From 9db190c7e736ec8d063187d4241b59feaf7dc2d1 Mon Sep 17 00:00:00 2001
From: Pierre Schmitz <pierre@archlinux.de>
Date: Wed, 22 Jun 2011 11:28:20 +0200
Subject: update to MediaWiki 1.17.0

---
 includes/diff/Diff.php                |  580 ----------
 includes/diff/DifferenceEngine.php    | 2002 +++++++++++++++------------------
 includes/diff/DifferenceInterface.php | 1024 -----------------
 includes/diff/WikiDiff.php            | 1241 ++++++++++++++++++++
 includes/diff/WikiDiff3.php           |  586 ++++++++++
 5 files changed, 2753 insertions(+), 2680 deletions(-)
 delete mode 100644 includes/diff/Diff.php
 delete mode 100644 includes/diff/DifferenceInterface.php
 create mode 100644 includes/diff/WikiDiff.php
 create mode 100644 includes/diff/WikiDiff3.php

(limited to 'includes/diff')

diff --git a/includes/diff/Diff.php b/includes/diff/Diff.php
deleted file mode 100644
index 538c2d83..00000000
--- a/includes/diff/Diff.php
+++ /dev/null
@@ -1,580 +0,0 @@
-<?php
-/* Copyright (C) 2008 Guy Van den Broeck <guy@guyvdb.eu>
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation; either version 2 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program; if not, write to the Free Software
- * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
- * or see http://www.gnu.org/
- */
-
-/**
- * This diff implementation is mainly lifted from the LCS algorithm of the Eclipse project which
- * in turn is based on Myers' "An O(ND) difference algorithm and its variations"
- * (http://citeseer.ist.psu.edu/myers86ond.html) with range compression (see Wu et al.'s
- * "An O(NP) Sequence Comparison Algorithm").
- *
- * This implementation supports an upper bound on the excution time.
- *
- * Complexity: O((M + N)D) worst case time, O(M + N + D^2) expected time, O(M + N) space
- *
- * @author Guy Van den Broeck
- * @ingroup DifferenceEngine
- */
-class WikiDiff3 {
-
-	//Input variables
-	private $from;
-	private $to;
-	private $m;
-	private $n;
-
-	private $tooLong;
-	private $powLimit;
-
-	//State variables
-	private $maxDifferences;
-	private $lcsLengthCorrectedForHeuristic = false;
-
-	//Output variables
-	public $length;
-	public $removed;
-	public $added;
-	public $heuristicUsed;
-
-	function __construct($tooLong = 2000000, $powLimit = 1.45){
-		$this->tooLong = $tooLong;
-		$this->powLimit = $powLimit;
-	}
-
-	public function diff(/*array*/ $from, /*array*/ $to){
-		//remember initial lengths
-		$m = sizeof($from);
-		$n = count($to);
-
-		$this->heuristicUsed = false;
-
-		//output
-		$removed = $m > 0 ? array_fill(0, $m, true) : array();
-		$added = $n > 0 ? array_fill(0, $n, true) : array();
-
-		//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 = array();
-
-		//remove tokens not in both sequences
-		$shared = array();
-		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);
-
-		$this->removed = $this->m > 0 ? array_fill(0, $this->m, true) : array();
-		$this->added = $this->n > 0 ? array_fill(0, $this->n, true) : array();
-
-		if ($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
-				$this->maxDifferences = floor(pow($this->maxDifferences, $this->powLimit - 1.0));
-				wfDebug("Limiting max number of differences to $this->maxDifferences\n");
-			}
-
-			/*
-			 * 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);
-			$V = array($temp, $temp);
-			$snake = array(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 = $added;
-	}
-
-	function diff_range($from_lines, $to_lines) {
-		// Diff and store locally
-		$this->diff($from_lines, $to_lines);
-		unset($from_lines, $to_lines);
-
-		$ranges = array();
-		$xi = $yi = 0;
-		while ($xi < $this->m || $yi < $this->n) {
-			// Matching "snake".
-			while ($xi < $this->m && $yi < $this->n
-					&& !$this->removed[$xi]
-					&& !$this->added[$yi]) {
-				++$xi;
-				++$yi;
-			}
-			// Find deletes & adds.
-			$xstart = $xi;
-			while ($xi < $this->m && $this->removed[$xi]) {
-				++$xi;
-			}
-
-			$ystart = $yi;
-			while ($yi < $this->n && $this->added[$yi]) {
-				++$yi;
-			}
-
-			if ($xi > $xstart || $yi > $ystart) {
-				$ranges[] = new RangeDifference($xstart, $xi,
-								$ystart, $yi);
-			}
-		}
-		return $ranges;
-	}
-
-	private function lcs_rec($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
-		$len = $snake[2];
-		$startx = $snake[0];
-		$starty = $snake[1];
-
-		// the middle snake is part of the LCS, store it
-		for ($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, $snake);
-		} else if ($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) == 1) {
-			$value_to_add_forward = 1;
-		} else {
-			$value_to_add_forward = 0;
-		}
-
-		if (($N & 1) == 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) == 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 = $k - 1;
-					} else if ($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;
-						$value_to_add_backward = 0;
-					} else if ($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 = $k - 1;
-					} else if ($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;
-						$value_to_add_backward = 0;
-					} else if ($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.\n");
-		$this->heuristicUsed = true;
-		return 5; /*
-		* HACK: since we didn't really finish the LCS computation
-		* we don't really know the length of the SES. We don't do
-		* anything with the result anyway, unless it's <=1. We know
-		* for a fact SES > 1 so 5 is as good a number as any to
-		* return here
-		*/
-	}
-
-	private static function findMostProgress($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 = array(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] = $progress;
-			} else if ($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] = $progress;
-			} else if ($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 $max_progress[floor($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;
-	}
-
-}
-
-/**
- * Alternative representation of a set of changes, by the index
- * ranges that are changed.
- * 
- * @ingroup DifferenceEngine
- */
-class RangeDifference {
-
-	public $leftstart;
-	public $leftend;
-	public $leftlength;
-
-	public $rightstart;
-	public $rightend;
-	public $rightlength;
-
-	function __construct($leftstart, $leftend, $rightstart, $rightend){
-		$this->leftstart = $leftstart;
-		$this->leftend = $leftend;
-		$this->leftlength = $leftend - $leftstart;
-		$this->rightstart = $rightstart;
-		$this->rightend = $rightend;
-		$this->rightlength = $rightend - $rightstart;
-	}
-}
diff --git a/includes/diff/DifferenceEngine.php b/includes/diff/DifferenceEngine.php
index 184d1fc2..edf35a92 100644
--- a/includes/diff/DifferenceEngine.php
+++ b/includes/diff/DifferenceEngine.php
@@ -1,1240 +1,1090 @@
 <?php
 /**
- * @defgroup DifferenceEngine DifferenceEngine
- */
-
-// A PHP diff engine for phpwiki. (Taken from phpwiki-1.3.3)
-//
-// Copyright (C) 2000, 2001 Geoffrey T. Dairiki <dairiki@dairiki.org>
-// You may copy this code freely under the conditions of the GPL.
-//
-
-define('USE_ASSERTS', function_exists('assert'));
-
-/**
- * @todo document
- * @private
+ * User interface for the difference engine
+ *
+ * @file
  * @ingroup DifferenceEngine
  */
-class _DiffOp {
-	var $type;
-	var $orig;
-	var $closing;
-
-	function reverse() {
-		trigger_error('pure virtual', E_USER_ERROR);
-	}
-
-	function norig() {
-		return $this->orig ? sizeof($this->orig) : 0;
-	}
-
-	function nclosing() {
-		return $this->closing ? sizeof($this->closing) : 0;
-	}
-}
 
 /**
- * @todo document
- * @private
- * @ingroup DifferenceEngine
+ * Constant to indicate diff cache compatibility.
+ * Bump this when changing the diff formatting in a way that
+ * fixes important bugs or such to force cached diff views to
+ * clear.
  */
-class _DiffOp_Copy extends _DiffOp {
-	var $type = 'copy';
-
-	function _DiffOp_Copy ($orig, $closing = false) {
-		if (!is_array($closing))
-		$closing = $orig;
-		$this->orig = $orig;
-		$this->closing = $closing;
-	}
-
-	function reverse() {
-		return new _DiffOp_Copy($this->closing, $this->orig);
-	}
-}
+define( 'MW_DIFF_VERSION', '1.11a' );
 
 /**
  * @todo document
- * @private
  * @ingroup DifferenceEngine
  */
-class _DiffOp_Delete extends _DiffOp {
-	var $type = 'delete';
+class DifferenceEngine {
+	/**#@+
+	 * @private
+	 */
+	var $mOldid, $mNewid, $mTitle;
+	var $mOldtitle, $mNewtitle, $mPagetitle;
+	var $mOldtext, $mNewtext;
+	var $mOldPage, $mNewPage;
+	var $mRcidMarkPatrolled;
+	var $mOldRev, $mNewRev;
+	var $mRevisionsLoaded = false; // Have the revisions been loaded
+	var $mTextLoaded = 0; // How many text blobs have been loaded, 0, 1 or 2?
+	var $mCacheHit = false; // Was the diff fetched from cache?
 
-	function _DiffOp_Delete ($lines) {
-		$this->orig = $lines;
-		$this->closing = false;
-	}
+	/**
+	 * Set this to true to add debug info to the HTML output.
+	 * Warning: this may cause RSS readers to spuriously mark articles as "new"
+	 * (bug 20601)
+	 */
+	var $enableDebugComment = false;
 
-	function reverse() {
-		return new _DiffOp_Add($this->orig);
-	}
-}
+	// If true, line X is not displayed when X is 1, for example to increase
+	// readability and conserve space with many small diffs.
+	protected $mReducedLineNumbers = false;
 
-/**
- * @todo document
- * @private
- * @ingroup DifferenceEngine
- */
-class _DiffOp_Add extends _DiffOp {
-	var $type = 'add';
+	protected $unhide = false; # show rev_deleted content if allowed
+	/**#@-*/
 
-	function _DiffOp_Add ($lines) {
-		$this->closing = $lines;
-		$this->orig = false;
+	/**
+	 * Constructor
+	 * @param $titleObj Title object that the diff is associated with
+	 * @param $old Integer: old ID we want to show and diff with.
+	 * @param $new String: either 'prev' or 'next'.
+	 * @param $rcid Integer: ??? FIXME (default 0)
+	 * @param $refreshCache boolean If set, refreshes the diff cache
+	 * @param $unhide boolean If set, allow viewing deleted revs
+	 */
+	function __construct( $titleObj = null, $old = 0, $new = 0, $rcid = 0,
+		$refreshCache = false, $unhide = false )
+	{
+		if ( $titleObj ) {
+			$this->mTitle = $titleObj;
+		} else {
+			global $wgTitle;
+			$this->mTitle = $wgTitle;
+		}
+		wfDebug( "DifferenceEngine old '$old' new '$new' rcid '$rcid'\n" );
+
+		if ( 'prev' === $new ) {
+			# Show diff between revision $old and the previous one.
+			# Get previous one from DB.
+			$this->mNewid = intval( $old );
+			$this->mOldid = $this->mTitle->getPreviousRevisionID( $this->mNewid );
+		} elseif ( 'next' === $new ) {
+			# Show diff between revision $old and the next one.
+			# Get next one from DB.
+			$this->mOldid = intval( $old );
+			$this->mNewid = $this->mTitle->getNextRevisionID( $this->mOldid );
+			if ( false === $this->mNewid ) {
+				# if no result, NewId points to the newest old revision. The only newer
+				# revision is cur, which is "0".
+				$this->mNewid = 0;
+			}
+		} else {
+			$this->mOldid = intval( $old );
+			$this->mNewid = intval( $new );
+			wfRunHooks( 'NewDifferenceEngine', array( &$titleObj, &$this->mOldid, &$this->mNewid, $old, $new ) );
+		}
+		$this->mRcidMarkPatrolled = intval( $rcid );  # force it to be an integer
+		$this->mRefreshCache = $refreshCache;
+		$this->unhide = $unhide;
 	}
 
-	function reverse() {
-		return new _DiffOp_Delete($this->closing);
+	function setReducedLineNumbers( $value = true ) {
+		$this->mReducedLineNumbers = $value;
 	}
-}
 
-/**
- * @todo document
- * @private
- * @ingroup DifferenceEngine
- */
-class _DiffOp_Change extends _DiffOp {
-	var $type = 'change';
-
-	function _DiffOp_Change ($orig, $closing) {
-		$this->orig = $orig;
-		$this->closing = $closing;
+	function getTitle() {
+		return $this->mTitle;
 	}
 
-	function reverse() {
-		return new _DiffOp_Change($this->closing, $this->orig);
+	function wasCacheHit() {
+		return $this->mCacheHit;
 	}
-}
 
-/**
- * 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 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
- *
- * closingly, some ideas (subdivision by NCHUNKS > 2, and some optimizations)
- * are my own.
- *
- * Line length limits for robustness added by Tim Starling, 2005-08-31
- * Alternative implementation added by Guy Van den Broeck, 2008-07-30
- *
- * @author Geoffrey T. Dairiki, Tim Starling, Guy Van den Broeck
- * @private
- * @ingroup DifferenceEngine
- */
-class _DiffEngine {
+	function getOldid() {
+		return $this->mOldid;
+	}
 
-	const MAX_XREF_LENGTH =  10000;
+	function getNewid() {
+		return $this->mNewid;
+	}
 
-	function diff ($from_lines, $to_lines){
+	function showDiffPage( $diffOnly = false ) {
+		global $wgUser, $wgOut, $wgUseExternalEditor, $wgUseRCPatrol;
 		wfProfileIn( __METHOD__ );
 
-		// Diff and store locally
-		$this->diff_local($from_lines, $to_lines);
-
-		// 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.
-		$n_from = sizeof($from_lines);
-		$n_to = sizeof($to_lines);
-
-		$edits = array();
-		$xi = $yi = 0;
-		while ($xi < $n_from || $yi < $n_to) {
-			USE_ASSERTS && assert($yi < $n_to || $this->xchanged[$xi]);
-			USE_ASSERTS && 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)
-			$edits[] = new _DiffOp_Copy($copy);
+		# Allow frames except in certain special cases
+		$wgOut->allowClickjacking();
+
+		# If external diffs are enabled both globally and for the user,
+		# we'll use the application/x-external-editor interface to call
+		# an external diff tool like kompare, kdiff3, etc.
+		if ( $wgUseExternalEditor && $wgUser->getOption( 'externaldiff' ) ) {
+			global $wgInputEncoding, $wgServer, $wgScript, $wgLang;
+			$wgOut->disable();
+			header ( "Content-type: application/x-external-editor; charset=" . $wgInputEncoding );
+			$url1 = $this->mTitle->getFullURL( array(
+				'action' => 'raw',
+				'oldid' => $this->mOldid
+			) );
+			$url2 = $this->mTitle->getFullURL( array(
+				'action' => 'raw',
+				'oldid' => $this->mNewid
+			) );
+			$special = $wgLang->getNsText( NS_SPECIAL );
+			$control = <<<CONTROL
+			[Process]
+			Type=Diff text
+			Engine=MediaWiki
+			Script={$wgServer}{$wgScript}
+			Special namespace={$special}
+
+			[File]
+			Extension=wiki
+			URL=$url1
+
+			[File 2]
+			Extension=wiki
+			URL=$url2
+CONTROL;
+			echo( $control );
+
+			wfProfileOut( __METHOD__ );
+			return;
+		}
 
-			// Find deletes & adds.
-			$delete = array();
-			while ($xi < $n_from && $this->xchanged[$xi])
-			$delete[] = $from_lines[$xi++];
+		$wgOut->setArticleFlag( false );
+		if ( !$this->loadRevisionData() ) {
+			$t = $this->mTitle->getPrefixedText();
+			$d = wfMsgExt( 'missingarticle-diff', array( 'escape' ), $this->mOldid, $this->mNewid );
+			$wgOut->setPagetitle( wfMsg( 'errorpagetitle' ) );
+			$wgOut->addWikiMsg( 'missing-article', "<nowiki>$t</nowiki>", $d );
+			wfProfileOut( __METHOD__ );
+			return;
+		}
 
-			$add = array();
-			while ($yi < $n_to && $this->ychanged[$yi])
-			$add[] = $to_lines[$yi++];
+		wfRunHooks( 'DiffViewHeader', array( $this, $this->mOldRev, $this->mNewRev ) );
 
-			if ($delete && $add)
-			$edits[] = new _DiffOp_Change($delete, $add);
-			elseif ($delete)
-			$edits[] = new _DiffOp_Delete($delete);
-			elseif ($add)
-			$edits[] = new _DiffOp_Add($add);
+		if ( $this->mNewRev->isCurrent() ) {
+			$wgOut->setArticleFlag( true );
 		}
-		wfProfileOut( __METHOD__ );
-		return $edits;
-	}
-
-	function diff_local ($from_lines, $to_lines) {
-		global $wgExternalDiffEngine;
-		wfProfileIn( __METHOD__);
-
-		if($wgExternalDiffEngine == 'wikidiff3'){
-			// wikidiff3
-			$wikidiff3 = new WikiDiff3();
-			$wikidiff3->diff($from_lines, $to_lines);
-			$this->xchanged = $wikidiff3->removed;
-			$this->ychanged = $wikidiff3->added;
-			unset($wikidiff3);
-		}else{
-			// old diff
-			$n_from = sizeof($from_lines);
-			$n_to = sizeof($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++) {
-				$xhash[$this->_line_hash($from_lines[$xi])] = 1;
-			}
+		# mOldid is false if the difference engine is called with a "vague" query for
+		# a diff between a version V and its previous version V' AND the version V
+		# is the first version of that article. In that case, V' does not exist.
+		if ( $this->mOldid === false ) {
+			$this->showFirstRevision();
+			$this->renderNewRevision();  // should we respect $diffOnly here or not?
+			wfProfileOut( __METHOD__ );
+			return;
+		}
 
-			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;
-			}
+		$wgOut->suppressQuickbar();
 
-			// Find the LCS.
-			$this->_compareseq(0, sizeof($this->xv), 0, sizeof($this->yv));
+		$oldTitle = $this->mOldPage->getPrefixedText();
+		$newTitle = $this->mNewPage->getPrefixedText();
+		if ( $oldTitle == $newTitle ) {
+			$wgOut->setPageTitle( $newTitle );
+		} else {
+			$wgOut->setPageTitle( $oldTitle . ', ' . $newTitle );
+		}
+		if ( $this->mNewPage->equals( $this->mOldPage ) ) {
+			$wgOut->setSubtitle( wfMsgExt( 'difference', array( 'parseinline' ) ) );
+		} else {
+			$wgOut->setSubtitle( wfMsgExt( 'difference-multipage', array( 'parseinline' ) ) );
+		}
+		$wgOut->setRobotPolicy( 'noindex,nofollow' );
+
+		if ( !$this->mOldPage->userCanRead() || !$this->mNewPage->userCanRead() ) {
+			$wgOut->loginToUse();
+			$wgOut->output();
+			$wgOut->disable();
+			wfProfileOut( __METHOD__ );
+			return;
 		}
-		wfProfileOut( __METHOD__ );
-	}
 
-	/**
-	 * Returns the whole line if it's small enough, or the MD5 hash otherwise
-	 */
-	function _line_hash( $line ) {
-		if ( strlen( $line ) > self::MAX_XREF_LENGTH ) {
-			return md5( $line );
+		$sk = $wgUser->getSkin();
+
+		// Check if page is editable
+		$editable = $this->mNewRev->getTitle()->userCan( 'edit' );
+		if ( $editable && $this->mNewRev->isCurrent() && $wgUser->isAllowed( 'rollback' ) ) {
+			$wgOut->preventClickjacking();
+			$rollback = '&#160;&#160;&#160;' . $sk->generateRollback( $this->mNewRev );
 		} else {
-			return $line;
+			$rollback = '';
 		}
-	}
 
-	/* 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.
-			$flip = true;
-			list ($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];
-				reset($matches);
-				while (list ($junk, $y) = each($matches))
-				if (empty($this->in_seq[$y])) {
-					$k = $this->_lcs_pos($y);
-					USE_ASSERTS && assert($k > 0);
-					$ymids[$k] = $ymids[$k-1];
-					break;
-				}
-				while (list ( /* $junk */, $y) = each($matches)) {
-					if ($y > $this->seq[$k-1]) {
-						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);
-						USE_ASSERTS && assert($k > 0);
-						$ymids[$k] = $ymids[$k-1];
-					}
+		// Prepare a change patrol link, if applicable
+		if ( $wgUseRCPatrol && $this->mTitle->userCan( 'patrol' ) ) {
+			// If we've been given an explicit change identifier, use it; saves time
+			if ( $this->mRcidMarkPatrolled ) {
+				$rcid = $this->mRcidMarkPatrolled;
+				$rc = RecentChange::newFromId( $rcid );
+				// Already patrolled?
+				$rcid = is_object( $rc ) && !$rc->getAttribute( 'rc_patrolled' ) ? $rcid : 0;
+			} else {
+				// Look for an unpatrolled change corresponding to this diff
+				$db = wfGetDB( DB_SLAVE );
+				$change = RecentChange::newFromConds(
+					array(
+					// Redundant user,timestamp condition so we can use the existing index
+						'rc_user_text'  => $this->mNewRev->getRawUserText(),
+						'rc_timestamp'  => $db->timestamp( $this->mNewRev->getTimestamp() ),
+						'rc_this_oldid' => $this->mNewid,
+						'rc_last_oldid' => $this->mOldid,
+						'rc_patrolled'  => 0
+					),
+					__METHOD__
+				);
+				if ( $change instanceof RecentChange ) {
+					$rcid = $change->mAttribs['rc_id'];
+					$this->mRcidMarkPatrolled = $rcid;
+				} else {
+					// None found
+					$rcid = 0;
 				}
 			}
+			// Build the link
+			if ( $rcid ) {
+				$wgOut->preventClickjacking();
+				$token = $wgUser->editToken( $rcid );
+				$patrol = ' <span class="patrollink">[' . $sk->link(
+					$this->mTitle,
+					wfMsgHtml( 'markaspatrolleddiff' ),
+					array(),
+					array(
+						'action' => 'markpatrolled',
+						'rcid' => $rcid,
+						'token' => $token,
+					),
+					array(
+						'known',
+						'noclasses'
+					)
+				) . ']</span>';
+			} else {
+				$patrol = '';
+			}
+		} else {
+			$patrol = '';
 		}
 
-		$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);
+		# Carry over 'diffonly' param via navigation links
+		if ( $diffOnly != $wgUser->getBoolOption( 'diffonly' ) ) {
+			$query['diffonly'] = $diffOnly;
 		}
-		$seps[] = $flip ? array($ylim, $xlim) : array($xlim, $ylim);
 
-		return array($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;
+		# Make "previous revision link"
+		$query['diff'] = 'prev';
+		$query['oldid'] = $this->mOldid;
+		# Cascade unhide param in links for easy deletion browsing
+		if ( $this->unhide ) {
+			$query['unhide'] = 1;
+		}
+		if ( !$this->mOldRev->getPrevious() ) {
+			$prevlink = '&#160;';
+		} else {
+			$prevlink = $sk->link(
+				$this->mTitle,
+				wfMsgHtml( 'previousdiff' ),
+				array(
+					'id' => 'differences-prevlink'
+				),
+				$query,
+				array(
+					'known',
+					'noclasses'
+				)
+			);
 		}
 
-		$beg = 1;
-		while ($beg < $end) {
-			$mid = (int)(($beg + $end) / 2);
-			if ( $ypos > $this->seq[$mid] )
-			$beg = $mid + 1;
-			else
-			$end = $mid;
+		# Make "next revision link"
+		$query['diff'] = 'next';
+		$query['oldid'] = $this->mNewid;
+		# Skip next link on the top revision
+		if ( $this->mNewRev->isCurrent() ) {
+			$nextlink = '&#160;';
+		} else {
+			$nextlink = $sk->link(
+				$this->mTitle,
+				wfMsgHtml( 'nextdiff' ),
+				array(
+					'id' => 'differences-nextlink'
+				),
+				$query,
+				array(
+					'known',
+					'noclasses'
+				)
+			);
 		}
 
-		USE_ASSERTS && assert($ypos != $this->seq[$end]);
+		$oldminor = '';
+		$newminor = '';
 
-		$this->in_seq[$this->seq[$end]] = false;
-		$this->seq[$end] = $ypos;
-		$this->in_seq[$ypos] = 1;
-		return $end;
-	}
+		if ( $this->mOldRev->isMinor() ) {
+			$oldminor = ChangesList::flag( 'minor' );
+		}
+		if ( $this->mNewRev->isMinor() ) {
+			$newminor = ChangesList::flag( 'minor' );
+		}
 
-	/* 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, $ylim) {
-		// Slide down the bottom initial diagonal.
-		while ($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;
+		# Handle RevisionDelete links...
+		$ldel = $this->revisionDeleteLink( $this->mOldRev );
+		$rdel = $this->revisionDeleteLink( $this->mNewRev );
+
+		$oldHeader = '<div id="mw-diff-otitle1"><strong>' . $this->mOldtitle . '</strong></div>' .
+			'<div id="mw-diff-otitle2">' .
+				$sk->revUserTools( $this->mOldRev, !$this->unhide ) . '</div>' .
+			'<div id="mw-diff-otitle3">' . $oldminor .
+				$sk->revComment( $this->mOldRev, !$diffOnly, !$this->unhide ) . $ldel . '</div>' .
+			'<div id="mw-diff-otitle4">' . $prevlink . '</div>';
+		$newHeader = '<div id="mw-diff-ntitle1"><strong>' . $this->mNewtitle . '</strong></div>' .
+			'<div id="mw-diff-ntitle2">' . $sk->revUserTools( $this->mNewRev, !$this->unhide ) .
+				" $rollback</div>" .
+			'<div id="mw-diff-ntitle3">' . $newminor .
+				$sk->revComment( $this->mNewRev, !$diffOnly, !$this->unhide ) . $rdel . '</div>' .
+			'<div id="mw-diff-ntitle4">' . $nextlink . $patrol . '</div>';
+
+		# Check if this user can see the revisions
+		$allowed = $this->mOldRev->userCan( Revision::DELETED_TEXT )
+			&& $this->mNewRev->userCan( Revision::DELETED_TEXT );
+		# Check if one of the revisions is deleted/suppressed
+		$deleted = $suppressed = false;
+		if ( $this->mOldRev->isDeleted( Revision::DELETED_TEXT ) ) {
+			$deleted = true; // old revisions text is hidden
+			if ( $this->mOldRev->isDeleted( Revision::DELETED_RESTRICTED ) )
+				$suppressed = true; // also suppressed
+		}
+		if ( $this->mNewRev->isDeleted( Revision::DELETED_TEXT ) ) {
+			$deleted = true; // new revisions text is hidden
+			if ( $this->mNewRev->isDeleted( Revision::DELETED_RESTRICTED ) )
+				$suppressed = true; // also suppressed
+		}
+		# If the diff cannot be shown due to a deleted revision, then output
+		# the diff header and links to unhide (if available)...
+		if ( $deleted && ( !$this->unhide || !$allowed ) ) {
+			$this->showDiffStyle();
+			$multi = $this->getMultiNotice();
+			$wgOut->addHTML( $this->addHeader( '', $oldHeader, $newHeader, $multi ) );
+			if ( !$allowed ) {
+				$msg = $suppressed ? 'rev-suppressed-no-diff' : 'rev-deleted-no-diff';
+				# Give explanation for why revision is not visible
+				$wgOut->wrapWikiMsg( "<div class='mw-warning plainlinks'>\n$1\n</div>\n",
+					array( $msg ) );
+			} else {
+				# Give explanation and add a link to view the diff...
+				$link = $this->mTitle->getFullUrl( array(
+					'diff' => $this->mNewid,
+					'oldid' => $this->mOldid,
+					'unhide' => 1
+				) );
+				$msg = $suppressed ? 'rev-suppressed-unhide-diff' : 'rev-deleted-unhide-diff';
+				$wgOut->wrapWikiMsg( "<div class='mw-warning plainlinks'>\n$1\n</div>\n", array( $msg, $link ) );
+			}
+		# Otherwise, output a regular diff...
 		} else {
-			// Use the partitions to split this problem into subproblems.
-			reset($seps);
-			$pt1 = $seps[0];
-			while ($pt2 = next($seps)) {
-				$this->_compareseq ($pt1[0], $pt2[0], $pt1[1], $pt2[1]);
-				$pt1 = $pt2;
+			# Add deletion notice if the user is viewing deleted content
+			$notice = '';
+			if ( $deleted ) {
+				$msg = $suppressed ? 'rev-suppressed-diff-view' : 'rev-deleted-diff-view';
+				$notice = "<div class='mw-warning plainlinks'>\n" . wfMsgExt( $msg, 'parseinline' ) . "</div>\n";
+			}
+			$this->showDiff( $oldHeader, $newHeader, $notice );
+			if ( !$diffOnly ) {
+				$this->renderNewRevision();
 			}
 		}
+		wfProfileOut( __METHOD__ );
 	}
 
-	/* 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) {
-		wfProfileIn( __METHOD__ );
-		$i = 0;
-		$j = 0;
-
-		USE_ASSERTS && assert('sizeof($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]) {
-				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])
-					$start--;
-					USE_ASSERTS && assert('$j > 0');
-					while ($other_changed[--$j])
-					continue;
-					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])
-					$i++;
-
-					USE_ASSERTS && assert('$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;
-				USE_ASSERTS && assert('$j > 0');
-				while ($other_changed[--$j])
-				continue;
-				USE_ASSERTS && assert('$j >= 0 && !$other_changed[$j]');
+	protected function revisionDeleteLink( $rev ) {
+		global $wgUser;
+		$link = '';
+		$canHide = $wgUser->isAllowed( 'deleterevision' );
+		// Show del/undel link if:
+		// (a) the user can delete revisions, or
+		// (b) the user can view deleted revision *and* this one is deleted
+		if ( $canHide || ( $rev->getVisibility() && $wgUser->isAllowed( 'deletedhistory' ) ) ) {
+			$sk = $wgUser->getSkin();
+			if ( !$rev->userCan( Revision::DELETED_RESTRICTED ) ) {
+				$link = $sk->revDeleteLinkDisabled( $canHide ); // revision was hidden from sysops
+			} else {
+				$query = array(
+					'type' 	 => 'revision',
+					'target' => $rev->mTitle->getPrefixedDbkey(),
+					'ids' 	 => $rev->getId()
+				);
+				$link = $sk->revDeleteLink( $query,
+					$rev->isDeleted( Revision::DELETED_RESTRICTED ), $canHide );
 			}
+			$link = '&#160;&#160;&#160;' . $link . ' ';
 		}
-		wfProfileOut( __METHOD__ );
+		return $link;
 	}
-}
-
-/**
- * Class representing a 'diff' between two sequences of strings.
- * @todo document
- * @private
- * @ingroup DifferenceEngine
- */
-class Diff
-{
-	var $edits;
 
 	/**
-	 * Constructor.
-	 * Computes diff between sequences of strings.
-	 *
-	 * @param $from_lines array An array of strings.
-	 *		  (Typically these are lines from a file.)
-	 * @param $to_lines array An array of strings.
+	 * Show the new revision of the page.
 	 */
-	function Diff($from_lines, $to_lines) {
-		$eng = new _DiffEngine;
-		$this->edits = $eng->diff($from_lines, $to_lines);
-		//$this->_check($from_lines, $to_lines);
-	}
+	function renderNewRevision() {
+		global $wgOut, $wgUser;
+		wfProfileIn( __METHOD__ );
 
-	/**
-	 * Compute reversed Diff.
-	 *
-	 * SYNOPSIS:
-	 *
-	 *	$diff = new Diff($lines1, $lines2);
-	 *	$rev = $diff->reverse();
-	 * @return object A Diff object representing the inverse of the
-	 *				  original diff.
-	 */
-	function reverse () {
-		$rev = $this;
-		$rev->edits = array();
-		foreach ($this->edits as $edit) {
-			$rev->edits[] = $edit->reverse();
+		$wgOut->addHTML( "<hr /><h2>{$this->mPagetitle}</h2>\n" );
+		# Add deleted rev tag if needed
+		if ( !$this->mNewRev->userCan( Revision::DELETED_TEXT ) ) {
+			$wgOut->wrapWikiMsg( "<div class='mw-warning plainlinks'>\n$1\n</div>\n", 'rev-deleted-text-permission' );
+		} else if ( $this->mNewRev->isDeleted( Revision::DELETED_TEXT ) ) {
+			$wgOut->wrapWikiMsg( "<div class='mw-warning plainlinks'>\n$1\n</div>\n", 'rev-deleted-text-view' );
 		}
-		return $rev;
-	}
 
-	/**
-	 * Check for empty diff.
-	 *
-	 * @return bool True iff two sequences were identical.
-	 */
-	function isEmpty () {
-		foreach ($this->edits as $edit) {
-			if ($edit->type != 'copy')
-			return false;
+		$pCache = true;
+		if ( !$this->mNewRev->isCurrent() ) {
+			$oldEditSectionSetting = $wgOut->parserOptions()->setEditSection( false );
+			$pCache = false;
 		}
-		return true;
-	}
 
-	/**
-	 * Compute the length of the Longest Common Subsequence (LCS).
-	 *
-	 * This is mostly for diagnostic purposed.
-	 *
-	 * @return int The length of the LCS.
-	 */
-	function lcs () {
-		$lcs = 0;
-		foreach ($this->edits as $edit) {
-			if ($edit->type == 'copy')
-			$lcs += sizeof($edit->orig);
+		$this->loadNewText();
+		if ( is_object( $this->mNewRev ) ) {
+			$wgOut->setRevisionId( $this->mNewRev->getId() );
 		}
-		return $lcs;
-	}
-
-	/**
-	 * Get the original set of lines.
-	 *
-	 * This reconstructs the $from_lines parameter passed to the
-	 * constructor.
-	 *
-	 * @return array The original sequence of strings.
-	 */
-	function orig() {
-		$lines = array();
 
-		foreach ($this->edits as $edit) {
-			if ($edit->orig)
-			array_splice($lines, sizeof($lines), 0, $edit->orig);
+		if ( $this->mTitle->isCssJsSubpage() || $this->mTitle->isCssOrJsPage() ) {
+			// Stolen from Article::view --AG 2007-10-11
+			// Give hooks a chance to customise the output
+			if ( wfRunHooks( 'ShowRawCssJs', array( $this->mNewtext, $this->mTitle, $wgOut ) ) ) {
+				// Wrap the whole lot in a <pre> and don't parse
+				$m = array();
+				preg_match( '!\.(css|js)$!u', $this->mTitle->getText(), $m );
+				$wgOut->addHTML( "<pre class=\"mw-code mw-{$m[1]}\" dir=\"ltr\">\n" );
+				$wgOut->addHTML( htmlspecialchars( $this->mNewtext ) );
+				$wgOut->addHTML( "\n</pre>\n" );
+			}
+		} elseif ( wfRunHooks( 'ArticleContentOnDiff', array( $this, $wgOut ) ) ) {
+			if ( $pCache ) {
+				$article = new Article( $this->mTitle, 0 );
+				$pOutput = ParserCache::singleton()->get( $article, $wgOut->parserOptions() );
+				if ( $pOutput ) {
+					$wgOut->addParserOutput( $pOutput );
+				} else {
+					$article->doViewParse();
+				}
+			} else {
+				$wgOut->addWikiTextTidy( $this->mNewtext );
+			}
 		}
-		return $lines;
-	}
 
-	/**
-	 * Get the closing set of lines.
-	 *
-	 * This reconstructs the $to_lines parameter passed to the
-	 * constructor.
-	 *
-	 * @return array The sequence of strings.
-	 */
-	function closing() {
-		$lines = array();
+		if ( is_object( $this->mNewRev ) && !$this->mNewRev->isCurrent() ) {
+			$wgOut->parserOptions()->setEditSection( $oldEditSectionSetting );
+		}
 
-		foreach ($this->edits as $edit) {
-			if ($edit->closing)
-			array_splice($lines, sizeof($lines), 0, $edit->closing);
+		# Add redundant patrol link on bottom...
+		if ( $this->mRcidMarkPatrolled && $this->mTitle->quickUserCan( 'patrol' ) ) {
+			$sk = $wgUser->getSkin();
+			$token = $wgUser->editToken( $this->mRcidMarkPatrolled );
+			$wgOut->preventClickjacking();
+			$wgOut->addHTML(
+				"<div class='patrollink'>[" . $sk->link(
+					$this->mTitle,
+					wfMsgHtml( 'markaspatrolleddiff' ),
+					array(),
+					array(
+						'action' => 'markpatrolled',
+						'rcid' => $this->mRcidMarkPatrolled,
+						'token' => $token,
+					)
+				) . ']</div>'
+			 );
 		}
-		return $lines;
+
+		wfProfileOut( __METHOD__ );
 	}
 
 	/**
-	 * Check a Diff for validity.
-	 *
-	 * This is here only for debugging purposes.
+	 * Show the first revision of an article. Uses normal diff headers in
+	 * contrast to normal "old revision" display style.
 	 */
-	function _check ($from_lines, $to_lines) {
+	function showFirstRevision() {
+		global $wgOut, $wgUser;
 		wfProfileIn( __METHOD__ );
-		if (serialize($from_lines) != serialize($this->orig()))
-		trigger_error("Reconstructed original doesn't match", E_USER_ERROR);
-		if (serialize($to_lines) != serialize($this->closing()))
-		trigger_error("Reconstructed closing doesn't match", E_USER_ERROR);
 
-		$rev = $this->reverse();
-		if (serialize($to_lines) != serialize($rev->orig()))
-		trigger_error("Reversed original doesn't match", E_USER_ERROR);
-		if (serialize($from_lines) != serialize($rev->closing()))
-		trigger_error("Reversed closing doesn't match", E_USER_ERROR);
+		# Get article text from the DB
+		#
+		if ( ! $this->loadNewText() ) {
+			$t = $this->mTitle->getPrefixedText();
+			$d = wfMsgExt( 'missingarticle-diff', array( 'escape' ), $this->mOldid, $this->mNewid );
+			$wgOut->setPagetitle( wfMsg( 'errorpagetitle' ) );
+			$wgOut->addWikiMsg( 'missing-article', "<nowiki>$t</nowiki>", $d );
+			wfProfileOut( __METHOD__ );
+			return;
+		}
+		if ( $this->mNewRev->isCurrent() ) {
+			$wgOut->setArticleFlag( true );
+		}
+
+		# Check if user is allowed to look at this page. If not, bail out.
+		#
+		if ( !$this->mTitle->userCanRead() ) {
+			$wgOut->loginToUse();
+			$wgOut->output();
+			wfProfileOut( __METHOD__ );
+			throw new MWException( "Permission Error: you do not have access to view this page" );
+		}
 
+		# Prepare the header box
+		#
+		$sk = $wgUser->getSkin();
 
-		$prevtype = 'none';
-		foreach ($this->edits as $edit) {
-			if ( $prevtype == $edit->type )
-			trigger_error("Edit sequence is non-optimal", E_USER_ERROR);
-			$prevtype = $edit->type;
+		$next = $this->mTitle->getNextRevisionID( $this->mNewid );
+		if ( !$next ) {
+			$nextlink = '';
+		} else {
+			$nextlink = '<br />' . $sk->link(
+				$this->mTitle,
+				wfMsgHtml( 'nextdiff' ),
+				array(
+					'id' => 'differences-nextlink'
+				),
+				array(
+					'diff' => 'next',
+					'oldid' => $this->mNewid,
+				),
+				array(
+					'known',
+					'noclasses'
+				)
+			);
 		}
+		$header = "<div class=\"firstrevisionheader\" style=\"text-align: center\">" .
+			$sk->revUserTools( $this->mNewRev ) . "<br />" . $sk->revComment( $this->mNewRev ) . $nextlink . "</div>\n";
+
+		$wgOut->addHTML( $header );
+
+		$wgOut->setSubtitle( wfMsgExt( 'difference', array( 'parseinline' ) ) );
+		$wgOut->setRobotPolicy( 'noindex,nofollow' );
 
-		$lcs = $this->lcs();
-		trigger_error('Diff okay: LCS = '.$lcs, E_USER_NOTICE);
 		wfProfileOut( __METHOD__ );
 	}
-}
 
-/**
- * @todo document, bad name.
- * @private
- * @ingroup DifferenceEngine
- */
-class MappedDiff extends Diff
-{
 	/**
-	 * Constructor.
-	 *
-	 * Computes diff between sequences of strings.
-	 *
-	 * This can be used to compute things like
-	 * case-insensitve diffs, or diffs which ignore
-	 * changes in white-space.
-	 *
-	 * @param $from_lines array An array of strings.
-	 *	(Typically these are lines from a file.)
-	 *
-	 * @param $to_lines array An array of strings.
-	 *
-	 * @param $mapped_from_lines array This array should
-	 *	have the same size number of elements as $from_lines.
-	 *	The elements in $mapped_from_lines and
-	 *	$mapped_to_lines are what is actually compared
-	 *	when computing the diff.
-	 *
-	 * @param $mapped_to_lines array This array should
-	 *	have the same number of elements as $to_lines.
+	 * Get the diff text, send it to $wgOut
+	 * Returns false if the diff could not be generated, otherwise returns true
 	 */
-	function MappedDiff($from_lines, $to_lines,
-	$mapped_from_lines, $mapped_to_lines) {
-		wfProfileIn( __METHOD__ );
-
-		assert(sizeof($from_lines) == sizeof($mapped_from_lines));
-		assert(sizeof($to_lines) == sizeof($mapped_to_lines));
-
-		$this->Diff($mapped_from_lines, $mapped_to_lines);
-
-		$xi = $yi = 0;
-		for ($i = 0; $i < sizeof($this->edits); $i++) {
-			$orig = &$this->edits[$i]->orig;
-			if (is_array($orig)) {
-				$orig = array_slice($from_lines, $xi, sizeof($orig));
-				$xi += sizeof($orig);
-			}
-
-			$closing = &$this->edits[$i]->closing;
-			if (is_array($closing)) {
-				$closing = array_slice($to_lines, $yi, sizeof($closing));
-				$yi += sizeof($closing);
-			}
+	function showDiff( $otitle, $ntitle, $notice = '' ) {
+		global $wgOut;
+		$diff = $this->getDiff( $otitle, $ntitle, $notice );
+		if ( $diff === false ) {
+			$wgOut->addWikiMsg( 'missing-article', "<nowiki>(fixme, bug)</nowiki>", '' );
+			return false;
+		} else {
+			$this->showDiffStyle();
+			$wgOut->addHTML( $diff );
+			return true;
 		}
-		wfProfileOut( __METHOD__ );
 	}
-}
 
-/**
- * A class to format Diffs
- *
- * This class formats the diff in classic diff format.
- * It is intended that this class be customized via inheritance,
- * to obtain fancier outputs.
- * @todo document
- * @private
- * @ingroup DifferenceEngine
- */
-class DiffFormatter {
 	/**
-	 * Number of leading context "lines" to preserve.
-	 *
-	 * This should be left at zero for this class, but subclasses
-	 * may want to set this to other values.
+	 * Add style sheets and supporting JS for diff display.
 	 */
-	var $leading_context_lines = 0;
+	function showDiffStyle() {
+		global $wgOut;
+		$wgOut->addModuleStyles( 'mediawiki.legacy.diff' );
+		$wgOut->addModuleScripts( 'mediawiki.legacy.diff' );
+	}
 
 	/**
-	 * Number of trailing context "lines" to preserve.
+	 * Get complete diff table, including header
 	 *
-	 * This should be left at zero for this class, but subclasses
-	 * may want to set this to other values.
+	 * @param $otitle Title: old title
+	 * @param $ntitle Title: new title
+	 * @param $notice String: HTML between diff header and body
+	 * @return mixed
 	 */
-	var $trailing_context_lines = 0;
+	function getDiff( $otitle, $ntitle, $notice = '' ) {
+		$body = $this->getDiffBody();
+		if ( $body === false ) {
+			return false;
+		} else {
+			$multi = $this->getMultiNotice();
+			return $this->addHeader( $body, $otitle, $ntitle, $multi, $notice );
+		}
+	}
 
 	/**
-	 * Format a diff.
+	 * Get the diff table body, without header
 	 *
-	 * @param $diff object A Diff object.
-	 * @return string The formatted output.
+	 * @return mixed (string/false)
 	 */
-	function format($diff) {
+	public function getDiffBody() {
+		global $wgMemc;
 		wfProfileIn( __METHOD__ );
-
-		$xi = $yi = 1;
-		$block = false;
-		$context = array();
-
-		$nlead = $this->leading_context_lines;
-		$ntrail = $this->trailing_context_lines;
-
-		$this->_start_diff();
-
-		foreach ($diff->edits as $edit) {
-			if ($edit->type == 'copy') {
-				if (is_array($block)) {
-					if (sizeof($edit->orig) <= $nlead + $ntrail) {
-						$block[] = $edit;
-					}
-					else{
-						if ($ntrail) {
-							$context = array_slice($edit->orig, 0, $ntrail);
-							$block[] = new _DiffOp_Copy($context);
-						}
-						$this->_block($x0, $ntrail + $xi - $x0,
-						$y0, $ntrail + $yi - $y0,
-						$block);
-						$block = false;
-					}
-				}
-				$context = $edit->orig;
-			}
-			else {
-				if (! is_array($block)) {
-					$context = array_slice($context, sizeof($context) - $nlead);
-					$x0 = $xi - sizeof($context);
-					$y0 = $yi - sizeof($context);
-					$block = array();
-					if ($context)
-					$block[] = new _DiffOp_Copy($context);
+		$this->mCacheHit = true;
+		// Check if the diff should be hidden from this user
+		if ( !$this->loadRevisionData() ) {
+			wfProfileOut( __METHOD__ );
+			return false;
+		} elseif ( $this->mOldRev && !$this->mOldRev->userCan( Revision::DELETED_TEXT ) ) {
+			wfProfileOut( __METHOD__ );
+			return false;
+		} elseif ( $this->mNewRev && !$this->mNewRev->userCan( Revision::DELETED_TEXT ) ) {
+			wfProfileOut( __METHOD__ );
+			return false;
+		}
+		// Short-circuit
+		if ( $this->mOldRev && $this->mNewRev
+			&& $this->mOldRev->getID() == $this->mNewRev->getID() )
+		{
+			wfProfileOut( __METHOD__ );
+			return '';
+		}
+		// Cacheable?
+		$key = false;
+		if ( $this->mOldid && $this->mNewid ) {
+			$key = wfMemcKey( 'diff', 'version', MW_DIFF_VERSION,
+				'oldid', $this->mOldid, 'newid', $this->mNewid );
+			// Try cache
+			if ( !$this->mRefreshCache ) {
+				$difftext = $wgMemc->get( $key );
+				if ( $difftext ) {
+					wfIncrStats( 'diff_cache_hit' );
+					$difftext = $this->localiseLineNumbers( $difftext );
+					$difftext .= "\n<!-- diff cache key $key -->\n";
+					wfProfileOut( __METHOD__ );
+					return $difftext;
 				}
-				$block[] = $edit;
-			}
+			} // don't try to load but save the result
+		}
+		$this->mCacheHit = false;
 
-			if ($edit->orig)
-			$xi += sizeof($edit->orig);
-			if ($edit->closing)
-			$yi += sizeof($edit->closing);
+		// Loadtext is permission safe, this just clears out the diff
+		if ( !$this->loadText() ) {
+			wfProfileOut( __METHOD__ );
+			return false;
 		}
 
-		if (is_array($block))
-		$this->_block($x0, $xi - $x0,
-		$y0, $yi - $y0,
-		$block);
+		$difftext = $this->generateDiffBody( $this->mOldtext, $this->mNewtext );
 
-		$end = $this->_end_diff();
+		// Save to cache for 7 days
+		if ( !wfRunHooks( 'AbortDiffCache', array( &$this ) ) ) {
+			wfIncrStats( 'diff_uncacheable' );
+		} elseif ( $key !== false && $difftext !== false ) {
+			wfIncrStats( 'diff_cache_miss' );
+			$wgMemc->set( $key, $difftext, 7 * 86400 );
+		} else {
+			wfIncrStats( 'diff_uncacheable' );
+		}
+		// Replace line numbers with the text in the user's language
+		if ( $difftext !== false ) {
+			$difftext = $this->localiseLineNumbers( $difftext );
+		}
 		wfProfileOut( __METHOD__ );
-		return $end;
+		return $difftext;
 	}
 
-	function _block($xbeg, $xlen, $ybeg, $ylen, &$edits) {
-		wfProfileIn( __METHOD__ );
-		$this->_start_block($this->_block_header($xbeg, $xlen, $ybeg, $ylen));
-		foreach ($edits as $edit) {
-			if ($edit->type == 'copy')
-			$this->_context($edit->orig);
-			elseif ($edit->type == 'add')
-			$this->_added($edit->closing);
-			elseif ($edit->type == 'delete')
-			$this->_deleted($edit->orig);
-			elseif ($edit->type == 'change')
-			$this->_changed($edit->orig, $edit->closing);
-			else
-			trigger_error('Unknown edit type', E_USER_ERROR);
-		}
-		$this->_end_block();
-		wfProfileOut( __METHOD__ );
+	/**
+	 * Make sure the proper modules are loaded before we try to
+	 * make the diff
+	 */
+	private function initDiffEngines() {
+		global $wgExternalDiffEngine;
+		if ( $wgExternalDiffEngine == 'wikidiff' && !function_exists( 'wikidiff_do_diff' ) ) {
+			wfProfileIn( __METHOD__ . '-php_wikidiff.so' );
+			wfSuppressWarnings();
+			dl( 'php_wikidiff.so' );
+			wfRestoreWarnings();
+			wfProfileOut( __METHOD__ . '-php_wikidiff.so' );
+		}
+		else if ( $wgExternalDiffEngine == 'wikidiff2' && !function_exists( 'wikidiff2_do_diff' ) ) {
+			wfProfileIn( __METHOD__ . '-php_wikidiff2.so' );
+			wfSuppressWarnings();
+			wfDl( 'wikidiff2' );
+			wfRestoreWarnings();
+			wfProfileOut( __METHOD__ . '-php_wikidiff2.so' );
+		}
 	}
 
-	function _start_diff() {
-		ob_start();
-	}
+	/**
+	 * Generate a diff, no caching
+	 *
+	 * @param $otext String: old text, must be already segmented
+	 * @param $ntext String: new text, must be already segmented
+	 */
+	function generateDiffBody( $otext, $ntext ) {
+		global $wgExternalDiffEngine, $wgContLang;
 
-	function _end_diff() {
-		$val = ob_get_contents();
-		ob_end_clean();
-		return $val;
-	}
+		$otext = str_replace( "\r\n", "\n", $otext );
+		$ntext = str_replace( "\r\n", "\n", $ntext );
 
-	function _block_header($xbeg, $xlen, $ybeg, $ylen) {
-		if ($xlen > 1)
-		$xbeg .= "," . ($xbeg + $xlen - 1);
-		if ($ylen > 1)
-		$ybeg .= "," . ($ybeg + $ylen - 1);
+		$this->initDiffEngines();
 
-		return $xbeg . ($xlen ? ($ylen ? 'c' : 'd') : 'a') . $ybeg;
-	}
+		if ( $wgExternalDiffEngine == 'wikidiff' && function_exists( 'wikidiff_do_diff' ) ) {
+			# For historical reasons, external diff engine expects
+			# input text to be HTML-escaped already
+			$otext = htmlspecialchars ( $wgContLang->segmentForDiff( $otext ) );
+			$ntext = htmlspecialchars ( $wgContLang->segmentForDiff( $ntext ) );
+			return $wgContLang->unsegmentForDiff( wikidiff_do_diff( $otext, $ntext, 2 ) ) .
+			$this->debug( 'wikidiff1' );
+		}
 
-	function _start_block($header) {
-		echo $header . "\n";
-	}
+		if ( $wgExternalDiffEngine == 'wikidiff2' && function_exists( 'wikidiff2_do_diff' ) ) {
+			# Better external diff engine, the 2 may some day be dropped
+			# This one does the escaping and segmenting itself
+			wfProfileIn( 'wikidiff2_do_diff' );
+			$text = wikidiff2_do_diff( $otext, $ntext, 2 );
+			$text .= $this->debug( 'wikidiff2' );
+			wfProfileOut( 'wikidiff2_do_diff' );
+			return $text;
+		}
+		if ( $wgExternalDiffEngine != 'wikidiff3' && $wgExternalDiffEngine !== false ) {
+			# Diff via the shell
+			global $wgTmpDirectory;
+			$tempName1 = tempnam( $wgTmpDirectory, 'diff_' );
+			$tempName2 = tempnam( $wgTmpDirectory, 'diff_' );
+
+			$tempFile1 = fopen( $tempName1, "w" );
+			if ( !$tempFile1 ) {
+				wfProfileOut( __METHOD__ );
+				return false;
+			}
+			$tempFile2 = fopen( $tempName2, "w" );
+			if ( !$tempFile2 ) {
+				wfProfileOut( __METHOD__ );
+				return false;
+			}
+			fwrite( $tempFile1, $otext );
+			fwrite( $tempFile2, $ntext );
+			fclose( $tempFile1 );
+			fclose( $tempFile2 );
+			$cmd = wfEscapeShellArg( $wgExternalDiffEngine, $tempName1, $tempName2 );
+			wfProfileIn( __METHOD__ . "-shellexec" );
+			$difftext = wfShellExec( $cmd );
+			$difftext .= $this->debug( "external $wgExternalDiffEngine" );
+			wfProfileOut( __METHOD__ . "-shellexec" );
+			unlink( $tempName1 );
+			unlink( $tempName2 );
+			wfProfileOut( __METHOD__ );
+			return $difftext;
+		}
 
-	function _end_block() {
+		# Native PHP diff
+		$ota = explode( "\n", $wgContLang->segmentForDiff( $otext ) );
+		$nta = explode( "\n", $wgContLang->segmentForDiff( $ntext ) );
+		$diffs = new Diff( $ota, $nta );
+		$formatter = new TableDiffFormatter();
+		$difftext = $wgContLang->unsegmentForDiff( $formatter->format( $diffs ) ) .
+		wfProfileOut( __METHOD__ );
+		return $difftext;
+		$this->debug();
 	}
 
-	function _lines($lines, $prefix = ' ') {
-		foreach ($lines as $line)
-		echo "$prefix $line\n";
+	/**
+	 * Generate a debug comment indicating diff generating time,
+	 * server node, and generator backend.
+	 */
+	protected function debug( $generator = "internal" ) {
+		global $wgShowHostnames;
+		if ( !$this->enableDebugComment ) {
+			return '';
+		}
+		$data = array( $generator );
+		if ( $wgShowHostnames ) {
+			$data[] = wfHostname();
+		}
+		$data[] = wfTimestamp( TS_DB );
+		return "<!-- diff generator: " .
+		implode( " ",
+		array_map(
+					"htmlspecialchars",
+		$data ) ) .
+			" -->\n";
 	}
 
-	function _context($lines) {
-		$this->_lines($lines);
+	/**
+	 * Replace line numbers with the text in the user's language
+	 */
+	function localiseLineNumbers( $text ) {
+		return preg_replace_callback( '/<!--LINE (\d+)-->/',
+		array( &$this, 'localiseLineNumbersCb' ), $text );
 	}
 
-	function _added($lines) {
-		$this->_lines($lines, '>');
-	}
-	function _deleted($lines) {
-		$this->_lines($lines, '<');
+	function localiseLineNumbersCb( $matches ) {
+		global $wgLang;
+		if ( $matches[1] === '1' && $this->mReducedLineNumbers ) return '';
+		return wfMsgExt( 'lineno', 'escape', $wgLang->formatNum( $matches[1] ) );
 	}
 
-	function _changed($orig, $closing) {
-		$this->_deleted($orig);
-		echo "---\n";
-		$this->_added($closing);
-	}
-}
 
-/**
- * A formatter that outputs unified diffs
- * @ingroup DifferenceEngine
- */
+	/**
+	 * If there are revisions between the ones being compared, return a note saying so.
+	 * @return string
+	 */
+	function getMultiNotice() {
+		if ( !is_object( $this->mOldRev ) || !is_object( $this->mNewRev ) ) {
+			return '';
+		} elseif ( !$this->mOldPage->equals( $this->mNewPage ) ) {
+			// Comparing two different pages? Count would be meaningless.
+			return '';
+		}
 
-class UnifiedDiffFormatter extends DiffFormatter {
-	var $leading_context_lines = 2;
-	var $trailing_context_lines = 2;
+		$oldid = $this->mOldRev->getId();
+		$newid = $this->mNewRev->getId();
+		if ( $oldid > $newid ) {
+			$tmp = $oldid; $oldid = $newid; $newid = $tmp;
+		}
 
-	function _added($lines) {
-		$this->_lines($lines, '+');
-	}
-	function _deleted($lines) {
-		$this->_lines($lines, '-');
-	}
-	function _changed($orig, $closing) {
-		$this->_deleted($orig);
-		$this->_added($closing);
-	}
-	function _block_header($xbeg, $xlen, $ybeg, $ylen) {
-		return "@@ -$xbeg,$xlen +$ybeg,$ylen @@";
+		$nEdits = $this->mTitle->countRevisionsBetween( $oldid, $newid );
+		if ( $nEdits > 0 ) {
+			$limit = 100;
+			// We use ($limit + 1) so we can detect if there are > 100 authors
+			// in a given revision range. In that case, diff-multi-manyusers is used.
+			$numUsers = $this->mTitle->countAuthorsBetween( $oldid, $newid, $limit + 1 );
+			return self::intermediateEditsMsg( $nEdits, $numUsers, $limit );
+		}
+		return ''; // nothing
 	}
-}
 
-/**
- * A pseudo-formatter that just passes along the Diff::$edits array
- * @ingroup DifferenceEngine
- */
-class ArrayDiffFormatter extends DiffFormatter {
-	function format($diff) {
-		$oldline = 1;
-		$newline = 1;
-		$retval = array();
-		foreach($diff->edits as $edit)
-		switch($edit->type) {
-			case 'add':
-				foreach($edit->closing as $l) {
-					$retval[] = array(
-							'action' => 'add',
-							'new'=> $l,
-							'newline' => $newline++
-					);
-				}
-				break;
-			case 'delete':
-				foreach($edit->orig as $l) {
-					$retval[] = array(
-							'action' => 'delete',
-							'old' => $l,
-							'oldline' => $oldline++,
-					);
-				}
-				break;
-			case 'change':
-				foreach($edit->orig as $i => $l) {
-					$retval[] = array(
-							'action' => 'change',
-							'old' => $l,
-							'new' => @$edit->closing[$i],
-							'oldline' => $oldline++,
-							'newline' => $newline++,
-					);
-				}
-				break;
-			case 'copy':
-				$oldline += count($edit->orig);
-				$newline += count($edit->orig);
+	/**
+	 * Get a notice about how many intermediate edits and users there are
+	 * @param $numEdits int
+	 * @param $numUsers int
+	 * @param $limit int
+	 * @return string
+	 */
+	public static function intermediateEditsMsg( $numEdits, $numUsers, $limit ) {
+		global $wgLang;
+		if ( $numUsers > $limit ) {
+			$msg = 'diff-multi-manyusers';
+			$numUsers = $limit;
+		} else {
+			$msg = 'diff-multi';
 		}
-		return $retval;
+		return wfMsgExt( $msg, 'parseinline',
+			$wgLang->formatnum( $numEdits ), $wgLang->formatnum( $numUsers ) );
 	}
-}
 
-/**
- *	Additions by Axel Boldt follow, partly taken from diff.php, phpwiki-1.3.3
- *
- */
-
-define('NBSP', '&#160;'); // iso-8859-x non-breaking space.
-
-/**
- * @todo document
- * @private
- * @ingroup DifferenceEngine
- */
-class _HWLDF_WordAccumulator {
-	function _HWLDF_WordAccumulator () {
-		$this->_lines = array();
-		$this->_line = '';
-		$this->_group = '';
-		$this->_tag = '';
-	}
+	/**
+	 * Add the header to a diff body
+	 */
+	static function addHeader( $diff, $otitle, $ntitle, $multi = '', $notice = '' ) {
+		$header = "<table class='diff'>";
+		if ( $diff ) { // Safari/Chrome show broken output if cols not used
+			$header .= "
+			<col class='diff-marker' />
+			<col class='diff-content' />
+			<col class='diff-marker' />
+			<col class='diff-content' />";
+			$colspan = 2;
+			$multiColspan = 4;
+		} else {
+			$colspan = 1;
+			$multiColspan = 2;
+		}
+		$header .= "
+		<tr valign='top'>
+		<td colspan='$colspan' class='diff-otitle'>{$otitle}</td>
+		<td colspan='$colspan' class='diff-ntitle'>{$ntitle}</td>
+		</tr>";
+
+		if ( $multi != '' ) {
+			$header .= "<tr><td colspan='{$multiColspan}' align='center' class='diff-multi'>{$multi}</td></tr>";
+		}
+		if ( $notice != '' ) {
+			$header .= "<tr><td colspan='{$multiColspan}' align='center'>{$notice}</td></tr>";
+		}
 
-	function _flushGroup ($new_tag) {
-		if ($this->_group !== '') {
-			if ($this->_tag == 'ins')
-			$this->_line .= '<ins class="diffchange diffchange-inline">' .
-			htmlspecialchars ( $this->_group ) . '</ins>';
-			elseif ($this->_tag == 'del')
-			$this->_line .= '<del class="diffchange diffchange-inline">' .
-			htmlspecialchars ( $this->_group ) . '</del>';
-			else
-			$this->_line .= htmlspecialchars ( $this->_group );
-		}
-		$this->_group = '';
-		$this->_tag = $new_tag;
+		return $header . $diff . "</table>";
 	}
 
-	function _flushLine ($new_tag) {
-		$this->_flushGroup($new_tag);
-		if ($this->_line != '')
-		array_push ( $this->_lines, $this->_line );
-		else
-		# make empty lines visible by inserting an NBSP
-		array_push ( $this->_lines, NBSP );
-		$this->_line = '';
+	/**
+	 * Use specified text instead of loading from the database
+	 */
+	function setText( $oldText, $newText ) {
+		$this->mOldtext = $oldText;
+		$this->mNewtext = $newText;
+		$this->mTextLoaded = 2;
+		$this->mRevisionsLoaded = true;
 	}
 
-	function addWords ($words, $tag = '') {
-		if ($tag != $this->_tag)
-		$this->_flushGroup($tag);
-
-		foreach ($words as $word) {
-			// new-line should only come as first char of word.
-			if ($word == '')
-			continue;
-			if ($word[0] == "\n") {
-				$this->_flushLine($tag);
-				$word = substr($word, 1);
-			}
-			assert(!strstr($word, "\n"));
-			$this->_group .= $word;
+	/**
+	 * Load revision metadata for the specified articles. If newid is 0, then compare
+	 * the old article in oldid to the current article; if oldid is 0, then
+	 * compare the current article to the immediately previous one (ignoring the
+	 * value of newid).
+	 *
+	 * If oldid is false, leave the corresponding revision object set
+	 * to false. This is impossible via ordinary user input, and is provided for
+	 * API convenience.
+	 */
+	function loadRevisionData() {
+		global $wgLang, $wgUser;
+		if ( $this->mRevisionsLoaded ) {
+			return true;
+		} else {
+			// Whether it succeeds or fails, we don't want to try again
+			$this->mRevisionsLoaded = true;
 		}
-	}
 
-	function getLines() {
-		$this->_flushLine('~done');
-		return $this->_lines;
-	}
-}
-
-/**
- * @todo document
- * @private
- * @ingroup DifferenceEngine
- */
-class WordLevelDiff extends MappedDiff {
-	const MAX_LINE_LENGTH = 10000;
-
-	function WordLevelDiff ($orig_lines, $closing_lines) {
-		wfProfileIn( __METHOD__ );
-
-		list ($orig_words, $orig_stripped) = $this->_split($orig_lines);
-		list ($closing_words, $closing_stripped) = $this->_split($closing_lines);
-
-		$this->MappedDiff($orig_words, $closing_words,
-		$orig_stripped, $closing_stripped);
-		wfProfileOut( __METHOD__ );
-	}
+		// Load the new revision object
+		$this->mNewRev = $this->mNewid
+			? Revision::newFromId( $this->mNewid )
+			: Revision::newFromTitle( $this->mTitle );
+		if ( !$this->mNewRev instanceof Revision )
+			return false;
 
-	function _split($lines) {
-		wfProfileIn( __METHOD__ );
+		// Update the new revision ID in case it was 0 (makes life easier doing UI stuff)
+		$this->mNewid = $this->mNewRev->getId();
+
+		// Check if page is editable
+		$editable = $this->mNewRev->getTitle()->userCan( 'edit' );
+
+		// Set assorted variables
+		$timestamp = $wgLang->timeanddate( $this->mNewRev->getTimestamp(), true );
+		$dateofrev = $wgLang->date( $this->mNewRev->getTimestamp(), true );
+		$timeofrev = $wgLang->time( $this->mNewRev->getTimestamp(), true );
+		$this->mNewPage = $this->mNewRev->getTitle();
+		if ( $this->mNewRev->isCurrent() ) {
+			$newLink = $this->mNewPage->escapeLocalUrl( array(
+				'oldid' => $this->mNewid
+			) );
+			$this->mPagetitle = htmlspecialchars( wfMsg(
+				'currentrev-asof',
+				$timestamp,
+				$dateofrev,
+				$timeofrev
+			) );
+			$newEdit = $this->mNewPage->escapeLocalUrl( array(
+				'action' => 'edit'
+			) );
+
+			$this->mNewtitle = "<a href='$newLink'>{$this->mPagetitle}</a>";
+			$this->mNewtitle .= " (<a href='$newEdit'>" . wfMsgHtml( $editable ? 'editold' : 'viewsourceold' ) . "</a>)";
+		} else {
+			$newLink = $this->mNewPage->escapeLocalUrl( array(
+				'oldid' => $this->mNewid
+			) );
+			$newEdit = $this->mNewPage->escapeLocalUrl( array(
+				'action' => 'edit',
+				'oldid' => $this->mNewid
+			) );
+			$this->mPagetitle = htmlspecialchars( wfMsg(
+				'revisionasof',
+				$timestamp,
+				$dateofrev,
+				$timeofrev
+			) );
+
+			$this->mNewtitle = "<a href='$newLink'>{$this->mPagetitle}</a>";
+			$this->mNewtitle .= " (<a href='$newEdit'>" . wfMsgHtml( $editable ? 'editold' : 'viewsourceold' ) . "</a>)";
+		}
+		if ( !$this->mNewRev->userCan( Revision::DELETED_TEXT ) ) {
+			$this->mNewtitle = "<span class='history-deleted'>{$this->mPagetitle}</span>";
+		} else if ( $this->mNewRev->isDeleted( Revision::DELETED_TEXT ) ) {
+			$this->mNewtitle = "<span class='history-deleted'>{$this->mNewtitle}</span>";
+		}
 
-		$words = array();
-		$stripped = array();
-		$first = true;
-		foreach ( $lines as $line ) {
-			# If the line is too long, just pretend the entire line is one big word
-			# This prevents resource exhaustion problems
-			if ( $first ) {
-				$first = false;
+		// Load the old revision object
+		$this->mOldRev = false;
+		if ( $this->mOldid ) {
+			$this->mOldRev = Revision::newFromId( $this->mOldid );
+		} elseif ( $this->mOldid === 0 ) {
+			$rev = $this->mNewRev->getPrevious();
+			if ( $rev ) {
+				$this->mOldid = $rev->getId();
+				$this->mOldRev = $rev;
 			} else {
-				$words[] = "\n";
-				$stripped[] = "\n";
+				// No previous revision; mark to show as first-version only.
+				$this->mOldid = false;
+				$this->mOldRev = false;
 			}
-			if ( strlen( $line ) > self::MAX_LINE_LENGTH ) {
-				$words[] = $line;
-				$stripped[] = $line;
-			} else {
-				$m = array();
-				if (preg_match_all('/ ( [^\S\n]+ | [0-9_A-Za-z\x80-\xff]+ | . ) (?: (?!< \n) [^\S\n])? /xs',
-				$line, $m))
-				{
-					$words = array_merge( $words, $m[0] );
-					$stripped = array_merge( $stripped, $m[1] );
-				}
-			}
-		}
-		wfProfileOut( __METHOD__ );
-		return array($words, $stripped);
-	}
-
-	function orig () {
-		wfProfileIn( __METHOD__ );
-		$orig = new _HWLDF_WordAccumulator;
+		} /* elseif ( $this->mOldid === false ) leave mOldRev false; */
 
-		foreach ($this->edits as $edit) {
-			if ($edit->type == 'copy')
-			$orig->addWords($edit->orig);
-			elseif ($edit->orig)
-			$orig->addWords($edit->orig, 'del');
+		if ( is_null( $this->mOldRev ) ) {
+			return false;
 		}
-		$lines = $orig->getLines();
-		wfProfileOut( __METHOD__ );
-		return $lines;
-	}
 
-	function closing () {
-		wfProfileIn( __METHOD__ );
-		$closing = new _HWLDF_WordAccumulator;
+		if ( $this->mOldRev ) {
+			$this->mOldPage = $this->mOldRev->getTitle();
+
+			$t = $wgLang->timeanddate( $this->mOldRev->getTimestamp(), true );
+			$dateofrev = $wgLang->date( $this->mOldRev->getTimestamp(), true );
+			$timeofrev = $wgLang->time( $this->mOldRev->getTimestamp(), true );
+			$oldLink = $this->mOldPage->escapeLocalUrl( array(
+				'oldid' => $this->mOldid
+			) );
+			$oldEdit = $this->mOldPage->escapeLocalUrl( array(
+				'action' => 'edit',
+				'oldid' => $this->mOldid
+			) );
+			$this->mOldPagetitle = htmlspecialchars( wfMsg( 'revisionasof', $t, $dateofrev, $timeofrev ) );
+
+			$this->mOldtitle = "<a href='$oldLink'>{$this->mOldPagetitle}</a>"
+			. " (<a href='$oldEdit'>" . wfMsgHtml( $editable ? 'editold' : 'viewsourceold' ) . "</a>)";
+			// Add an "undo" link
+			$newUndo = $this->mNewPage->escapeLocalUrl( array(
+				'action' => 'edit',
+				'undoafter' => $this->mOldid,
+				'undo' => $this->mNewid
+			) );
+			$htmlLink = htmlspecialchars( wfMsg( 'editundo' ) );
+			$htmlTitle = Xml::expandAttributes( array( 'title' => $wgUser->getSkin()->titleAttrib( 'undo' ) ) );
+			if ( $editable && !$this->mOldRev->isDeleted( Revision::DELETED_TEXT ) && !$this->mNewRev->isDeleted( Revision::DELETED_TEXT ) ) {
+				$this->mNewtitle .= " (<a href='$newUndo' $htmlTitle>" . $htmlLink . "</a>)";
+			}
 
-		foreach ($this->edits as $edit) {
-			if ($edit->type == 'copy')
-			$closing->addWords($edit->closing);
-			elseif ($edit->closing)
-			$closing->addWords($edit->closing, 'ins');
+			if ( !$this->mOldRev->userCan( Revision::DELETED_TEXT ) ) {
+				$this->mOldtitle = '<span class="history-deleted">' . $this->mOldPagetitle . '</span>';
+			} else if ( $this->mOldRev->isDeleted( Revision::DELETED_TEXT ) ) {
+				$this->mOldtitle = '<span class="history-deleted">' . $this->mOldtitle . '</span>';
+			}
 		}
-		$lines = $closing->getLines();
-		wfProfileOut( __METHOD__ );
-		return $lines;
-	}
-}
-
-/**
- * Wikipedia Table style diff formatter.
- * @todo document
- * @private
- * @ingroup DifferenceEngine
- */
-class TableDiffFormatter extends DiffFormatter {
-	function TableDiffFormatter() {
-		$this->leading_context_lines = 2;
-		$this->trailing_context_lines = 2;
-	}
-
-	public static function escapeWhiteSpace( $msg ) {
-		$msg = preg_replace( '/^ /m', '&nbsp; ', $msg );
-		$msg = preg_replace( '/ $/m', ' &nbsp;', $msg );
-		$msg = preg_replace( '/  /', '&nbsp; ', $msg );
-		return $msg;
-	}
 
-	function _block_header( $xbeg, $xlen, $ybeg, $ylen ) {
-		$r = '<tr><td colspan="2" class="diff-lineno"><!--LINE '.$xbeg."--></td>\n" .
-		  '<td colspan="2" class="diff-lineno"><!--LINE '.$ybeg."--></td></tr>\n";
-		return $r;
-	}
-
-	function _start_block( $header ) {
-		echo $header;
-	}
-
-	function _end_block() {
-	}
-
-	function _lines( $lines, $prefix=' ', $color='white' ) {
-	}
-
-	# HTML-escape parameter before calling this
-	function addedLine( $line ) {
-		return $this->wrapLine( '+', 'diff-addedline', $line );
-	}
-
-	# HTML-escape parameter before calling this
-	function deletedLine( $line ) {
-		return $this->wrapLine( '-', 'diff-deletedline', $line );
-	}
-
-	# HTML-escape parameter before calling this
-	function contextLine( $line ) {
-		return $this->wrapLine( ' ', 'diff-context', $line );
+		return true;
 	}
 
-	private function wrapLine( $marker, $class, $line ) {
-		if( $line !== '' ) {
-			// The <div> wrapper is needed for 'overflow: auto' style to scroll properly
-			$line = Xml::tags( 'div', null, $this->escapeWhiteSpace( $line ) );
+	/**
+	 * Load the text of the revisions, as well as revision data.
+	 */
+	function loadText() {
+		if ( $this->mTextLoaded == 2 ) {
+			return true;
+		} else {
+			// Whether it succeeds or fails, we don't want to try again
+			$this->mTextLoaded = 2;
 		}
-		return "<td class='diff-marker'>$marker</td><td class='$class'>$line</td>";
-	}
 
-	function emptyLine() {
-		return '<td colspan="2">&nbsp;</td>';
-	}
-
-	function _added( $lines ) {
-		foreach ($lines as $line) {
-			echo '<tr>' . $this->emptyLine() .
-			$this->addedLine( '<ins class="diffchange">' .
-			htmlspecialchars ( $line ) . '</ins>' ) . "</tr>\n";
+		if ( !$this->loadRevisionData() ) {
+			return false;
 		}
-	}
-
-	function _deleted($lines) {
-		foreach ($lines as $line) {
-			echo '<tr>' . $this->deletedLine( '<del class="diffchange">' .
-			htmlspecialchars ( $line ) . '</del>' ) .
-			$this->emptyLine() . "</tr>\n";
+		if ( $this->mOldRev ) {
+			$this->mOldtext = $this->mOldRev->getText( Revision::FOR_THIS_USER );
+			if ( $this->mOldtext === false ) {
+				return false;
+			}
 		}
-	}
-
-	function _context( $lines ) {
-		foreach ($lines as $line) {
-			echo '<tr>' .
-			$this->contextLine( htmlspecialchars ( $line ) ) .
-			$this->contextLine( htmlspecialchars ( $line ) ) . "</tr>\n";
+		if ( $this->mNewRev ) {
+			$this->mNewtext = $this->mNewRev->getText( Revision::FOR_THIS_USER );
+			if ( $this->mNewtext === false ) {
+				return false;
+			}
 		}
+		return true;
 	}
 
-	function _changed( $orig, $closing ) {
-		wfProfileIn( __METHOD__ );
-
-		$diff = new WordLevelDiff( $orig, $closing );
-		$del = $diff->orig();
-		$add = $diff->closing();
-
-		# Notice that WordLevelDiff returns HTML-escaped output.
-		# Hence, we will be calling addedLine/deletedLine without HTML-escaping.
-
-		while ( $line = array_shift( $del ) ) {
-			$aline = array_shift( $add );
-			echo '<tr>' . $this->deletedLine( $line ) .
-			$this->addedLine( $aline ) . "</tr>\n";
+	/**
+	 * Load the text of the new revision, not the old one
+	 */
+	function loadNewText() {
+		if ( $this->mTextLoaded >= 1 ) {
+			return true;
+		} else {
+			$this->mTextLoaded = 1;
 		}
-		foreach ($add as $line) {	# If any leftovers
-			echo '<tr>' . $this->emptyLine() .
-			$this->addedLine( $line ) . "</tr>\n";
+		if ( !$this->loadRevisionData() ) {
+			return false;
 		}
-		wfProfileOut( __METHOD__ );
+		$this->mNewtext = $this->mNewRev->getText( Revision::FOR_THIS_USER );
+		return true;
 	}
 }
diff --git a/includes/diff/DifferenceInterface.php b/includes/diff/DifferenceInterface.php
deleted file mode 100644
index 0e9ca9f6..00000000
--- a/includes/diff/DifferenceInterface.php
+++ /dev/null
@@ -1,1024 +0,0 @@
-<?php
-/**
- * @defgroup DifferenceEngine DifferenceEngine
- */
- 
-/**
- * Constant to indicate diff cache compatibility.
- * Bump this when changing the diff formatting in a way that
- * fixes important bugs or such to force cached diff views to
- * clear.
- */
-define( 'MW_DIFF_VERSION', '1.11a' );
-
-/**
- * @todo document
- * @ingroup DifferenceEngine
- */
-class DifferenceEngine {
-	/**#@+
-	 * @private
-	 */
-	var $mOldid, $mNewid, $mTitle;
-	var $mOldtitle, $mNewtitle, $mPagetitle;
-	var $mOldtext, $mNewtext;
-	var $mOldPage, $mNewPage;
-	var $mRcidMarkPatrolled;
-	var $mOldRev, $mNewRev;
-	var $mRevisionsLoaded = false; // Have the revisions been loaded
-	var $mTextLoaded = 0; // How many text blobs have been loaded, 0, 1 or 2?
-	var $mCacheHit = false; // Was the diff fetched from cache?
-
-	/**
-	 * Set this to true to add debug info to the HTML output.
-	 * Warning: this may cause RSS readers to spuriously mark articles as "new"
-	 * (bug 20601)
-	 */
-	var $enableDebugComment = false;
-
-	// If true, line X is not displayed when X is 1, for example to increase
-	// readability and conserve space with many small diffs.
-	protected $mReducedLineNumbers = false;
-
-	protected $unhide = false;
-	/**#@-*/
-
-	/**
-	 * Constructor
-	 * @param $titleObj Title object that the diff is associated with
-	 * @param $old Integer: old ID we want to show and diff with.
-	 * @param $new String: either 'prev' or 'next'.
-	 * @param $rcid Integer: ??? FIXME (default 0)
-	 * @param $refreshCache boolean If set, refreshes the diff cache
-	 * @param $unhide boolean If set, allow viewing deleted revs
-	 */
-	function __construct( $titleObj = null, $old = 0, $new = 0, $rcid = 0,
-		$refreshCache = false, $unhide = false )
-	{
-		if ( $titleObj ) {
-			$this->mTitle = $titleObj;
-		} else {
-			global $wgTitle;
-			$this->mTitle = $wgTitle;
-		}
-		wfDebug("DifferenceEngine old '$old' new '$new' rcid '$rcid'\n");
-
-		if ( 'prev' === $new ) {
-			# Show diff between revision $old and the previous one.
-			# Get previous one from DB.
-			$this->mNewid = intval($old);
-			$this->mOldid = $this->mTitle->getPreviousRevisionID( $this->mNewid );
-		} elseif ( 'next' === $new ) {
-			# Show diff between revision $old and the next one.
-			# Get next one from DB.
-			$this->mOldid = intval($old);
-			$this->mNewid = $this->mTitle->getNextRevisionID( $this->mOldid );
-			if ( false === $this->mNewid ) {
-				# if no result, NewId points to the newest old revision. The only newer
-				# revision is cur, which is "0".
-				$this->mNewid = 0;
-			}
-		} else {
-			$this->mOldid = intval($old);
-			$this->mNewid = intval($new);
-			wfRunHooks( 'NewDifferenceEngine', array(&$titleObj, &$this->mOldid, &$this->mNewid, $old, $new) ); 
-		}
-		$this->mRcidMarkPatrolled = intval($rcid);  # force it to be an integer
-		$this->mRefreshCache = $refreshCache;
-		$this->unhide = $unhide;
-	}
-
-	function setReducedLineNumbers( $value = true ) {
-		$this->mReducedLineNumbers = $value;
-	}
-
-	function getTitle() {
-		return $this->mTitle;
-	}
-	
-	function wasCacheHit() {
-		return $this->mCacheHit;
-	}
-	
-	function getOldid() {
-		return $this->mOldid;
-	}
-	
-	function getNewid() {
-		return $this->mNewid;
-	}
-
-	function showDiffPage( $diffOnly = false ) {
-		global $wgUser, $wgOut, $wgUseExternalEditor, $wgUseRCPatrol;
-		wfProfileIn( __METHOD__ );
-
-		# Allow frames except in certain special cases
-		$wgOut->allowClickjacking();
-
-		# If external diffs are enabled both globally and for the user,
-		# we'll use the application/x-external-editor interface to call
-		# an external diff tool like kompare, kdiff3, etc.
-		if($wgUseExternalEditor && $wgUser->getOption('externaldiff')) {
-			global $wgInputEncoding,$wgServer,$wgScript,$wgLang;
-			$wgOut->disable();
-			header ( "Content-type: application/x-external-editor; charset=".$wgInputEncoding );
-			$url1=$this->mTitle->getFullURL( array(
-				'action' => 'raw',
-				'oldid' => $this->mOldid
-			) );
-			$url2=$this->mTitle->getFullURL( array(
-				'action' => 'raw',
-				'oldid' => $this->mNewid
-			) );
-			$special=$wgLang->getNsText(NS_SPECIAL);
-			$control=<<<CONTROL
-			[Process]
-			Type=Diff text
-			Engine=MediaWiki
-			Script={$wgServer}{$wgScript}
-			Special namespace={$special}
-
-			[File]
-			Extension=wiki
-			URL=$url1
-
-			[File 2]
-			Extension=wiki
-			URL=$url2
-CONTROL;
-			echo($control);
-			return;
-		}
-
-		$wgOut->setArticleFlag( false );
-		if ( !$this->loadRevisionData() ) {
-			$t = $this->mTitle->getPrefixedText();
-			$d = wfMsgExt( 'missingarticle-diff', array( 'escape' ), $this->mOldid, $this->mNewid );
-			$wgOut->setPagetitle( wfMsg( 'errorpagetitle' ) );
-			$wgOut->addWikiMsg( 'missing-article', "<nowiki>$t</nowiki>", $d );
-			wfProfileOut( __METHOD__ );
-			return;
-		}
-
-		wfRunHooks( 'DiffViewHeader', array( $this, $this->mOldRev, $this->mNewRev ) );
-
-		if ( $this->mNewRev->isCurrent() ) {
-			$wgOut->setArticleFlag( true );
-		}
-
-		# mOldid is false if the difference engine is called with a "vague" query for
-		# a diff between a version V and its previous version V' AND the version V
-		# is the first version of that article. In that case, V' does not exist.
-		if ( $this->mOldid === false ) {
-			$this->showFirstRevision();
-			$this->renderNewRevision();  // should we respect $diffOnly here or not?
-			wfProfileOut( __METHOD__ );
-			return;
-		}
-
-		$wgOut->suppressQuickbar();
-
-		$oldTitle = $this->mOldPage->getPrefixedText();
-		$newTitle = $this->mNewPage->getPrefixedText();
-		if( $oldTitle == $newTitle ) {
-			$wgOut->setPageTitle( $newTitle );
-		} else {
-			$wgOut->setPageTitle( $oldTitle . ', ' . $newTitle );
-		}
-		$wgOut->setSubtitle( wfMsgExt( 'difference', array( 'parseinline' ) ) );
-		$wgOut->setRobotPolicy( 'noindex,nofollow' );
-
-		if ( !$this->mOldPage->userCanRead() || !$this->mNewPage->userCanRead() ) {
-			$wgOut->loginToUse();
-			$wgOut->output();
-			$wgOut->disable();
-			wfProfileOut( __METHOD__ );
-			return;
-		}
-
-		$sk = $wgUser->getSkin();
-
-		// Check if page is editable
-		$editable = $this->mNewRev->getTitle()->userCan( 'edit' );
-		if ( $editable && $this->mNewRev->isCurrent() && $wgUser->isAllowed( 'rollback' ) ) {
-			$wgOut->preventClickjacking();
-			$rollback = '&nbsp;&nbsp;&nbsp;' . $sk->generateRollback( $this->mNewRev );
-		} else {
-			$rollback = '';
-		}
-
-		// Prepare a change patrol link, if applicable
-		if( $wgUseRCPatrol && $this->mTitle->userCan('patrol') ) {
-			// If we've been given an explicit change identifier, use it; saves time
-			if( $this->mRcidMarkPatrolled ) {
-				$rcid = $this->mRcidMarkPatrolled;
-				$rc = RecentChange::newFromId( $rcid );
-				// Already patrolled?
-				$rcid = is_object($rc) && !$rc->getAttribute('rc_patrolled') ? $rcid : 0;
-			} else {
-				// Look for an unpatrolled change corresponding to this diff
-				$db = wfGetDB( DB_SLAVE );
-				$change = RecentChange::newFromConds(
-					array(
-					// Redundant user,timestamp condition so we can use the existing index
-						'rc_user_text'  => $this->mNewRev->getRawUserText(),
-						'rc_timestamp'  => $db->timestamp( $this->mNewRev->getTimestamp() ),
-						'rc_this_oldid' => $this->mNewid,
-						'rc_last_oldid' => $this->mOldid,
-						'rc_patrolled'  => 0
-					),
-					__METHOD__
-				);
-				if( $change instanceof RecentChange ) {
-					$rcid = $change->mAttribs['rc_id'];
-					$this->mRcidMarkPatrolled = $rcid;
-				} else {
-					// None found
-					$rcid = 0;
-				}
-			}
-			// Build the link
-			if( $rcid ) {
-				$patrol = ' <span class="patrollink">[' . $sk->link(
-					$this->mTitle, 
-					wfMsgHtml( 'markaspatrolleddiff' ),
-					array(),
-					array(
-						'action' => 'markpatrolled',
-						'rcid' => $rcid
-					),
-					array(
-						'known',
-						'noclasses'
-					)
-				) . ']</span>';
-			} else {
-				$patrol = '';
-			}
-		} else {
-			$patrol = '';
-		}
-
-		# Carry over 'diffonly' param via navigation links
-		if( $diffOnly != $wgUser->getBoolOption('diffonly') ) {
-			$query['diffonly'] = $diffOnly;
-		}
-
-		# Make "previous revision link"
-		$query['diff'] = 'prev';
-		$query['oldid'] = $this->mOldid;
-		# Cascade unhide param in links for easy deletion browsing
-		if( $this->unhide ) {
-			$query['unhide'] = 1;
-		}
-		$prevlink = $sk->link(
-			$this->mTitle,
-			wfMsgHtml( 'previousdiff' ),
-			array(
-				'id' => 'differences-prevlink'
-			),
-			$query,
-			array(
-				'known',
-				'noclasses'
-			)
-		);
-
-		# Make "next revision link"
-		$query['diff'] = 'next';
-		$query['oldid'] = $this->mNewid;
-		# Skip next link on the top revision
-		if( $this->mNewRev->isCurrent() ) {
-			$nextlink = '&nbsp;';
-		} else {
-			$nextlink = $sk->link(
-				$this->mTitle,
-				wfMsgHtml( 'nextdiff' ),
-				array(
-					'id' => 'differences-nextlink'
-				),
-				$query,
-				array(
-					'known',
-					'noclasses'
-				)
-			);
-		}
-
-		$oldminor = '';
-		$newminor = '';
-
-		if( $this->mOldRev->isMinor() ) {
-			$oldminor = ChangesList::flag( 'minor' );
-		}
-		if( $this->mNewRev->isMinor() ) {
-			$newminor = ChangesList::flag( 'minor' );
-		}
-
-		# Handle RevisionDelete links...
-		$ldel = $this->revisionDeleteLink( $this->mOldRev );
-		$rdel = $this->revisionDeleteLink( $this->mNewRev );
-
-		$oldHeader = '<div id="mw-diff-otitle1"><strong>'.$this->mOldtitle.'</strong></div>' .
-			'<div id="mw-diff-otitle2">' .
-				$sk->revUserTools( $this->mOldRev, !$this->unhide ).'</div>' .
-			'<div id="mw-diff-otitle3">' . $oldminor .
-				$sk->revComment( $this->mOldRev, !$diffOnly, !$this->unhide ).$ldel.'</div>' .
-			'<div id="mw-diff-otitle4">' . $prevlink .'</div>';
-		$newHeader = '<div id="mw-diff-ntitle1"><strong>'.$this->mNewtitle.'</strong></div>' .
-			'<div id="mw-diff-ntitle2">' . $sk->revUserTools( $this->mNewRev, !$this->unhide ) .
-				" $rollback</div>" .
-			'<div id="mw-diff-ntitle3">' . $newminor .
-				$sk->revComment( $this->mNewRev, !$diffOnly, !$this->unhide ).$rdel.'</div>' .
-			'<div id="mw-diff-ntitle4">' . $nextlink . $patrol . '</div>';
-
-		# Check if this user can see the revisions
-		$allowed = $this->mOldRev->userCan(Revision::DELETED_TEXT)
-			&& $this->mNewRev->userCan(Revision::DELETED_TEXT);
-		# Check if one of the revisions is deleted/suppressed
-		$deleted = $suppressed = false;
-		if( $this->mOldRev->isDeleted(Revision::DELETED_TEXT) ) {
-			$deleted = true; // old revisions text is hidden
-			if( $this->mOldRev->isDeleted(Revision::DELETED_RESTRICTED) )
-				$suppressed = true; // also suppressed
-		}
-		if( $this->mNewRev->isDeleted(Revision::DELETED_TEXT) ) {
-			$deleted = true; // new revisions text is hidden
-			if( $this->mNewRev->isDeleted(Revision::DELETED_RESTRICTED) )
-				$suppressed = true; // also suppressed
-		}
-		# If the diff cannot be shown due to a deleted revision, then output
-		# the diff header and links to unhide (if available)...
-		if( $deleted && (!$this->unhide || !$allowed) ) {
-			$this->showDiffStyle();
-			$multi = $this->getMultiNotice();
-			$wgOut->addHTML( $this->addHeader( '', $oldHeader, $newHeader, $multi ) );
-			if( !$allowed ) {
-				$msg = $suppressed ? 'rev-suppressed-no-diff' : 'rev-deleted-no-diff';
-				# Give explanation for why revision is not visible
-				$wgOut->wrapWikiMsg( "<div class='mw-warning plainlinks'>\n$1</div>\n",
-					array( $msg ) );
-			} else {
-				# Give explanation and add a link to view the diff...
-				$link = $this->mTitle->getFullUrl( array(
-					'diff' => $this->mNewid,
-					'oldid' => $this->mOldid,
-					'unhide' => 1
-				) );
-				$msg = $suppressed ? 'rev-suppressed-unhide-diff' : 'rev-deleted-unhide-diff';
-				$wgOut->wrapWikiMsg( "<div class='mw-warning plainlinks'>\n$1</div>\n", array( $msg, $link ) );
-			}
-		# Otherwise, output a regular diff...
-		} else {
-			# Add deletion notice if the user is viewing deleted content
-			$notice = '';
-			if( $deleted ) {
-				$msg = $suppressed ? 'rev-suppressed-diff-view' : 'rev-deleted-diff-view';
-				$notice = "<div class='mw-warning plainlinks'>\n".wfMsgExt($msg,'parseinline')."</div>\n";
-			}
-			$this->showDiff( $oldHeader, $newHeader, $notice );
-			if( !$diffOnly ) {
-				$this->renderNewRevision();
-			}
-		}
-		wfProfileOut( __METHOD__ );
-	}
-	
-	protected function revisionDeleteLink( $rev ) {
-		global $wgUser;
-		$link = '';
-		$canHide = $wgUser->isAllowed( 'deleterevision' );
-		// Show del/undel link if:
-		// (a) the user can delete revisions, or
-		// (b) the user can view deleted revision *and* this one is deleted
-		if( $canHide || ($rev->getVisibility() && $wgUser->isAllowed( 'deletedhistory' )) ) {
-			$sk = $wgUser->getSkin();
-			if( !$rev->userCan( Revision::DELETED_RESTRICTED ) ) {
-				$link = $sk->revDeleteLinkDisabled( $canHide ); // revision was hidden from sysops
-			} else {
-				$query = array(
-					'type' 	 => 'revision',
-					'target' => $rev->mTitle->getPrefixedDbkey(),
-					'ids' 	 => $rev->getId()
-				);
-				$link = $sk->revDeleteLink( $query,
-					$rev->isDeleted( Revision::DELETED_RESTRICTED ), $canHide );
-			}
-			$link = '&nbsp;&nbsp;&nbsp;' . $link . ' ';
-		}
-		return $link;
-	}
-
-	/**
-	 * Show the new revision of the page.
-	 */
-	function renderNewRevision() {
-		global $wgOut, $wgUser;
-		wfProfileIn( __METHOD__ );
-
-		$wgOut->addHTML( "<hr /><h2>{$this->mPagetitle}</h2>\n" );
-		# Add deleted rev tag if needed
-		if( !$this->mNewRev->userCan(Revision::DELETED_TEXT) ) {
-			$wgOut->wrapWikiMsg( "<div class='mw-warning plainlinks'>\n$1</div>\n", 'rev-deleted-text-permission' );
-		} else if( $this->mNewRev->isDeleted(Revision::DELETED_TEXT) ) {
-			$wgOut->wrapWikiMsg( "<div class='mw-warning plainlinks'>\n$1</div>\n", 'rev-deleted-text-view' );
-		}
-
-		if( !$this->mNewRev->isCurrent() ) {
-			$oldEditSectionSetting = $wgOut->parserOptions()->setEditSection( false );
-		}
-
-		$this->loadNewText();
-		if( is_object( $this->mNewRev ) ) {
-			$wgOut->setRevisionId( $this->mNewRev->getId() );
-		}
-
-		if( $this->mTitle->isCssJsSubpage() || $this->mTitle->isCssOrJsPage() ) {
-			// Stolen from Article::view --AG 2007-10-11
-			// Give hooks a chance to customise the output
-			if( wfRunHooks( 'ShowRawCssJs', array( $this->mNewtext, $this->mTitle, $wgOut ) ) ) {
-				// Wrap the whole lot in a <pre> and don't parse
-				$m = array();
-				preg_match( '!\.(css|js)$!u', $this->mTitle->getText(), $m );
-				$wgOut->addHTML( "<pre class=\"mw-code mw-{$m[1]}\" dir=\"ltr\">\n" );
-				$wgOut->addHTML( htmlspecialchars( $this->mNewtext ) );
-				$wgOut->addHTML( "\n</pre>\n" );
-			}
-		} else {
-			$wgOut->addWikiTextTidy( $this->mNewtext );
-		}
-
-		if( is_object( $this->mNewRev ) && !$this->mNewRev->isCurrent() ) {
-			$wgOut->parserOptions()->setEditSection( $oldEditSectionSetting );
-		}
-		# Add redundant patrol link on bottom...
-		if( $this->mRcidMarkPatrolled && $this->mTitle->quickUserCan('patrol') ) {
-			$sk = $wgUser->getSkin();
-			$wgOut->addHTML(
-				"<div class='patrollink'>[" . $sk->link(
-					$this->mTitle,
-					wfMsgHtml( 'markaspatrolleddiff' ),
-					array(),
-					array(
-						'action' => 'markpatrolled',
-						'rcid' => $this->mRcidMarkPatrolled
-					)
-				) . ']</div>'
-			 );
-		}
-
-		wfProfileOut( __METHOD__ );
-	}
-
-	/**
-	 * Show the first revision of an article. Uses normal diff headers in
-	 * contrast to normal "old revision" display style.
-	 */
-	function showFirstRevision() {
-		global $wgOut, $wgUser;
-		wfProfileIn( __METHOD__ );
-
-		# Get article text from the DB
-		#
-		if ( ! $this->loadNewText() ) {
-			$t = $this->mTitle->getPrefixedText();
-			$d = wfMsgExt( 'missingarticle-diff', array( 'escape' ), $this->mOldid, $this->mNewid );
-			$wgOut->setPagetitle( wfMsg( 'errorpagetitle' ) );
-			$wgOut->addWikiMsg( 'missing-article', "<nowiki>$t</nowiki>", $d );
-			wfProfileOut( __METHOD__ );
-			return;
-		}
-		if ( $this->mNewRev->isCurrent() ) {
-			$wgOut->setArticleFlag( true );
-		}
-
-		# Check if user is allowed to look at this page. If not, bail out.
-		#
-		if ( !$this->mTitle->userCanRead() ) {
-			$wgOut->loginToUse();
-			$wgOut->output();
-			wfProfileOut( __METHOD__ );
-			throw new MWException("Permission Error: you do not have access to view this page");
-		}
-
-		# Prepare the header box
-		#
-		$sk = $wgUser->getSkin();
-
-		$next = $this->mTitle->getNextRevisionID( $this->mNewid );
-		if( !$next ) {
-			$nextlink = '';
-		} else {
-			$nextlink = '<br />' . $sk->link(
-				$this->mTitle,
-				wfMsgHtml( 'nextdiff' ),
-				array(
-					'id' => 'differences-nextlink'
-				),
-				array(
-					'diff' => 'next',
-					'oldid' => $this->mNewid,
-				),
-				array(
-					'known',
-					'noclasses'
-				)
-			);
-		}
-		$header = "<div class=\"firstrevisionheader\" style=\"text-align: center\">" .
-			$sk->revUserTools( $this->mNewRev ) . "<br />" . $sk->revComment( $this->mNewRev ) . $nextlink . "</div>\n";
-
-		$wgOut->addHTML( $header );
-
-		$wgOut->setSubtitle( wfMsgExt( 'difference', array( 'parseinline' ) ) );
-		$wgOut->setRobotPolicy( 'noindex,nofollow' );
-
-		wfProfileOut( __METHOD__ );
-	}
-
-	/**
-	 * Get the diff text, send it to $wgOut
-	 * Returns false if the diff could not be generated, otherwise returns true
-	 */
-	function showDiff( $otitle, $ntitle, $notice = '' ) {
-		global $wgOut;
-		$diff = $this->getDiff( $otitle, $ntitle, $notice );
-		if ( $diff === false ) {
-			$wgOut->addWikiMsg( 'missing-article', "<nowiki>(fixme, bug)</nowiki>", '' );
-			return false;
-		} else {
-			$this->showDiffStyle();
-			$wgOut->addHTML( $diff );
-			return true;
-		}
-	}
-
-	/**
-	 * Add style sheets and supporting JS for diff display.
-	 */
-	function showDiffStyle() {
-		global $wgStylePath, $wgStyleVersion, $wgOut;
-		$wgOut->addStyle( 'common/diff.css' );
-
-		// JS is needed to detect old versions of Mozilla to work around an annoyance bug.
-		$wgOut->addScript( "<script type=\"text/javascript\" src=\"$wgStylePath/common/diff.js?$wgStyleVersion\"></script>" );
-	}
-
-	/**
-	 * Get complete diff table, including header
-	 *
-	 * @param Title $otitle Old title
-	 * @param Title $ntitle New title
-	 * @param string $notice HTML between diff header and body
-	 * @return mixed
-	 */
-	function getDiff( $otitle, $ntitle, $notice = '' ) {
-		$body = $this->getDiffBody();
-		if ( $body === false ) {
-			return false;
-		} else {
-			$multi = $this->getMultiNotice();
-			return $this->addHeader( $body, $otitle, $ntitle, $multi, $notice );
-		}
-	}
-
-	/**
-	 * Get the diff table body, without header
-	 *
-	 * @return mixed
-	 */
-	function getDiffBody() {
-		global $wgMemc;
-		wfProfileIn( __METHOD__ );
-		$this->mCacheHit = true;
-		// Check if the diff should be hidden from this user
-		if ( !$this->loadRevisionData() )
-			return '';
-		if ( $this->mOldRev && !$this->mOldRev->userCan(Revision::DELETED_TEXT) ) {
-			return '';
-		} else if ( $this->mNewRev && !$this->mNewRev->userCan(Revision::DELETED_TEXT) ) {
-			return '';
-		} else if ( $this->mOldRev && $this->mNewRev && $this->mOldRev->getID() == $this->mNewRev->getID() ) {
-			return '';
-		}
-		// Cacheable?
-		$key = false;
-		if ( $this->mOldid && $this->mNewid ) {
-			$key = wfMemcKey( 'diff', 'version', MW_DIFF_VERSION, 'oldid', $this->mOldid, 'newid', $this->mNewid );
-			// Try cache
-			if ( !$this->mRefreshCache ) {
-				$difftext = $wgMemc->get( $key );
-				if ( $difftext ) {
-					wfIncrStats( 'diff_cache_hit' );
-					$difftext = $this->localiseLineNumbers( $difftext );
-					$difftext .= "\n<!-- diff cache key $key -->\n";
-					wfProfileOut( __METHOD__ );
-					return $difftext;
-				}
-			} // don't try to load but save the result
-		}
-		$this->mCacheHit = false;
-
-		// Loadtext is permission safe, this just clears out the diff
-		if ( !$this->loadText() ) {
-			wfProfileOut( __METHOD__ );
-			return false;
-		}
-
-		$difftext = $this->generateDiffBody( $this->mOldtext, $this->mNewtext );
-
-		// Save to cache for 7 days
-		if ( !wfRunHooks( 'AbortDiffCache', array( &$this ) ) ) {
-			wfIncrStats( 'diff_uncacheable' );
-		} else if ( $key !== false && $difftext !== false ) {
-			wfIncrStats( 'diff_cache_miss' );
-			$wgMemc->set( $key, $difftext, 7*86400 );
-		} else {
-			wfIncrStats( 'diff_uncacheable' );
-		}
-		// Replace line numbers with the text in the user's language
-		if ( $difftext !== false ) {
-			$difftext = $this->localiseLineNumbers( $difftext );
-		}
-		wfProfileOut( __METHOD__ );
-		return $difftext;
-	}
-
-	/**
-	 * Make sure the proper modules are loaded before we try to
-	 * make the diff
-	 */
-	private function initDiffEngines() {
-		global $wgExternalDiffEngine;
-		if ( $wgExternalDiffEngine == 'wikidiff' && !function_exists( 'wikidiff_do_diff' ) ) {
-			wfProfileIn( __METHOD__ . '-php_wikidiff.so' );
-			wfSuppressWarnings();
-			dl( 'php_wikidiff.so' );
-			wfRestoreWarnings();
-			wfProfileOut( __METHOD__ . '-php_wikidiff.so' );
-		}
-		else if ( $wgExternalDiffEngine == 'wikidiff2' && !function_exists( 'wikidiff2_do_diff' ) ) {
-			wfProfileIn( __METHOD__ . '-php_wikidiff2.so' );
-			wfSuppressWarnings();
-			dl( 'php_wikidiff2.so' );
-			wfRestoreWarnings();
-			wfProfileOut( __METHOD__ . '-php_wikidiff2.so' );
-		}
-	}
-
-	/**
-	 * Generate a diff, no caching
-	 * $otext and $ntext must be already segmented
-	 */
-	function generateDiffBody( $otext, $ntext ) {
-		global $wgExternalDiffEngine, $wgContLang;
-
-		$otext = str_replace( "\r\n", "\n", $otext );
-		$ntext = str_replace( "\r\n", "\n", $ntext );
-
-		$this->initDiffEngines();
-
-		if ( $wgExternalDiffEngine == 'wikidiff' && function_exists( 'wikidiff_do_diff' ) ) {
-			# For historical reasons, external diff engine expects
-			# input text to be HTML-escaped already
-			$otext = htmlspecialchars ( $wgContLang->segmentForDiff( $otext ) );
-			$ntext = htmlspecialchars ( $wgContLang->segmentForDiff( $ntext ) );
-			return $wgContLang->unsegementForDiff( wikidiff_do_diff( $otext, $ntext, 2 ) ) .
-			$this->debug( 'wikidiff1' );
-		}
-
-		if ( $wgExternalDiffEngine == 'wikidiff2' && function_exists( 'wikidiff2_do_diff' ) ) {
-			# Better external diff engine, the 2 may some day be dropped
-			# This one does the escaping and segmenting itself
-			wfProfileIn( 'wikidiff2_do_diff' );
-			$text = wikidiff2_do_diff( $otext, $ntext, 2 );
-			$text .= $this->debug( 'wikidiff2' );
-			wfProfileOut( 'wikidiff2_do_diff' );
-			return $text;
-		}
-		if ( $wgExternalDiffEngine != 'wikidiff3' && $wgExternalDiffEngine !== false ) {
-			# Diff via the shell
-			global $wgTmpDirectory;
-			$tempName1 = tempnam( $wgTmpDirectory, 'diff_' );
-			$tempName2 = tempnam( $wgTmpDirectory, 'diff_' );
-
-			$tempFile1 = fopen( $tempName1, "w" );
-			if ( !$tempFile1 ) {
-				wfProfileOut( __METHOD__ );
-				return false;
-			}
-			$tempFile2 = fopen( $tempName2, "w" );
-			if ( !$tempFile2 ) {
-				wfProfileOut( __METHOD__ );
-				return false;
-			}
-			fwrite( $tempFile1, $otext );
-			fwrite( $tempFile2, $ntext );
-			fclose( $tempFile1 );
-			fclose( $tempFile2 );
-			$cmd = wfEscapeShellArg( $wgExternalDiffEngine, $tempName1, $tempName2 );
-			wfProfileIn( __METHOD__ . "-shellexec" );
-			$difftext = wfShellExec( $cmd );
-			$difftext .= $this->debug( "external $wgExternalDiffEngine" );
-			wfProfileOut( __METHOD__ . "-shellexec" );
-			unlink( $tempName1 );
-			unlink( $tempName2 );
-			return $difftext;
-		}
-
-		# Native PHP diff
-		$ota = explode( "\n", $wgContLang->segmentForDiff( $otext ) );
-		$nta = explode( "\n", $wgContLang->segmentForDiff( $ntext ) );
-		$diffs = new Diff( $ota, $nta );
-		$formatter = new TableDiffFormatter();
-		return $wgContLang->unsegmentForDiff( $formatter->format( $diffs ) ) .
-		$this->debug();
-	}
-
-	/**
-	 * Generate a debug comment indicating diff generating time,
-	 * server node, and generator backend.
-	 */
-	protected function debug( $generator="internal" ) {
-		global $wgShowHostnames;
-		if ( !$this->enableDebugComment ) {
-			return '';
-		}
-		$data = array( $generator );
-		if( $wgShowHostnames ) {
-			$data[] = wfHostname();
-		}
-		$data[] = wfTimestamp( TS_DB );
-		return "<!-- diff generator: " .
-		implode( " ",
-		array_map(
-					"htmlspecialchars",
-		$data ) ) .
-			" -->\n";
-	}
-
-	/**
-	 * Replace line numbers with the text in the user's language
-	 */
-	function localiseLineNumbers( $text ) {
-		return preg_replace_callback( '/<!--LINE (\d+)-->/',
-		array( &$this, 'localiseLineNumbersCb' ), $text );
-	}
-
-	function localiseLineNumbersCb( $matches ) {
-		global $wgLang;
-		if ( $matches[1] === '1' && $this->mReducedLineNumbers ) return '';
-		return wfMsgExt( 'lineno', 'escape', $wgLang->formatNum( $matches[1] ) );
-	}
-
-
-	/**
-	 * If there are revisions between the ones being compared, return a note saying so.
-	 */
-	function getMultiNotice() {
-		if ( !is_object($this->mOldRev) || !is_object($this->mNewRev) )
-		return '';
-
-		if( !$this->mOldPage->equals( $this->mNewPage ) ) {
-			// Comparing two different pages? Count would be meaningless.
-			return '';
-		}
-
-		$oldid = $this->mOldRev->getId();
-		$newid = $this->mNewRev->getId();
-		if ( $oldid > $newid ) {
-			$tmp = $oldid; $oldid = $newid; $newid = $tmp;
-		}
-
-		$n = $this->mTitle->countRevisionsBetween( $oldid, $newid );
-		if ( !$n )
-		return '';
-
-		return wfMsgExt( 'diff-multi', array( 'parseinline' ), $n );
-	}
-
-
-	/**
-	 * Add the header to a diff body
-	 */
-	static function addHeader( $diff, $otitle, $ntitle, $multi = '', $notice = '' ) {
-		$header = "<table class='diff'>";
-		if( $diff ) { // Safari/Chrome show broken output if cols not used
-			$header .= "
-			<col class='diff-marker' />
-			<col class='diff-content' />
-			<col class='diff-marker' />
-			<col class='diff-content' />";
-			$colspan = 2;
-			$multiColspan = 4;
-		} else {
-			$colspan = 1;
-			$multiColspan = 2;
-		}
-		$header .= "
-		<tr valign='top'>
-		<td colspan='$colspan' class='diff-otitle'>{$otitle}</td>
-		<td colspan='$colspan' class='diff-ntitle'>{$ntitle}</td>
-		</tr>";
-
-		if ( $multi != '' ) {
-			$header .= "<tr><td colspan='{$multiColspan}' align='center' class='diff-multi'>{$multi}</td></tr>";
-		}
-		if ( $notice != '' ) {
-			$header .= "<tr><td colspan='{$multiColspan}' align='center'>{$notice}</td></tr>";
-		}
-
-		return $header . $diff . "</table>";
-	}
-
-	/**
-	 * Use specified text instead of loading from the database
-	 */
-	function setText( $oldText, $newText ) {
-		$this->mOldtext = $oldText;
-		$this->mNewtext = $newText;
-		$this->mTextLoaded = 2;
-		$this->mRevisionsLoaded = true;
-	}
-
-	/**
-	 * Load revision metadata for the specified articles. If newid is 0, then compare
-	 * the old article in oldid to the current article; if oldid is 0, then
-	 * compare the current article to the immediately previous one (ignoring the
-	 * value of newid).
-	 *
-	 * If oldid is false, leave the corresponding revision object set
-	 * to false. This is impossible via ordinary user input, and is provided for
-	 * API convenience.
-	 */
-	function loadRevisionData() {
-		global $wgLang, $wgUser;
-		if ( $this->mRevisionsLoaded ) {
-			return true;
-		} else {
-			// Whether it succeeds or fails, we don't want to try again
-			$this->mRevisionsLoaded = true;
-		}
-
-		// Load the new revision object
-		$this->mNewRev = $this->mNewid
-			? Revision::newFromId( $this->mNewid )
-			: Revision::newFromTitle( $this->mTitle );
-		if( !$this->mNewRev instanceof Revision )
-			return false;
-
-		// Update the new revision ID in case it was 0 (makes life easier doing UI stuff)
-		$this->mNewid = $this->mNewRev->getId();
-
-		// Check if page is editable
-		$editable = $this->mNewRev->getTitle()->userCan( 'edit' );
-
-		// Set assorted variables
-		$timestamp = $wgLang->timeanddate( $this->mNewRev->getTimestamp(), true );
-		$dateofrev = $wgLang->date( $this->mNewRev->getTimestamp(), true );
-		$timeofrev = $wgLang->time( $this->mNewRev->getTimestamp(), true );
-		$this->mNewPage = $this->mNewRev->getTitle();
-		if( $this->mNewRev->isCurrent() ) {
-			$newLink = $this->mNewPage->escapeLocalUrl( array(
-				'oldid' => $this->mNewid
-			) );
-			$this->mPagetitle = htmlspecialchars( wfMsg(
-				'currentrev-asof',
-				$timestamp,
-				$dateofrev,
-				$timeofrev
-			) );
-			$newEdit = $this->mNewPage->escapeLocalUrl( array(
-				'action' => 'edit'
-			) );
-
-			$this->mNewtitle = "<a href='$newLink'>{$this->mPagetitle}</a>";
-			$this->mNewtitle .= " (<a href='$newEdit'>" . wfMsgHtml( $editable ? 'editold' : 'viewsourceold' ) . "</a>)";
-		} else {
-			$newLink = $this->mNewPage->escapeLocalUrl( array(
-				'oldid' => $this->mNewid
-			) );
-			$newEdit = $this->mNewPage->escapeLocalUrl( array(
-				'action' => 'edit',
-				'oldid' => $this->mNewid
-			) );
-			$this->mPagetitle = htmlspecialchars( wfMsg(
-				'revisionasof',
-				$timestamp,
-				$dateofrev,
-				$timeofrev
-			) );
-
-			$this->mNewtitle = "<a href='$newLink'>{$this->mPagetitle}</a>";
-			$this->mNewtitle .= " (<a href='$newEdit'>" . wfMsgHtml( $editable ? 'editold' : 'viewsourceold' ) . "</a>)";
-		}
-		if( !$this->mNewRev->userCan(Revision::DELETED_TEXT) ) {
-			$this->mNewtitle = "<span class='history-deleted'>{$this->mPagetitle}</span>";
-		} else if ( $this->mNewRev->isDeleted(Revision::DELETED_TEXT) ) {
-			$this->mNewtitle = "<span class='history-deleted'>{$this->mNewtitle}</span>";
-		}
-
-		// Load the old revision object
-		$this->mOldRev = false;
-		if( $this->mOldid ) {
-			$this->mOldRev = Revision::newFromId( $this->mOldid );
-		} elseif ( $this->mOldid === 0 ) {
-			$rev = $this->mNewRev->getPrevious();
-			if( $rev ) {
-				$this->mOldid = $rev->getId();
-				$this->mOldRev = $rev;
-			} else {
-				// No previous revision; mark to show as first-version only.
-				$this->mOldid = false;
-				$this->mOldRev = false;
-			}
-		}/* elseif ( $this->mOldid === false ) leave mOldRev false; */
-
-		if( is_null( $this->mOldRev ) ) {
-			return false;
-		}
-
-		if ( $this->mOldRev ) {
-			$this->mOldPage = $this->mOldRev->getTitle();
-
-			$t = $wgLang->timeanddate( $this->mOldRev->getTimestamp(), true );
-			$dateofrev = $wgLang->date( $this->mOldRev->getTimestamp(), true );
-			$timeofrev = $wgLang->time( $this->mOldRev->getTimestamp(), true );
-			$oldLink = $this->mOldPage->escapeLocalUrl( array(
-				'oldid' => $this->mOldid
-			) );
-			$oldEdit = $this->mOldPage->escapeLocalUrl( array(
-				'action' => 'edit',
-				'oldid' => $this->mOldid
-			) );
-			$this->mOldPagetitle = htmlspecialchars( wfMsg( 'revisionasof', $t, $dateofrev, $timeofrev ) );
-
-			$this->mOldtitle = "<a href='$oldLink'>{$this->mOldPagetitle}</a>"
-			. " (<a href='$oldEdit'>" . wfMsgHtml( $editable ? 'editold' : 'viewsourceold' ) . "</a>)";
-			// Add an "undo" link
-			$newUndo = $this->mNewPage->escapeLocalUrl( array(
-				'action' => 'edit',
-				'undoafter' => $this->mOldid,
-				'undo' => $this->mNewid
-			) );
-			$htmlLink = htmlspecialchars( wfMsg( 'editundo' ) );
-			$htmlTitle = $wgUser->getSkin()->tooltip( 'undo' );
-			if( $editable && !$this->mOldRev->isDeleted( Revision::DELETED_TEXT ) && !$this->mNewRev->isDeleted( Revision::DELETED_TEXT ) ) {
-				$this->mNewtitle .= " (<a href='$newUndo' $htmlTitle>" . $htmlLink . "</a>)";
-			}
-
-			if( !$this->mOldRev->userCan( Revision::DELETED_TEXT ) ) {
-				$this->mOldtitle = '<span class="history-deleted">' . $this->mOldPagetitle . '</span>';
-			} else if( $this->mOldRev->isDeleted( Revision::DELETED_TEXT ) ) {
-				$this->mOldtitle = '<span class="history-deleted">' . $this->mOldtitle . '</span>';
-			}
-		}
-
-		return true;
-	}
-
-	/**
-	 * Load the text of the revisions, as well as revision data.
-	 */
-	function loadText() {
-		if ( $this->mTextLoaded == 2 ) {
-			return true;
-		} else {
-			// Whether it succeeds or fails, we don't want to try again
-			$this->mTextLoaded = 2;
-		}
-
-		if ( !$this->loadRevisionData() ) {
-			return false;
-		}
-		if ( $this->mOldRev ) {
-			$this->mOldtext = $this->mOldRev->getText( Revision::FOR_THIS_USER );
-			if ( $this->mOldtext === false ) {
-				return false;
-			}
-		}
-		if ( $this->mNewRev ) {
-			$this->mNewtext = $this->mNewRev->getText( Revision::FOR_THIS_USER );
-			if ( $this->mNewtext === false ) {
-				return false;
-			}
-		}
-		return true;
-	}
-
-	/**
-	 * Load the text of the new revision, not the old one
-	 */
-	function loadNewText() {
-		if ( $this->mTextLoaded >= 1 ) {
-			return true;
-		} else {
-			$this->mTextLoaded = 1;
-		}
-		if ( !$this->loadRevisionData() ) {
-			return false;
-		}
-		$this->mNewtext = $this->mNewRev->getText( Revision::FOR_THIS_USER );
-		return true;
-	}
-}
diff --git a/includes/diff/WikiDiff.php b/includes/diff/WikiDiff.php
new file mode 100644
index 00000000..2d904c96
--- /dev/null
+++ b/includes/diff/WikiDiff.php
@@ -0,0 +1,1241 @@
+<?php
+/**
+ * A PHP diff engine for phpwiki. (Taken from phpwiki-1.3.3)
+ *
+ * Copyright © 2000, 2001 Geoffrey T. Dairiki <dairiki@dairiki.org>
+ * You may copy this code freely under the conditions of the GPL.
+ *
+ * @file
+ * @ingroup DifferenceEngine
+ * @defgroup DifferenceEngine DifferenceEngine
+ */
+
+/**
+ * @todo document
+ * @private
+ * @ingroup DifferenceEngine
+ */
+class _DiffOp {
+	var $type;
+	var $orig;
+	var $closing;
+
+	function reverse() {
+		trigger_error( 'pure virtual', E_USER_ERROR );
+	}
+
+	function norig() {
+		return $this->orig ? sizeof( $this->orig ) : 0;
+	}
+
+	function nclosing() {
+		return $this->closing ? sizeof( $this->closing ) : 0;
+	}
+}
+
+/**
+ * @todo document
+ * @private
+ * @ingroup DifferenceEngine
+ */
+class _DiffOp_Copy extends _DiffOp {
+	var $type = 'copy';
+
+	function __construct ( $orig, $closing = false ) {
+		if ( !is_array( $closing ) )
+		$closing = $orig;
+		$this->orig = $orig;
+		$this->closing = $closing;
+	}
+
+	function reverse() {
+		return new _DiffOp_Copy( $this->closing, $this->orig );
+	}
+}
+
+/**
+ * @todo document
+ * @private
+ * @ingroup DifferenceEngine
+ */
+class _DiffOp_Delete extends _DiffOp {
+	var $type = 'delete';
+
+	function __construct ( $lines ) {
+		$this->orig = $lines;
+		$this->closing = false;
+	}
+
+	function reverse() {
+		return new _DiffOp_Add( $this->orig );
+	}
+}
+
+/**
+ * @todo document
+ * @private
+ * @ingroup DifferenceEngine
+ */
+class _DiffOp_Add extends _DiffOp {
+	var $type = 'add';
+
+	function __construct ( $lines ) {
+		$this->closing = $lines;
+		$this->orig = false;
+	}
+
+	function reverse() {
+		return new _DiffOp_Delete( $this->closing );
+	}
+}
+
+/**
+ * @todo document
+ * @private
+ * @ingroup DifferenceEngine
+ */
+class _DiffOp_Change extends _DiffOp {
+	var $type = 'change';
+
+	function __construct ( $orig, $closing ) {
+		$this->orig = $orig;
+		$this->closing = $closing;
+	}
+
+	function reverse() {
+		return new _DiffOp_Change( $this->closing, $this->orig );
+	}
+}
+
+/**
+ * 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 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
+ *
+ * closingly, some ideas (subdivision by NCHUNKS > 2, and some optimizations)
+ * are my own.
+ *
+ * Line length limits for robustness added by Tim Starling, 2005-08-31
+ * Alternative implementation added by Guy Van den Broeck, 2008-07-30
+ *
+ * @author Geoffrey T. Dairiki, Tim Starling, Guy Van den Broeck
+ * @private
+ * @ingroup DifferenceEngine
+ */
+class _DiffEngine {
+
+	const MAX_XREF_LENGTH =  10000;
+
+	function diff ( $from_lines, $to_lines ) {
+		wfProfileIn( __METHOD__ );
+
+		// Diff and store locally
+		$this->diff_local( $from_lines, $to_lines );
+
+		// 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.
+		$n_from = sizeof( $from_lines );
+		$n_to = sizeof( $to_lines );
+
+		$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 _DiffOp_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 _DiffOp_Change( $delete, $add );
+			elseif ( $delete )
+			$edits[] = new _DiffOp_Delete( $delete );
+			elseif ( $add )
+			$edits[] = new _DiffOp_Add( $add );
+		}
+		wfProfileOut( __METHOD__ );
+		return $edits;
+	}
+
+	function diff_local ( $from_lines, $to_lines ) {
+		global $wgExternalDiffEngine;
+		wfProfileIn( __METHOD__ );
+
+		if ( $wgExternalDiffEngine == 'wikidiff3' ) {
+			// wikidiff3
+			$wikidiff3 = new WikiDiff3();
+			$wikidiff3->diff( $from_lines, $to_lines );
+			$this->xchanged = $wikidiff3->removed;
+			$this->ychanged = $wikidiff3->added;
+			unset( $wikidiff3 );
+		} else {
+			// old diff
+			$n_from = sizeof( $from_lines );
+			$n_to = sizeof( $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++ ) {
+				$xhash[$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 ) );
+		}
+		wfProfileOut( __METHOD__ );
+	}
+
+	/**
+	 * Returns the whole line if it's small enough, or the MD5 hash otherwise
+	 */
+	function _line_hash( $line ) {
+		if ( strlen( $line ) > self::MAX_XREF_LENGTH ) {
+			return md5( $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.
+	 */
+	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;
+			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];
+				reset( $matches );
+				while ( list( , $y ) = each( $matches ) ) {
+					if ( empty( $this->in_seq[$y] ) ) {
+						$k = $this->_lcs_pos( $y );
+						assert( $k > 0 );
+						$ymids[$k] = $ymids[$k -1];
+						break;
+					}
+				}
+				while ( list ( , $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;
+					} else if ( empty( $this->in_seq[$y] ) ) {
+						$k = $this->_lcs_pos( $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 _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;
+		}
+
+		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 ) {
+		// 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;
+			}
+		}
+	}
+
+	/* 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 ) {
+		wfProfileIn( __METHOD__ );
+		$i = 0;
+		$j = 0;
+
+		assert( 'sizeof($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] ) {
+				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]' );
+			}
+		}
+		wfProfileOut( __METHOD__ );
+	}
+}
+
+/**
+ * Class representing a 'diff' between two sequences of strings.
+ * @todo document
+ * @private
+ * @ingroup DifferenceEngine
+ */
+class Diff
+{
+	var $edits;
+
+	/**
+	 * Constructor.
+	 * Computes diff between sequences of strings.
+	 *
+	 * @param $from_lines array An array of strings.
+	 *		  (Typically these are lines from a file.)
+	 * @param $to_lines array An array of strings.
+	 */
+	function __construct( $from_lines, $to_lines ) {
+		$eng = new _DiffEngine;
+		$this->edits = $eng->diff( $from_lines, $to_lines );
+		// $this->_check($from_lines, $to_lines);
+	}
+
+	/**
+	 * Compute reversed Diff.
+	 *
+	 * SYNOPSIS:
+	 *
+	 *	$diff = new Diff($lines1, $lines2);
+	 *	$rev = $diff->reverse();
+	 * @return object A Diff object representing the inverse of the
+	 *				  original diff.
+	 */
+	function reverse () {
+		$rev = $this;
+		$rev->edits = array();
+		foreach ( $this->edits as $edit ) {
+			$rev->edits[] = $edit->reverse();
+		}
+		return $rev;
+	}
+
+	/**
+	 * Check for empty diff.
+	 *
+	 * @return bool True iff two sequences were identical.
+	 */
+	function isEmpty () {
+		foreach ( $this->edits as $edit ) {
+			if ( $edit->type != 'copy' )
+			return false;
+		}
+		return true;
+	}
+
+	/**
+	 * Compute the length of the Longest Common Subsequence (LCS).
+	 *
+	 * This is mostly for diagnostic purposed.
+	 *
+	 * @return int The length of the LCS.
+	 */
+	function lcs () {
+		$lcs = 0;
+		foreach ( $this->edits as $edit ) {
+			if ( $edit->type == 'copy' )
+			$lcs += sizeof( $edit->orig );
+		}
+		return $lcs;
+	}
+
+	/**
+	 * Get the original set of lines.
+	 *
+	 * This reconstructs the $from_lines parameter passed to the
+	 * constructor.
+	 *
+	 * @return array The original sequence of strings.
+	 */
+	function orig() {
+		$lines = array();
+
+		foreach ( $this->edits as $edit ) {
+			if ( $edit->orig )
+			array_splice( $lines, sizeof( $lines ), 0, $edit->orig );
+		}
+		return $lines;
+	}
+
+	/**
+	 * Get the closing set of lines.
+	 *
+	 * This reconstructs the $to_lines parameter passed to the
+	 * constructor.
+	 *
+	 * @return array The sequence of strings.
+	 */
+	function closing() {
+		$lines = array();
+
+		foreach ( $this->edits as $edit ) {
+			if ( $edit->closing )
+			array_splice( $lines, sizeof( $lines ), 0, $edit->closing );
+		}
+		return $lines;
+	}
+
+	/**
+	 * Check a Diff for validity.
+	 *
+	 * This is here only for debugging purposes.
+	 */
+	function _check ( $from_lines, $to_lines ) {
+		wfProfileIn( __METHOD__ );
+		if ( serialize( $from_lines ) != serialize( $this->orig() ) )
+		trigger_error( "Reconstructed original doesn't match", E_USER_ERROR );
+		if ( serialize( $to_lines ) != serialize( $this->closing() ) )
+		trigger_error( "Reconstructed closing doesn't match", E_USER_ERROR );
+
+		$rev = $this->reverse();
+		if ( serialize( $to_lines ) != serialize( $rev->orig() ) )
+		trigger_error( "Reversed original doesn't match", E_USER_ERROR );
+		if ( serialize( $from_lines ) != serialize( $rev->closing() ) )
+		trigger_error( "Reversed closing doesn't match", E_USER_ERROR );
+
+
+		$prevtype = 'none';
+		foreach ( $this->edits as $edit ) {
+			if ( $prevtype == $edit->type )
+			trigger_error( "Edit sequence is non-optimal", E_USER_ERROR );
+			$prevtype = $edit->type;
+		}
+
+		$lcs = $this->lcs();
+		trigger_error( 'Diff okay: LCS = ' . $lcs, E_USER_NOTICE );
+		wfProfileOut( __METHOD__ );
+	}
+}
+
+/**
+ * @todo document, bad name.
+ * @private
+ * @ingroup DifferenceEngine
+ */
+class MappedDiff extends Diff
+{
+	/**
+	 * Constructor.
+	 *
+	 * Computes diff between sequences of strings.
+	 *
+	 * This can be used to compute things like
+	 * case-insensitve diffs, or diffs which ignore
+	 * changes in white-space.
+	 *
+	 * @param $from_lines array An array of strings.
+	 *	(Typically these are lines from a file.)
+	 *
+	 * @param $to_lines array An array of strings.
+	 *
+	 * @param $mapped_from_lines array This array should
+	 *	have the same size number of elements as $from_lines.
+	 *	The elements in $mapped_from_lines and
+	 *	$mapped_to_lines are what is actually compared
+	 *	when computing the diff.
+	 *
+	 * @param $mapped_to_lines array This array should
+	 *	have the same number of elements as $to_lines.
+	 */
+	function __construct( $from_lines, $to_lines,
+	$mapped_from_lines, $mapped_to_lines ) {
+		wfProfileIn( __METHOD__ );
+
+		assert( sizeof( $from_lines ) == sizeof( $mapped_from_lines ) );
+		assert( sizeof( $to_lines ) == sizeof( $mapped_to_lines ) );
+
+		parent::__construct( $mapped_from_lines, $mapped_to_lines );
+
+		$xi = $yi = 0;
+		for ( $i = 0; $i < sizeof( $this->edits ); $i++ ) {
+			$orig = &$this->edits[$i]->orig;
+			if ( is_array( $orig ) ) {
+				$orig = array_slice( $from_lines, $xi, sizeof( $orig ) );
+				$xi += sizeof( $orig );
+			}
+
+			$closing = &$this->edits[$i]->closing;
+			if ( is_array( $closing ) ) {
+				$closing = array_slice( $to_lines, $yi, sizeof( $closing ) );
+				$yi += sizeof( $closing );
+			}
+		}
+		wfProfileOut( __METHOD__ );
+	}
+}
+
+/**
+ * A class to format Diffs
+ *
+ * This class formats the diff in classic diff format.
+ * It is intended that this class be customized via inheritance,
+ * to obtain fancier outputs.
+ * @todo document
+ * @private
+ * @ingroup DifferenceEngine
+ */
+class DiffFormatter {
+	/**
+	 * Number of leading context "lines" to preserve.
+	 *
+	 * This should be left at zero for this class, but subclasses
+	 * may want to set this to other values.
+	 */
+	var $leading_context_lines = 0;
+
+	/**
+	 * Number of trailing context "lines" to preserve.
+	 *
+	 * This should be left at zero for this class, but subclasses
+	 * may want to set this to other values.
+	 */
+	var $trailing_context_lines = 0;
+
+	/**
+	 * Format a diff.
+	 *
+	 * @param $diff object A Diff object.
+	 * @return string The formatted output.
+	 */
+	function format( $diff ) {
+		wfProfileIn( __METHOD__ );
+
+		$xi = $yi = 1;
+		$block = false;
+		$context = array();
+
+		$nlead = $this->leading_context_lines;
+		$ntrail = $this->trailing_context_lines;
+
+		$this->_start_diff();
+
+		foreach ( $diff->edits as $edit ) {
+			if ( $edit->type == 'copy' ) {
+				if ( is_array( $block ) ) {
+					if ( sizeof( $edit->orig ) <= $nlead + $ntrail ) {
+						$block[] = $edit;
+					}
+					else {
+						if ( $ntrail ) {
+							$context = array_slice( $edit->orig, 0, $ntrail );
+							$block[] = new _DiffOp_Copy( $context );
+						}
+						$this->_block( $x0, $ntrail + $xi - $x0,
+						$y0, $ntrail + $yi - $y0,
+						$block );
+						$block = false;
+					}
+				}
+				$context = $edit->orig;
+			}
+			else {
+				if ( ! is_array( $block ) ) {
+					$context = array_slice( $context, sizeof( $context ) - $nlead );
+					$x0 = $xi - sizeof( $context );
+					$y0 = $yi - sizeof( $context );
+					$block = array();
+					if ( $context )
+					$block[] = new _DiffOp_Copy( $context );
+				}
+				$block[] = $edit;
+			}
+
+			if ( $edit->orig )
+			$xi += sizeof( $edit->orig );
+			if ( $edit->closing )
+			$yi += sizeof( $edit->closing );
+		}
+
+		if ( is_array( $block ) )
+		$this->_block( $x0, $xi - $x0,
+		$y0, $yi - $y0,
+		$block );
+
+		$end = $this->_end_diff();
+		wfProfileOut( __METHOD__ );
+		return $end;
+	}
+
+	function _block( $xbeg, $xlen, $ybeg, $ylen, &$edits ) {
+		wfProfileIn( __METHOD__ );
+		$this->_start_block( $this->_block_header( $xbeg, $xlen, $ybeg, $ylen ) );
+		foreach ( $edits as $edit ) {
+			if ( $edit->type == 'copy' )
+			$this->_context( $edit->orig );
+			elseif ( $edit->type == 'add' )
+			$this->_added( $edit->closing );
+			elseif ( $edit->type == 'delete' )
+			$this->_deleted( $edit->orig );
+			elseif ( $edit->type == 'change' )
+			$this->_changed( $edit->orig, $edit->closing );
+			else
+			trigger_error( 'Unknown edit type', E_USER_ERROR );
+		}
+		$this->_end_block();
+		wfProfileOut( __METHOD__ );
+	}
+
+	function _start_diff() {
+		ob_start();
+	}
+
+	function _end_diff() {
+		$val = ob_get_contents();
+		ob_end_clean();
+		return $val;
+	}
+
+	function _block_header( $xbeg, $xlen, $ybeg, $ylen ) {
+		if ( $xlen > 1 )
+		$xbeg .= "," . ( $xbeg + $xlen - 1 );
+		if ( $ylen > 1 )
+		$ybeg .= "," . ( $ybeg + $ylen - 1 );
+
+		return $xbeg . ( $xlen ? ( $ylen ? 'c' : 'd' ) : 'a' ) . $ybeg;
+	}
+
+	function _start_block( $header ) {
+		echo $header . "\n";
+	}
+
+	function _end_block() {
+	}
+
+	function _lines( $lines, $prefix = ' ' ) {
+		foreach ( $lines as $line )
+		echo "$prefix $line\n";
+	}
+
+	function _context( $lines ) {
+		$this->_lines( $lines );
+	}
+
+	function _added( $lines ) {
+		$this->_lines( $lines, '>' );
+	}
+	function _deleted( $lines ) {
+		$this->_lines( $lines, '<' );
+	}
+
+	function _changed( $orig, $closing ) {
+		$this->_deleted( $orig );
+		echo "---\n";
+		$this->_added( $closing );
+	}
+}
+
+/**
+ * A formatter that outputs unified diffs
+ * @ingroup DifferenceEngine
+ */
+
+class UnifiedDiffFormatter extends DiffFormatter {
+	var $leading_context_lines = 2;
+	var $trailing_context_lines = 2;
+
+	function _added( $lines ) {
+		$this->_lines( $lines, '+' );
+	}
+	function _deleted( $lines ) {
+		$this->_lines( $lines, '-' );
+	}
+	function _changed( $orig, $closing ) {
+		$this->_deleted( $orig );
+		$this->_added( $closing );
+	}
+	function _block_header( $xbeg, $xlen, $ybeg, $ylen ) {
+		return "@@ -$xbeg,$xlen +$ybeg,$ylen @@";
+	}
+}
+
+/**
+ * A pseudo-formatter that just passes along the Diff::$edits array
+ * @ingroup DifferenceEngine
+ */
+class ArrayDiffFormatter extends DiffFormatter {
+	function format( $diff ) {
+		$oldline = 1;
+		$newline = 1;
+		$retval = array();
+		foreach ( $diff->edits as $edit )
+		switch( $edit->type ) {
+			case 'add':
+				foreach ( $edit->closing as $l ) {
+					$retval[] = array(
+							'action' => 'add',
+							'new' => $l,
+							'newline' => $newline++
+					);
+				}
+				break;
+			case 'delete':
+				foreach ( $edit->orig as $l ) {
+					$retval[] = array(
+							'action' => 'delete',
+							'old' => $l,
+							'oldline' => $oldline++,
+					);
+				}
+				break;
+			case 'change':
+				foreach ( $edit->orig as $i => $l ) {
+					$retval[] = array(
+							'action' => 'change',
+							'old' => $l,
+							'new' => @$edit->closing[$i],
+							'oldline' => $oldline++,
+							'newline' => $newline++,
+					);
+				}
+				break;
+			case 'copy':
+				$oldline += count( $edit->orig );
+				$newline += count( $edit->orig );
+		}
+		return $retval;
+	}
+}
+
+/**
+ *	Additions by Axel Boldt follow, partly taken from diff.php, phpwiki-1.3.3
+ *
+ */
+
+define( 'NBSP', '&#160;' ); // iso-8859-x non-breaking space.
+
+/**
+ * @todo document
+ * @private
+ * @ingroup DifferenceEngine
+ */
+class _HWLDF_WordAccumulator {
+	function __construct () {
+		$this->_lines = array();
+		$this->_line = '';
+		$this->_group = '';
+		$this->_tag = '';
+	}
+
+	function _flushGroup ( $new_tag ) {
+		if ( $this->_group !== '' ) {
+			if ( $this->_tag == 'ins' )
+			$this->_line .= '<ins class="diffchange diffchange-inline">' .
+			htmlspecialchars ( $this->_group ) . '</ins>';
+			elseif ( $this->_tag == 'del' )
+			$this->_line .= '<del class="diffchange diffchange-inline">' .
+			htmlspecialchars ( $this->_group ) . '</del>';
+			else
+			$this->_line .= htmlspecialchars ( $this->_group );
+		}
+		$this->_group = '';
+		$this->_tag = $new_tag;
+	}
+
+	function _flushLine ( $new_tag ) {
+		$this->_flushGroup( $new_tag );
+		if ( $this->_line != '' )
+		array_push ( $this->_lines, $this->_line );
+		else
+		# make empty lines visible by inserting an NBSP
+		array_push ( $this->_lines, NBSP );
+		$this->_line = '';
+	}
+
+	function addWords ( $words, $tag = '' ) {
+		if ( $tag != $this->_tag )
+		$this->_flushGroup( $tag );
+
+		foreach ( $words as $word ) {
+			// new-line should only come as first char of word.
+			if ( $word == '' )
+			continue;
+			if ( $word[0] == "\n" ) {
+				$this->_flushLine( $tag );
+				$word = substr( $word, 1 );
+			}
+			assert( !strstr( $word, "\n" ) );
+			$this->_group .= $word;
+		}
+	}
+
+	function getLines() {
+		$this->_flushLine( '~done' );
+		return $this->_lines;
+	}
+}
+
+/**
+ * @todo document
+ * @private
+ * @ingroup DifferenceEngine
+ */
+class WordLevelDiff extends MappedDiff {
+	const MAX_LINE_LENGTH = 10000;
+
+	function __construct ( $orig_lines, $closing_lines ) {
+		wfProfileIn( __METHOD__ );
+
+		list ( $orig_words, $orig_stripped ) = $this->_split( $orig_lines );
+		list ( $closing_words, $closing_stripped ) = $this->_split( $closing_lines );
+
+		parent::__construct( $orig_words, $closing_words,
+		$orig_stripped, $closing_stripped );
+		wfProfileOut( __METHOD__ );
+	}
+
+	function _split( $lines ) {
+		wfProfileIn( __METHOD__ );
+
+		$words = array();
+		$stripped = array();
+		$first = true;
+		foreach ( $lines as $line ) {
+			# If the line is too long, just pretend the entire line is one big word
+			# This prevents resource exhaustion problems
+			if ( $first ) {
+				$first = false;
+			} else {
+				$words[] = "\n";
+				$stripped[] = "\n";
+			}
+			if ( strlen( $line ) > self::MAX_LINE_LENGTH ) {
+				$words[] = $line;
+				$stripped[] = $line;
+			} else {
+				$m = array();
+				if ( preg_match_all( '/ ( [^\S\n]+ | [0-9_A-Za-z\x80-\xff]+ | . ) (?: (?!< \n) [^\S\n])? /xs',
+				$line, $m ) )
+				{
+					$words = array_merge( $words, $m[0] );
+					$stripped = array_merge( $stripped, $m[1] );
+				}
+			}
+		}
+		wfProfileOut( __METHOD__ );
+		return array( $words, $stripped );
+	}
+
+	function orig () {
+		wfProfileIn( __METHOD__ );
+		$orig = new _HWLDF_WordAccumulator;
+
+		foreach ( $this->edits as $edit ) {
+			if ( $edit->type == 'copy' )
+			$orig->addWords( $edit->orig );
+			elseif ( $edit->orig )
+			$orig->addWords( $edit->orig, 'del' );
+		}
+		$lines = $orig->getLines();
+		wfProfileOut( __METHOD__ );
+		return $lines;
+	}
+
+	function closing () {
+		wfProfileIn( __METHOD__ );
+		$closing = new _HWLDF_WordAccumulator;
+
+		foreach ( $this->edits as $edit ) {
+			if ( $edit->type == 'copy' )
+			$closing->addWords( $edit->closing );
+			elseif ( $edit->closing )
+			$closing->addWords( $edit->closing, 'ins' );
+		}
+		$lines = $closing->getLines();
+		wfProfileOut( __METHOD__ );
+		return $lines;
+	}
+}
+
+/**
+ * Wikipedia Table style diff formatter.
+ * @todo document
+ * @private
+ * @ingroup DifferenceEngine
+ */
+class TableDiffFormatter extends DiffFormatter {
+	function __construct() {
+		$this->leading_context_lines = 2;
+		$this->trailing_context_lines = 2;
+	}
+
+	public static function escapeWhiteSpace( $msg ) {
+		$msg = preg_replace( '/^ /m', '&#160; ', $msg );
+		$msg = preg_replace( '/ $/m', ' &#160;', $msg );
+		$msg = preg_replace( '/  /', '&#160; ', $msg );
+		return $msg;
+	}
+
+	function _block_header( $xbeg, $xlen, $ybeg, $ylen ) {
+		$r = '<tr><td colspan="2" class="diff-lineno"><!--LINE ' . $xbeg . "--></td>\n" .
+		  '<td colspan="2" class="diff-lineno"><!--LINE ' . $ybeg . "--></td></tr>\n";
+		return $r;
+	}
+
+	function _start_block( $header ) {
+		echo $header;
+	}
+
+	function _end_block() {
+	}
+
+	function _lines( $lines, $prefix = ' ', $color = 'white' ) {
+	}
+
+	# HTML-escape parameter before calling this
+	function addedLine( $line ) {
+		return $this->wrapLine( '+', 'diff-addedline', $line );
+	}
+
+	# HTML-escape parameter before calling this
+	function deletedLine( $line ) {
+		return $this->wrapLine( '&minus;', 'diff-deletedline', $line );
+	}
+
+	# HTML-escape parameter before calling this
+	function contextLine( $line ) {
+		return $this->wrapLine( '&#160;', 'diff-context', $line );
+	}
+
+	private function wrapLine( $marker, $class, $line ) {
+		if ( $line !== '' ) {
+			// The <div> wrapper is needed for 'overflow: auto' style to scroll properly
+			$line = Xml::tags( 'div', null, $this->escapeWhiteSpace( $line ) );
+		}
+		return "<td class='diff-marker'>$marker</td><td class='$class'>$line</td>";
+	}
+
+	function emptyLine() {
+		return '<td colspan="2">&#160;</td>';
+	}
+
+	function _added( $lines ) {
+		foreach ( $lines as $line ) {
+			echo '<tr>' . $this->emptyLine() .
+			$this->addedLine( '<ins class="diffchange">' .
+			htmlspecialchars ( $line ) . '</ins>' ) . "</tr>\n";
+		}
+	}
+
+	function _deleted( $lines ) {
+		foreach ( $lines as $line ) {
+			echo '<tr>' . $this->deletedLine( '<del class="diffchange">' .
+			htmlspecialchars ( $line ) . '</del>' ) .
+			$this->emptyLine() . "</tr>\n";
+		}
+	}
+
+	function _context( $lines ) {
+		foreach ( $lines as $line ) {
+			echo '<tr>' .
+			$this->contextLine( htmlspecialchars ( $line ) ) .
+			$this->contextLine( htmlspecialchars ( $line ) ) . "</tr>\n";
+		}
+	}
+
+	function _changed( $orig, $closing ) {
+		wfProfileIn( __METHOD__ );
+
+		$diff = new WordLevelDiff( $orig, $closing );
+		$del = $diff->orig();
+		$add = $diff->closing();
+
+		# Notice that WordLevelDiff returns HTML-escaped output.
+		# Hence, we will be calling addedLine/deletedLine without HTML-escaping.
+
+		while ( $line = array_shift( $del ) ) {
+			$aline = array_shift( $add );
+			echo '<tr>' . $this->deletedLine( $line ) .
+			$this->addedLine( $aline ) . "</tr>\n";
+		}
+		foreach ( $add as $line ) {	# If any leftovers
+			echo '<tr>' . $this->emptyLine() .
+			$this->addedLine( $line ) . "</tr>\n";
+		}
+		wfProfileOut( __METHOD__ );
+	}
+}
diff --git a/includes/diff/WikiDiff3.php b/includes/diff/WikiDiff3.php
new file mode 100644
index 00000000..8def296d
--- /dev/null
+++ b/includes/diff/WikiDiff3.php
@@ -0,0 +1,586 @@
+<?php
+/**
+ * New version of the difference engine
+ *
+ * Copyright © 2008 Guy Van den Broeck <guy@guyvdb.eu>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @ingroup DifferenceEngine
+ */
+
+/**
+ * This diff implementation is mainly lifted from the LCS algorithm of the Eclipse project which
+ * in turn is based on Myers' "An O(ND) difference algorithm and its variations"
+ * (http://citeseer.ist.psu.edu/myers86ond.html) with range compression (see Wu et al.'s
+ * "An O(NP) Sequence Comparison Algorithm").
+ *
+ * This implementation supports an upper bound on the excution time.
+ *
+ * Complexity: O((M + N)D) worst case time, O(M + N + D^2) expected time, O(M + N) space
+ *
+ * @author Guy Van den Broeck
+ * @ingroup DifferenceEngine
+ */
+class WikiDiff3 {
+
+	// Input variables
+	private $from;
+	private $to;
+	private $m;
+	private $n;
+
+	private $tooLong;
+	private $powLimit;
+
+	// State variables
+	private $maxDifferences;
+	private $lcsLengthCorrectedForHeuristic = false;
+
+	// Output variables
+	public $length;
+	public $removed;
+	public $added;
+	public $heuristicUsed;
+
+	function __construct( $tooLong = 2000000, $powLimit = 1.45 ) {
+		$this->tooLong = $tooLong;
+		$this->powLimit = $powLimit;
+	}
+
+	public function diff( /*array*/ $from, /*array*/ $to ) {
+		// remember initial lengths
+		$m = sizeof( $from );
+		$n = count( $to );
+
+		$this->heuristicUsed = false;
+
+		// output
+		$removed = $m > 0 ? array_fill( 0, $m, true ) : array();
+		$added = $n > 0 ? array_fill( 0, $n, true ) : array();
+
+		// 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 = array();
+
+		// remove tokens not in both sequences
+		$shared = array();
+		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 );
+
+		$this->removed = $this->m > 0 ? array_fill( 0, $this->m, true ) : array();
+		$this->added = $this->n > 0 ? array_fill( 0, $this->n, true ) : array();
+
+		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( pow( $this->maxDifferences, $this->powLimit - 1.0 ) );
+				wfDebug( "Limiting max number of differences to $this->maxDifferences\n" );
+			}
+
+			/*
+			 * 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 = array( $temp, $temp );
+			$snake = array( 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;
+	}
+
+	function diff_range( $from_lines, $to_lines ) {
+		// Diff and store locally
+		$this->diff( $from_lines, $to_lines );
+		unset( $from_lines, $to_lines );
+
+		$ranges = array();
+		$xi = $yi = 0;
+		while ( $xi < $this->m || $yi < $this->n ) {
+			// Matching "snake".
+			while ( $xi < $this->m && $yi < $this->n
+					&& !$this->removed[$xi]
+					&& !$this->added[$yi] ) {
+				++$xi;
+				++$yi;
+			}
+			// Find deletes & adds.
+			$xstart = $xi;
+			while ( $xi < $this->m && $this->removed[$xi] ) {
+				++$xi;
+			}
+
+			$ystart = $yi;
+			while ( $yi < $this->n && $this->added[$yi] ) {
+				++$yi;
+			}
+
+			if ( $xi > $xstart || $yi > $ystart ) {
+				$ranges[] = new RangeDifference( $xstart, $xi,
+								$ystart, $yi );
+			}
+		}
+		return $ranges;
+	}
+
+	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
+		$len = $snake[2];
+		$startx = $snake[0];
+		$starty = $snake[1];
+
+		// 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 );
+		} else if ( $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 ) == 1 ) {
+			$value_to_add_forward = 1;
+		} else {
+			$value_to_add_forward = 0;
+		}
+
+		if ( ( $N & 1 ) == 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 ) == 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;
+					} else if ( $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;
+					} else if ( $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;
+					} else if ( $absy -$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;
+					} else if ( $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.\n" );
+		$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 = array( 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;
+			} else if ( $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;
+			} else if ( $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[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;
+	}
+
+}
+
+/**
+ * Alternative representation of a set of changes, by the index
+ * ranges that are changed.
+ *
+ * @ingroup DifferenceEngine
+ */
+class RangeDifference {
+
+	public $leftstart;
+	public $leftend;
+	public $leftlength;
+
+	public $rightstart;
+	public $rightend;
+	public $rightlength;
+
+	function __construct( $leftstart, $leftend, $rightstart, $rightend ) {
+		$this->leftstart = $leftstart;
+		$this->leftend = $leftend;
+		$this->leftlength = $leftend - $leftstart;
+		$this->rightstart = $rightstart;
+		$this->rightend = $rightend;
+		$this->rightlength = $rightend - $rightstart;
+	}
+}
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