DiffEngine.php

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<?php
use MediaWiki\Diff\ComplexityException;
 
// FIXME: Don't use assert() in this file
// phpcs:disable MediaWiki.Usage.ForbiddenFunctions.assert
 
class DiffEngine {
 
    // Input variables
    private $from;
    private $to;
    private $m;
    private $n;
 
    private $tooLong;
    private $powLimit;
 
    protected $bailoutComplexity = 0;
 
    // State variables
    private $maxDifferences;
    private $lcsLengthCorrectedForHeuristic = false;
 
    // Output variables
    public $length;
    public $removed;
    public $added;
    public $heuristicUsed;
 
    public function __construct( $tooLong = 2000000, $powLimit = 1.45 ) {
        $this->tooLong = $tooLong;
        $this->powLimit = $powLimit;
    }
 
    public function diff( $from_lines, $to_lines ) {
        // Diff and store locally
        $this->diffInternal( $from_lines, $to_lines );
 
        // Merge edits when possible
        $this->shiftBoundaries( $from_lines, $this->removed, $this->added );
        $this->shiftBoundaries( $to_lines, $this->added, $this->removed );
 
        // Compute the edit operations.
        $n_from = count( $from_lines );
        $n_to = count( $to_lines );
 
        $edits = [];
        $xi = $yi = 0;
        while ( $xi < $n_from || $yi < $n_to ) {
            assert( $yi < $n_to || $this->removed[$xi] );
            assert( $xi < $n_from || $this->added[$yi] );
 
            // Skip matching "snake".
            $copy = [];
            while ( $xi < $n_from && $yi < $n_to
                    && !$this->removed[$xi] && !$this->added[$yi]
            ) {
                $copy[] = $from_lines[$xi++];
                ++$yi;
            }
            if ( $copy ) {
                $edits[] = new DiffOpCopy( $copy );
            }
 
            // Find deletes & adds.
            $delete = [];
            while ( $xi < $n_from && $this->removed[$xi] ) {
                $delete[] = $from_lines[$xi++];
            }
 
            $add = [];
            while ( $yi < $n_to && $this->added[$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;
    }
 
    public function setBailoutComplexity( $value ) {
        $this->bailoutComplexity = $value;
    }
 
    private function shiftBoundaries( array $lines, array &$changed, array $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;
                    // @phan-suppress-next-line PhanPluginLoopVariableReuse
                    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] );
            }
        }
    }
 
    protected function diffInternal( array $from, array $to ) {
        // remember initial lengths
        $m = count( $from );
        $n = count( $to );
 
        $this->heuristicUsed = false;
 
        // output
        $removed = $m > 0 ? array_fill( 0, $m, true ) : [];
        $added = $n > 0 ? array_fill( 0, $n, true ) : [];
 
        // reduce the complexity for the next step (intentionally done twice)
        // remove common tokens at the start
        $i = 0;
        while ( $i < $m && $i < $n && $from[$i] === $to[$i] ) {
            $removed[$i] = $added[$i] = false;
            unset( $from[$i], $to[$i] );
            ++$i;
        }
 
        // remove common tokens at the end
        $j = 1;
        while ( $i + $j <= $m && $i + $j <= $n && $from[$m - $j] === $to[$n - $j] ) {
            $removed[$m - $j] = $added[$n - $j] = false;
            unset( $from[$m - $j], $to[$n - $j] );
            ++$j;
        }
 
        $this->from = $newFromIndex = $this->to = $newToIndex = [];
 
        // remove tokens not in both sequences
        $shared = [];
        foreach ( $from as $key ) {
            $shared[$key] = false;
        }
 
        foreach ( $to as $index => &$el ) {
            if ( array_key_exists( $el, $shared ) ) {
                // keep it
                $this->to[] = $el;
                $shared[$el] = true;
                $newToIndex[] = $index;
            }
        }
        foreach ( $from as $index => &$el ) {
            if ( $shared[$el] ) {
                // keep it
                $this->from[] = $el;
                $newFromIndex[] = $index;
            }
        }
 
        unset( $shared, $from, $to );
 
        $this->m = count( $this->from );
        $this->n = count( $this->to );
 
        if ( $this->bailoutComplexity > 0 && $this->m * $this->n > $this->bailoutComplexity ) {
            throw new ComplexityException();
        }
 
        $this->removed = $this->m > 0 ? array_fill( 0, $this->m, true ) : [];
        $this->added = $this->n > 0 ? array_fill( 0, $this->n, true ) : [];
 
        if ( $this->m == 0 || $this->n == 0 ) {
            $this->length = 0;
        } else {
            $this->maxDifferences = ceil( ( $this->m + $this->n ) / 2.0 );
            if ( $this->m * $this->n > $this->tooLong ) {
                // limit complexity to D^POW_LIMIT for long sequences
                $this->maxDifferences = floor( $this->maxDifferences ** ( $this->powLimit - 1.0 ) );
                wfDebug( "Limiting max number of differences to $this->maxDifferences" );
            }
 
            /*
             * The common prefixes and suffixes are always part of some LCS, include
             * them now to reduce our search space
             */
            $max = min( $this->m, $this->n );
            for ( $forwardBound = 0; $forwardBound < $max
                && $this->from[$forwardBound] === $this->to[$forwardBound];
                ++$forwardBound
            ) {
                $this->removed[$forwardBound] = $this->added[$forwardBound] = false;
            }
 
            $backBoundL1 = $this->m - 1;
            $backBoundL2 = $this->n - 1;
 
            while ( $backBoundL1 >= $forwardBound && $backBoundL2 >= $forwardBound
                && $this->from[$backBoundL1] === $this->to[$backBoundL2]
            ) {
                $this->removed[$backBoundL1--] = $this->added[$backBoundL2--] = false;
            }
 
            $temp = array_fill( 0, $this->m + $this->n + 1, 0 );
            $V = [ $temp, $temp ];
            $snake = [ 0, 0, 0 ];
 
            $this->length = $forwardBound + $this->m - $backBoundL1 - 1
                + $this->lcs_rec(
                    $forwardBound,
                    $backBoundL1,
                    $forwardBound,
                    $backBoundL2,
                    $V,
                    $snake
            );
        }
 
        $this->m = $m;
        $this->n = $n;
 
        $this->length += $i + $j - 1;
 
        foreach ( $this->removed as $key => &$removed_elem ) {
            if ( !$removed_elem ) {
                $removed[$newFromIndex[$key]] = false;
            }
        }
        foreach ( $this->added as $key => &$added_elem ) {
            if ( !$added_elem ) {
                $added[$newToIndex[$key]] = false;
            }
        }
        $this->removed = $removed;
        $this->added = $added;
    }
 
    private function lcs_rec( $bottoml1, $topl1, $bottoml2, $topl2, &$V, &$snake ) {
        // check that both sequences are non-empty
        if ( $bottoml1 > $topl1 || $bottoml2 > $topl2 ) {
            return 0;
        }
 
        $d = $this->find_middle_snake( $bottoml1, $topl1, $bottoml2,
            $topl2, $V, $snake );
 
        // need to store these so we don't lose them when they're
        // overwritten by the recursion
        list( $startx, $starty, $len ) = $snake;
 
        // the middle snake is part of the LCS, store it
        for ( $i = 0; $i < $len; ++$i ) {
            $this->removed[$startx + $i] = $this->added[$starty + $i] = false;
        }
 
        if ( $d > 1 ) {
            return $len
            + $this->lcs_rec( $bottoml1, $startx - 1, $bottoml2,
                $starty - 1, $V, $snake )
            + $this->lcs_rec( $startx + $len, $topl1, $starty + $len,
                $topl2, $V, $snake );
        } elseif ( $d == 1 ) {
            /*
             * In this case the sequences differ by exactly 1 line. We have
             * already saved all the lines after the difference in the for loop
             * above, now we need to save all the lines before the difference.
             */
            $max = min( $startx - $bottoml1, $starty - $bottoml2 );
            for ( $i = 0; $i < $max; ++$i ) {
                $this->removed[$bottoml1 + $i] =
                    $this->added[$bottoml2 + $i] = false;
            }
 
            return $max + $len;
        }
 
        return $len;
    }
 
    private function find_middle_snake( $bottoml1, $topl1, $bottoml2, $topl2, &$V, &$snake ) {
        $from = &$this->from;
        $to = &$this->to;
        $V0 = &$V[0];
        $V1 = &$V[1];
        $snake0 = &$snake[0];
        $snake1 = &$snake[1];
        $snake2 = &$snake[2];
        $bottoml1_min_1 = $bottoml1 - 1;
        $bottoml2_min_1 = $bottoml2 - 1;
        $N = $topl1 - $bottoml1_min_1;
        $M = $topl2 - $bottoml2_min_1;
        $delta = $N - $M;
        $maxabsx = $N + $bottoml1;
        $maxabsy = $M + $bottoml2;
        $limit = min( $this->maxDifferences, ceil( ( $N + $M ) / 2 ) );
 
        // value_to_add_forward: a 0 or 1 that we add to the start
        // offset to make it odd/even
        if ( $M & 1 ) {
            $value_to_add_forward = 1;
        } else {
            $value_to_add_forward = 0;
        }
 
        if ( $N & 1 ) {
            $value_to_add_backward = 1;
        } else {
            $value_to_add_backward = 0;
        }
 
        $start_forward = -$M;
        $end_forward = $N;
        $start_backward = -$N;
        $end_backward = $M;
 
        $limit_min_1 = $limit - 1;
        $limit_plus_1 = $limit + 1;
 
        $V0[$limit_plus_1] = 0;
        $V1[$limit_min_1] = $N;
        $limit = min( $this->maxDifferences, ceil( ( $N + $M ) / 2 ) );
 
        if ( $delta & 1 ) {
            for ( $d = 0; $d <= $limit; ++$d ) {
                $start_diag = max( $value_to_add_forward + $start_forward, -$d );
                $end_diag = min( $end_forward, $d );
                $value_to_add_forward = 1 - $value_to_add_forward;
 
                // compute forward furthest reaching paths
                for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
                    if ( $k == -$d || ( $k < $d
                            && $V0[$limit_min_1 + $k] < $V0[$limit_plus_1 + $k] )
                    ) {
                        $x = $V0[$limit_plus_1 + $k];
                    } else {
                        $x = $V0[$limit_min_1 + $k] + 1;
                    }
 
                    $absx = $snake0 = $x + $bottoml1;
                    $absy = $snake1 = $x - $k + $bottoml2;
 
                    while ( $absx < $maxabsx && $absy < $maxabsy && $from[$absx] === $to[$absy] ) {
                        ++$absx;
                        ++$absy;
                    }
                    $x = $absx - $bottoml1;
 
                    $snake2 = $absx - $snake0;
                    $V0[$limit + $k] = $x;
                    if ( $k >= $delta - $d + 1 && $k <= $delta + $d - 1
                        && $x >= $V1[$limit + $k - $delta]
                    ) {
                        return 2 * $d - 1;
                    }
 
                    // check to see if we can cut down the diagonal range
                    if ( $x >= $N && $end_forward > $k - 1 ) {
                        $end_forward = $k - 1;
                    } elseif ( $absy - $bottoml2 >= $M ) {
                        $start_forward = $k + 1;
                        $value_to_add_forward = 0;
                    }
                }
 
                $start_diag = max( $value_to_add_backward + $start_backward, -$d );
                $end_diag = min( $end_backward, $d );
                $value_to_add_backward = 1 - $value_to_add_backward;
 
                // compute backward furthest reaching paths
                for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
                    if ( $k == $d
                        || ( $k != -$d && $V1[$limit_min_1 + $k] < $V1[$limit_plus_1 + $k] )
                    ) {
                        $x = $V1[$limit_min_1 + $k];
                    } else {
                        $x = $V1[$limit_plus_1 + $k] - 1;
                    }
 
                    $y = $x - $k - $delta;
 
                    $snake2 = 0;
                    while ( $x > 0 && $y > 0
                        && $from[$x + $bottoml1_min_1] === $to[$y + $bottoml2_min_1]
                    ) {
                        --$x;
                        --$y;
                        ++$snake2;
                    }
                    $V1[$limit + $k] = $x;
 
                    // check to see if we can cut down our diagonal range
                    if ( $x <= 0 ) {
                        $start_backward = $k + 1;
                        $value_to_add_backward = 0;
                    } elseif ( $y <= 0 && $end_backward > $k - 1 ) {
                        $end_backward = $k - 1;
                    }
                }
            }
        } else {
            for ( $d = 0; $d <= $limit; ++$d ) {
                $start_diag = max( $value_to_add_forward + $start_forward, -$d );
                $end_diag = min( $end_forward, $d );
                $value_to_add_forward = 1 - $value_to_add_forward;
 
                // compute forward furthest reaching paths
                for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
                    if ( $k == -$d
                        || ( $k < $d && $V0[$limit_min_1 + $k] < $V0[$limit_plus_1 + $k] )
                    ) {
                        $x = $V0[$limit_plus_1 + $k];
                    } else {
                        $x = $V0[$limit_min_1 + $k] + 1;
                    }
 
                    $absx = $snake0 = $x + $bottoml1;
                    $absy = $snake1 = $x - $k + $bottoml2;
 
                    while ( $absx < $maxabsx && $absy < $maxabsy && $from[$absx] === $to[$absy] ) {
                        ++$absx;
                        ++$absy;
                    }
                    $x = $absx - $bottoml1;
                    $snake2 = $absx - $snake0;
                    $V0[$limit + $k] = $x;
 
                    // check to see if we can cut down the diagonal range
                    if ( $x >= $N && $end_forward > $k - 1 ) {
                        $end_forward = $k - 1;
                    } elseif ( $absy - $bottoml2 >= $M ) {
                        $start_forward = $k + 1;
                        $value_to_add_forward = 0;
                    }
                }
 
                $start_diag = max( $value_to_add_backward + $start_backward, -$d );
                $end_diag = min( $end_backward, $d );
                $value_to_add_backward = 1 - $value_to_add_backward;
 
                // compute backward furthest reaching paths
                for ( $k = $start_diag; $k <= $end_diag; $k += 2 ) {
                    if ( $k == $d
                        || ( $k != -$d && $V1[$limit_min_1 + $k] < $V1[$limit_plus_1 + $k] )
                    ) {
                        $x = $V1[$limit_min_1 + $k];
                    } else {
                        $x = $V1[$limit_plus_1 + $k] - 1;
                    }
 
                    $y = $x - $k - $delta;
 
                    $snake2 = 0;
                    while ( $x > 0 && $y > 0
                        && $from[$x + $bottoml1_min_1] === $to[$y + $bottoml2_min_1]
                    ) {
                        --$x;
                        --$y;
                        ++$snake2;
                    }
                    $V1[$limit + $k] = $x;
 
                    if ( $k >= -$delta - $d && $k <= $d - $delta
                        && $x <= $V0[$limit + $k + $delta]
                    ) {
                        $snake0 = $bottoml1 + $x;
                        $snake1 = $bottoml2 + $y;
 
                        return 2 * $d;
                    }
 
                    // check to see if we can cut down our diagonal range
                    if ( $x <= 0 ) {
                        $start_backward = $k + 1;
                        $value_to_add_backward = 0;
                    } elseif ( $y <= 0 && $end_backward > $k - 1 ) {
                        $end_backward = $k - 1;
                    }
                }
            }
        }
        /*
         * computing the true LCS is too expensive, instead find the diagonal
         * with the most progress and pretend a midle snake of length 0 occurs
         * there.
         */
 
        $most_progress = self::findMostProgress( $M, $N, $limit, $V );
 
        $snake0 = $bottoml1 + $most_progress[0];
        $snake1 = $bottoml2 + $most_progress[1];
        $snake2 = 0;
        wfDebug( "Computing the LCS is too expensive. Using a heuristic." );
        $this->heuristicUsed = true;
 
        return 5; /*
        * HACK: since we didn't really finish the LCS computation
        * we don't really know the length of the SES. We don't do
        * anything with the result anyway, unless it's <=1. We know
        * for a fact SES > 1 so 5 is as good a number as any to
        * return here
        */
    }
 
    private static function findMostProgress( $M, $N, $limit, $V ) {
        $delta = $N - $M;
 
        if ( ( $M & 1 ) == ( $limit & 1 ) ) {
            $forward_start_diag = max( -$M, -$limit );
        } else {
            $forward_start_diag = max( 1 - $M, -$limit );
        }
 
        $forward_end_diag = min( $N, $limit );
 
        if ( ( $N & 1 ) == ( $limit & 1 ) ) {
            $backward_start_diag = max( -$N, -$limit );
        } else {
            $backward_start_diag = max( 1 - $N, -$limit );
        }
 
        $backward_end_diag = -min( $M, $limit );
 
        $temp = [ 0, 0, 0 ];
 
        $max_progress = array_fill( 0, ceil( max( $forward_end_diag - $forward_start_diag,
                $backward_end_diag - $backward_start_diag ) / 2 ), $temp );
        $num_progress = 0; // the 1st entry is current, it is initialized
        // with 0s
 
        // first search the forward diagonals
        for ( $k = $forward_start_diag; $k <= $forward_end_diag; $k += 2 ) {
            $x = $V[0][$limit + $k];
            $y = $x - $k;
            if ( $x > $N || $y > $M ) {
                continue;
            }
 
            $progress = $x + $y;
            if ( $progress > $max_progress[0][2] ) {
                $num_progress = 0;
                $max_progress[0][0] = $x;
                $max_progress[0][1] = $y;
                $max_progress[0][2] = $progress;
            } elseif ( $progress == $max_progress[0][2] ) {
                ++$num_progress;
                $max_progress[$num_progress][0] = $x;
                $max_progress[$num_progress][1] = $y;
                $max_progress[$num_progress][2] = $progress;
            }
        }
 
        $max_progress_forward = true; // initially the maximum
        // progress is in the forward
        // direction
 
        // now search the backward diagonals
        for ( $k = $backward_start_diag; $k <= $backward_end_diag; $k += 2 ) {
            $x = $V[1][$limit + $k];
            $y = $x - $k - $delta;
            if ( $x < 0 || $y < 0 ) {
                continue;
            }
 
            $progress = $N - $x + $M - $y;
            if ( $progress > $max_progress[0][2] ) {
                $num_progress = 0;
                $max_progress_forward = false;
                $max_progress[0][0] = $x;
                $max_progress[0][1] = $y;
                $max_progress[0][2] = $progress;
            } elseif ( $progress == $max_progress[0][2] && !$max_progress_forward ) {
                ++$num_progress;
                $max_progress[$num_progress][0] = $x;
                $max_progress[$num_progress][1] = $y;
                $max_progress[$num_progress][2] = $progress;
            }
        }
 
        // return the middle diagonal with maximal progress.
        return $max_progress[(int)floor( $num_progress / 2 )];
    }
 
    public function getLcsLength() {
        if ( $this->heuristicUsed && !$this->lcsLengthCorrectedForHeuristic ) {
            $this->lcsLengthCorrectedForHeuristic = true;
            $this->length = $this->m - array_sum( $this->added );
        }
 
        return $this->length;
    }
 
}