<?php

/**
 * 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 analyze.c, from GNU
 * diffutils-2.7, which can be found at:
 *   ftp://gnudist.gnu.org/pub/gnu/diffutils/diffutils-2.7.tar.gz
 *
 * Finally, some ideas (subdivision by NCHUNKS > 2, and some optimizations)
 * are my own.
 *
 * @author Geoffrey T. Dairiki
 * @access private
 */
class DiffEngine
{
	const USE_ASSERTS = false;

	private array $xchanged;
	private array $ychanged;
	private array $xind;
	private array $yind;
	private array $xv;
	private array $yv;
	private int $lcs;
	private array $seq;
	private array $in_seq;

	function diff ($from_lines, $to_lines)
	{
		$n_from = count($from_lines);
		$n_to = count($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++)
		{
			$xhash[$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
		$this->_shift_boundaries($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 < $n_to)
		{
			self::USE_ASSERTS && assert($yi < $n_to || $this->xchanged[$xi]);
			self::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)
			{
				$edits[] = new DiffOp_Copy($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)
			{
				$edits[] = new DiffOp_Change($delete, $add);
			}
			elseif ($delete)
			{
				$edits[] = new DiffOp_Delete($delete);
			}
			elseif ($add)
			{
				$edits[] = new DiffOp_Add($add);
			}
		}

		return $edits;
	}

	/* 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.
	 */
	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);
							self::USE_ASSERTS && assert($k > 0);
							$ymids[$k] = $ymids[$k - 1];
							$found_empty = true;
						}
					}
					else
					{
						if ($y > $this->seq[$k - 1])
						{
							self::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;
						}
						else if (empty($this->in_seq[$y]))
						{
							$k = $this->_lcs_pos($y);
							self::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];
	}

	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;
			}
		}

		self::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 $this->{x,y}changed[], by
	 * storing a 1 in the element for each line that is an insertion
	 * or deletion (ie. is not in the LCS).
	 *
	 * The subsequence of file 0 is [XOFF, XLIM) and likewise for file 1.
	 *
	 * Note that XLIM, YLIM are exclusive bounds.
	 * All line numbers are origin-0 and discarded lines are not counted.
	 */
	function _compareseq ($xoff, $xlim, $yoff, $ylim): void
	{
		// Slide down the bottom initial diagonal.
		while ($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).
	 */
	function _shift_boundaries ($lines, &$changed, $other_changed): void
	{
		$i = 0;
		$j = 0;

		self::USE_ASSERTS && 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])
			{
				self::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])
					{
						$start--;
					}
					self::USE_ASSERTS && assert($j > 0);
					while ($other_changed[--$j])
					{
						continue;
					}
					self::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++;
					}

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