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];

								foreach ($matches as $y) {

										if (empty($this->in_seq[$y])) {

												$k = $this->_lcsPos($y);

												assert($k > 0);

												$ymids[$k] = $ymids[$k - 1];

												break;

										}

								}

								while (list($junk, $y) = each($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]');

						}

				}

		}