt;seq[$k]);
                        /* Optimization: this is a common case: next match is
                         * just replacing previous match. */
                        $this-&gt;in_seq[$this-&gt;seq[$k]] = false;
                        $this-&gt;seq[$k] = $y;
                        $this-&gt;in_seq[$y] = 1;
                    } elseif (empty($this-&gt;in_seq[$y])) {
                        $k = $this-&gt;_lcsPos($y);
                        assert($k &gt; 0);
                        $ymids[$k] = $ymids[$k - 1];
                    }
                    next($matches);
                }
            }
        }

        $seps[] = $flip ? array($yoff, $xoff) : array($xoff, $yoff);
        $ymid = $ymids[$this-&gt;lcs];
        for ($n = 0; $n &lt; $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-&gt;lcs, $seps);
    }

    function _lcsPos($ypos)
    {
        $end = $this-&gt;lcs;
        if ($end == 0 || $ypos &gt; $this-&gt;seq[$end]) {
            $this-&gt;seq[++$this-&gt;lcs] = $ypos;
            $this-&gt;in_seq[$ypos] = 1;
            return $this-&gt;lcs;
        }

        $beg = 1;
        while ($beg &lt; $end) {
            $mid = (int)(($beg + $end) / 2);
            if ($ypos &gt; $this-&gt;seq[$mid]) {
                $beg = $mid + 1;
            } else {
                $end = $mid;
            }
        }

        assert($ypos != $this-&gt;seq[$end]);

        $this-&gt;in_seq[$this-&gt;seq[$end]] = false;
        $this-&gt;seq[$end] = $ypos;
        $this-&gt;in_seq[$ypos] = 1;
        return $end;
    }

    /**
     * Finds LCS of two sequences.
     *
     * The results are recorded in the vectors $this-&gt;{x,y}changed[], by
     * storing a 1 in the element for each line that is an insertion or
     * deletion (ie. is not in the LCS).
     *
     * The subsequence of file 0 is (XOFF, XLIM) and likewise for file 1.
     *
     * Note that XLIM, YLIM are exclusive bounds.  All line numbers are
     * origin-0 and discarded lines are not counted.
     */
    function _compareseq ($xoff, $xlim, $yoff, $ylim)
    {
        /* Slide down the bottom initial diagonal. */
        while ($xoff &lt; $xlim &amp;&amp; $yoff &lt; $ylim
               &amp;&amp; $this-&gt;xv[$xoff] == $this-&gt;yv[$yoff]) {
            ++$xoff;
            ++$yoff;
        }

        /* Slide up the top initial diagonal. */
        while ($xlim &gt; $xoff &amp;&amp; $ylim &gt; $yoff
               &amp;&amp; $this-&gt;xv[$xlim - 1] == $this-&gt;yv[$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)
                = $this-&gt;_diag($xoff, $xlim, $yoff, $ylim, $nchunks);
        }

        if ($lcs == 0) {
            /* X and Y sequences have no common subsequence: mark all
             * changed. */
            while ($yoff &lt; $ylim) {
                $this-&gt;ychanged[$this-&gt;yind[$yoff++]] = 1;
            }
            while ($xoff &lt; $xlim) {
                $this-&gt;xchanged[$this-&gt;xind[$xoff++]] = 1;
            }
        } else {
            /* Use the partitions to split this problem into subproblems. */
            reset($seps);
            $pt1 = $seps[0];
            while ($pt2 = next($seps)) {
                $this-&gt;_compareseq ($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 &quot;change&quot;.
     *
     * This is extracted verbatim from analyze.c (GNU diffutils-2.7).
     */
    function _shiftBoundaries($lines, &amp;$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 &lt; $other_len &amp;&amp; $other_changed[$j]) {
                $j++;
            }

            while ($i &lt; $len &amp;&amp; ! $changed[$i]) {
                assert($j &lt; $other_len &amp;&amp; ! $other_changed[$j]);
                $i++; $j++;
                while ($j &lt; $other_len &amp;&amp; $other_changed[$j]) {
                    $j++;
                }
            }

            if ($i == $len) {
                break;
            }

            $start = $i;

            /* Find the end of this run of changes. */
            while (++$i &lt; $len &amp;&amp; $changed[$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 &gt; 0 &amp;&amp; $lines[$start - 1] == $lines[$i - 1]) {
                    $changed[--$start] = 1;
                    $changed[--$i] = false;
                    while ($start &gt; 0 &amp;&amp; $changed[$start - 1]) {
                        $start--;
                    }
                    assert($j &gt; 0);
                    while ($other_changed[--$j]) {
                        continue;
                    }
                    assert($j &gt;= 0 &amp;&amp; !$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 &lt; $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 &lt; $len &amp;&amp; $lines[$start] == $lines[$i]) {
                    $changed[$start++] = false;
                    $changed[$i++] = 1;
                    while ($i &lt; $len &amp;&amp; $changed[$i]) {
                        $i++;
                    }

                    assert($j &lt; $other_len &amp;&amp; ! $other_changed[$j]);
                    $j++;
                    if ($j &lt; $other_len &amp;&amp; $other_changed[$j]) {
                        $corresponding = $i;
                        while ($j &lt; $other_len &amp;&amp; $other_changed[$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 &lt; $i) {
                $changed[--$start] = 1;
                $changed[--$i] = 0;
                assert($j &gt; 0);
                while ($other_changed[--$j]) {
                    continue;
                }
                assert($j &gt;= 0 &amp;&amp; !$other_changed[$j]);
            }
        }
    }

}
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