class DiffEngine

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 = [];
        $this->xv = $this->yv = [];
        $this->xind = $this->yind = [];
        unset($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 = [];
        $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".
            $copy = [];
            while ($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.
            $delete = [];
            while ($xi < $n_from && $this->xchanged[$xi]) {
                $delete[] = $from_lines[$xi++];
            }
            $add = [];
            while ($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 a hash otherwise.
     */
    protected function _line_hash($line) {
        if (mb_strlen($line) > $this::MAX_XREF_LENGTH) {
            return hash('crc32b', $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;
            [
                $xoff,
                $xlim,
                $yoff,
                $ylim,
            ] = [
                $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 = [];
        $ymids[0] = [];
        $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];
                $found_empty = FALSE;
                foreach ($matches as $y) {
                    if (!$found_empty) {
                        if (empty($this->in_seq[$y])) {
                            $k = $this->_lcs_pos($y);
                            $this::USE_ASSERTS && assert($k > 0);
                            $ymids[$k] = $ymids[$k - 1];
                            $found_empty = TRUE;
                        }
                    }
                    else {
                        if ($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 ? [
            $yoff,
            $xoff,
        ] : [
            $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 ? [
                $y1,
                $x1,
            ] : [
                $x1,
                $y1,
            ];
        }
        $seps[] = $flip ? [
            $ylim,
            $xlim,
        ] : [
            $xlim,
            $ylim,
        ];
        return [
            $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;
            [
                $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]);
            }
        }
    }

}