class.WordLevelDiff.php

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<?php

// Copyright holded by MediaWiki contributers, Licensed under GPL version 2 or later


class DifferenceEngine
{
    public $mOldid;
    public $mNewid;
    public $mTitle;
    public $mOldtitle;
    public $mNewtitle;
    public $mPagetitle;
    public $mOldtext;
    public $mNewtext;
    public $mOldPage;
    public $mNewPage;
    public $mRcidMarkPatrolled;
    public $mOldRev;
    public $mNewRev;
    public $mRevisionsLoaded = false; // Have the revisions been loaded
    public $mTextLoaded = 0; // How many text blobs have been loaded, 0, 1 or 2?
    public function __construct($titleObj = null, $old = 0, $new = 0, $rcid = 0)
    {
        $this->mTitle = $titleObj;
        wfDebug("DifferenceEngine old '$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 previous one.
            # Get previous 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);
        }
        $this->mRcidMarkPatrolled = intval($rcid);  # force it to be an integer
    }

    public function showDiffPage($diffOnly = false)
    {
        global $wgUser, $wgOut, $wgUseExternalEditor, $wgUseRCPatrol;
        $fname = 'DifferenceEngine::showDiffPage';
        //wfProfileIn( $fname );

        # 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("action=raw&oldid=" . $this->mOldid);
            $url2 = $this->mTitle->getFullURL("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 = $this->mTitle->getPrefixedText() . " (Diff: {$this->mOldid}, {$this->mNewid})";
            $mtext = wfMsg('missingarticle', "<nowiki>$t</nowiki>");
            $wgOut->setPagetitle(wfMsg('errorpagetitle'));
            $wgOut->addWikitext($mtext);
            //wfProfileOut( $fname );
            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( $fname );
            return;
        }

        $wgOut->suppressQuickbar();

        $oldTitle = $this->mOldPage->getPrefixedText();
        $newTitle = $this->mNewPage->getPrefixedText();
        if ($oldTitle == $newTitle) {
            $wgOut->setPageTitle($newTitle);
        } else {
            $wgOut->setPageTitle($oldTitle . ', ' . $newTitle);
        }
        $wgOut->setSubtitle(wfMsg('difference'));
        $wgOut->setRobotpolicy('noindex,nofollow');

        if (!($this->mOldPage->userCanRead() && $this->mNewPage->userCanRead())) {
            $wgOut->loginToUse();
            $wgOut->output();
            //wfProfileOut( $fname );
            exit;
        }

        $sk = $wgUser->getSkin();

        if ($this->mNewRev->isCurrent() && $wgUser->isAllowed('rollback')) {
            $rollback = '&nbsp;&nbsp;&nbsp;' . $sk->generateRollback($this->mNewRev);
        } else {
            $rollback = '';
        }
        if ($wgUseRCPatrol && $this->mRcidMarkPatrolled != 0 && $wgUser->isAllowed('patrol')) {
            $patrol = ' [' . $sk->makeKnownLinkObj($this->mTitle, wfMsg('markaspatrolleddiff'), "action=markpatrolled&rcid={$this->mRcidMarkPatrolled}") . ']';
        } else {
            $patrol = '';
        }

        $prevlink = $sk->makeKnownLinkObj(
            $this->mTitle,
            wfMsgHtml('previousdiff'),
            'diff=prev&oldid=' . $this->mOldid,
            '',
            '',
            'id="differences-prevlink"'
        );
        if ($this->mNewRev->isCurrent()) {
            $nextlink = '&nbsp;';
        } else {
            $nextlink = $sk->makeKnownLinkObj(
                $this->mTitle,
                wfMsgHtml('nextdiff'),
                'diff=next&oldid=' . $this->mNewid,
                '',
                '',
                'id="differences-nextlink"'
            );
        }

        $oldminor = '';
        $newminor = '';

        if ($this->mOldRev->mMinorEdit == 1) {
            $oldminor = wfElement(
                'span',
                array( 'class' => 'minor' ),
                wfMsg('minoreditletter')
            ) . ' ';
        }

        if ($this->mNewRev->mMinorEdit == 1) {
            $newminor = wfElement(
                'span',
                array( 'class' => 'minor' ),
                wfMsg('minoreditletter')
            ) . ' ';
        }

        $oldHeader = "<strong>{$this->mOldtitle}</strong><br />" .
            $sk->revUserTools($this->mOldRev) . "<br />" .
            $oldminor . $sk->revComment($this->mOldRev, !$diffOnly) . "<br />" .
            $prevlink;
        $newHeader = "<strong>{$this->mNewtitle}</strong><br />" .
            $sk->revUserTools($this->mNewRev) . " $rollback<br />" .
            $newminor . $sk->revComment($this->mNewRev, !$diffOnly) . "<br />" .
            $nextlink . $patrol;

        $this->showDiff($oldHeader, $newHeader);

        if (!$diffOnly) {
            $this->renderNewRevision();
        }

        //wfProfileOut( $fname );
    }

    public function renderNewRevision()
    {
        global $wgOut;
        $fname = 'DifferenceEngine::renderNewRevision';
        //wfProfileIn( $fname );

        $wgOut->addHTML("<hr /><h2>{$this->mPagetitle}</h2>\n");
        #add deleted rev tag if needed
        if (!$this->mNewRev->userCan(Revision::DELETED_TEXT)) {
            $wgOut->addWikiText(wfMsg('rev-deleted-text-permission'));
        }

        if (!$this->mNewRev->isCurrent()) {
            $oldEditSectionSetting = $wgOut->parserOptions()->setEditSection(false);
        }

        $this->loadNewText();
        if (is_object($this->mNewRev)) {
            $wgOut->setRevisionId($this->mNewRev->getId());
        }

        $wgOut->addWikiTextTidy($this->mNewtext);

        if (!$this->mNewRev->isCurrent()) {
            $wgOut->parserOptions()->setEditSection($oldEditSectionSetting);
        }

        //wfProfileOut( $fname );
    }

    public function showFirstRevision()
    {
        global $wgOut, $wgUser;

        $fname = 'DifferenceEngine::showFirstRevision';
        //wfProfileIn( $fname );

        # Get article text from the DB
        #
        if (!$this->loadNewText()) {
            $t = $this->mTitle->getPrefixedText() . " (Diff: {$this->mOldid}, " .
              "{$this->mNewid})";
            $mtext = wfMsg('missingarticle', "<nowiki>$t</nowiki>");
            $wgOut->setPagetitle(wfMsg('errorpagetitle'));
            $wgOut->addWikitext($mtext);
            //wfProfileOut( $fname );
            return;
        }
        if ($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( $fname );
            exit;
        }

        # Prepare the header box
        #
        $sk = $wgUser->getSkin();

        $nextlink = $sk->makeKnownLinkObj($this->mTitle, wfMsgHtml('nextdiff'), 'diff=next&oldid=' . $this->mNewid, '', '', 'id="differences-nextlink"');
        $header = "<div class=\"firstrevisionheader\" style=\"text-align: center\"><strong>{$this->mOldtitle}</strong><br />" .
            $sk->revUserTools($this->mNewRev) . "<br />" .
            $sk->revComment($this->mNewRev) . "<br />" .
            $nextlink . "</div>\n";

        $wgOut->addHTML($header);

        $wgOut->setSubtitle(wfMsg('difference'));
        $wgOut->setRobotpolicy('noindex,nofollow');

        //wfProfileOut( $fname );
    }

    public function showDiff($otitle, $ntitle)
    {
        global $wgOut;
        $diff = $this->getDiff($otitle, $ntitle);
        if ($diff === false) {
            $wgOut->addWikitext(wfMsg('missingarticle', "<nowiki>(fixme, bug)</nowiki>"));
            return false;
        } else {
            $wgOut->addHTML($diff);
            return true;
        }
    }

    public function getDiff($otitle, $ntitle)
    {
        $body = $this->getDiffBody();
        if ($body === false) {
            return false;
        } else {
            $multi = $this->getMultiNotice();
            return $this->addHeader($body, $otitle, $ntitle, $multi);
        }
    }

    public function getDiffBody()
    {
        global $wgMemc;
        $fname = 'DifferenceEngine::getDiffBody';
        //wfProfileIn( $fname );
        
        // Cacheable?
        $key = false;
        if ($this->mOldid && $this->mNewid) {
            // Try cache
            $key = wfMemcKey('diff', 'oldid', $this->mOldid, 'newid', $this->mNewid);
            $difftext = $wgMemc->get($key);
            if ($difftext) {
                wfIncrStats('diff_cache_hit');
                $difftext = $this->localiseLineNumbers($difftext);
                $difftext .= "\n<!-- diff cache key $key -->\n";
                //wfProfileOut( $fname );
                return $difftext;
            }
        }

        #loadtext is permission safe, this just clears out the diff
        if (!$this->loadText()) {
            //wfProfileOut( $fname );
            return false;
        } elseif ($this->mOldRev && !$this->mOldRev->userCan(Revision::DELETED_TEXT)) {
            return '';
        } elseif ($this->mNewRev && !$this->mNewRev->userCan(Revision::DELETED_TEXT)) {
            return '';
        }

        $difftext = $this->generateDiffBody($this->mOldtext, $this->mNewtext);
        
        // Save to cache for 7 days
        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( $fname );
        return $difftext;
    }

    public function generateDiffBody($otext, $ntext)
    {
        global $wgExternalDiffEngine, $wgContLang;
        $fname = 'DifferenceEngine::generateDiffBody';

        $otext = str_replace("\r\n", "\n", $otext);
        $ntext = str_replace("\r\n", "\n", $ntext);
        
        if ($wgExternalDiffEngine == 'wikidiff') {
            # For historical reasons, external diff engine expects
            # input text to be HTML-escaped already
            $otext = htmlspecialchars($wgContLang->segmentForDiff($otext));
            $ntext = htmlspecialchars($wgContLang->segmentForDiff($ntext));
            if (!function_exists('wikidiff_do_diff')) {
                dl('php_wikidiff.so');
            }
            return $wgContLang->unsegementForDiff(wikidiff_do_diff($otext, $ntext, 2));
        }
        
        if ($wgExternalDiffEngine == 'wikidiff2') {
            # Better external diff engine, the 2 may some day be dropped
            # This one does the escaping and segmenting itself
            if (!function_exists('wikidiff2_do_diff')) {
                //wfProfileIn( "$fname-dl" );
                @dl('php_wikidiff2.so');
                //wfProfileOut( "$fname-dl" );
            }
            if (function_exists('wikidiff2_do_diff')) {
                //wfProfileIn( 'wikidiff2_do_diff' );
                $text = wikidiff2_do_diff($otext, $ntext, 2);
                //wfProfileOut( 'wikidiff2_do_diff' );
                return $text;
            }
        }
        if ($wgExternalDiffEngine !== false) {
            # Diff via the shell
            global $wgTmpDirectory;
            $tempName1 = tempnam($wgTmpDirectory, 'diff_');
            $tempName2 = tempnam($wgTmpDirectory, 'diff_');

            $tempFile1 = fopen($tempName1, "w");
            if (!$tempFile1) {
                //wfProfileOut( $fname );
                return false;
            }
            $tempFile2 = fopen($tempName2, "w");
            if (!$tempFile2) {
                //wfProfileOut( $fname );
                return false;
            }
            fwrite($tempFile1, $otext);
            fwrite($tempFile2, $ntext);
            fclose($tempFile1);
            fclose($tempFile2);
            $cmd = wfEscapeShellArg($wgExternalDiffEngine, $tempName1, $tempName2);
            //wfProfileIn( "$fname-shellexec" );
            $difftext = wfShellExec($cmd);
            //wfProfileOut( "$fname-shellexec" );
            unlink($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));
    }
        

    public function localiseLineNumbers($text)
    {
        return preg_replace_callback(
            '/<!--LINE (\d+)-->/',
            array( &$this, 'localiseLineNumbersCb' ),
            $text
        );
    }

    public function localiseLineNumbersCb($matches)
    {
        global $wgLang;
        return wfMsgExt('lineno', array('parseinline'), $wgLang->formatNum($matches[1]));
    }

    
    public 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);
    }


    public function addHeader($diff, $otitle, $ntitle, $multi = '')
    {
        global $wgOut;
    
        if ($this->mOldRev && $this->mOldRev->isDeleted(Revision::DELETED_TEXT)) {
            $otitle = '<span class="history-deleted">' . $otitle . '</span>';
        }
        if ($this->mNewRev && $this->mNewRev->isDeleted(Revision::DELETED_TEXT)) {
            $ntitle = '<span class="history-deleted">' . $ntitle . '</span>';
        }
        $header = "
                        <table border='0' width='98%' cellpadding='0' cellspacing='4' class='diff'>
                        <tr>
                                <td colspan='2' width='50%' align='center' class='diff-otitle'>{$otitle}</td>
                                <td colspan='2' width='50%' align='center' class='diff-ntitle'>{$ntitle}</td>
                        </tr>
                ";

        if ($multi != '') {
            $header .= "<tr><td colspan='4' align='center' class='diff-multi'>{$multi}</td></tr>";
        }

        return $header . $diff . "</table>";
    }

    public function setText($oldText, $newText)
    {
        $this->mOldtext = $oldText;
        $this->mNewtext = $newText;
        $this->mTextLoaded = 2;
    }

    public function loadRevisionData()
    {
        global $wgLang;
        if ($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
        if ($this->mNewid) {
            $this->mNewRev = Revision::newFromId($this->mNewid);
        } else {
            $this->mNewRev = Revision::newFromTitle($this->mTitle);
        }

        if (is_null($this->mNewRev)) {
            return false;
        }

        // Set assorted variables
        $timestamp = $wgLang->timeanddate($this->mNewRev->getTimestamp(), true);
        $this->mNewPage = $this->mNewRev->getTitle();
        if ($this->mNewRev->isCurrent()) {
            $newLink = $this->mNewPage->escapeLocalUrl();
            $this->mPagetitle = htmlspecialchars(wfMsg('currentrev'));
            $newEdit = $this->mNewPage->escapeLocalUrl('action=edit');

            $this->mNewtitle = "<a href='$newLink'>{$this->mPagetitle}</a> ($timestamp)"
                . " (<a href='$newEdit'>" . htmlspecialchars(wfMsg('editold')) . "</a>)";
        } else {
            $newLink = $this->mNewPage->escapeLocalUrl('oldid=' . $this->mNewid);
            $newEdit = $this->mNewPage->escapeLocalUrl('action=edit&oldid=' . $this->mNewid);
            $this->mPagetitle = htmlspecialchars(wfMsg('revisionasof', $timestamp));

            $this->mNewtitle = "<a href='$newLink'>{$this->mPagetitle}</a>"
                . " (<a href='$newEdit'>" . htmlspecialchars(wfMsg('editold')) . "</a>)";
        }

        // 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);
            $oldLink = $this->mOldPage->escapeLocalUrl('oldid=' . $this->mOldid);
            $oldEdit = $this->mOldPage->escapeLocalUrl('action=edit&oldid=' . $this->mOldid);
            $this->mOldtitle = "<a href='$oldLink'>" . htmlspecialchars(wfMsg('revisionasof', $t))
                . "</a> (<a href='$oldEdit'>" . htmlspecialchars(wfMsg('editold')) . "</a>)";
            //now that we considered old rev, we can make undo link (bug 8133, multi-edit undo)
            $newUndo = $this->mNewPage->escapeLocalUrl('action=edit&undoafter=' . $this->mOldid . '&undo=' . $this->mNewid);
            $this->mNewtitle .= " (<a href='$newUndo'>" . htmlspecialchars(wfMsg('editundo')) . "</a>)";
        }

        return true;
    }

    public 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) {
            // FIXME: permission tests
            $this->mOldtext = $this->mOldRev->revText();
            if ($this->mOldtext === false) {
                return false;
            }
        }
        if ($this->mNewRev) {
            $this->mNewtext = $this->mNewRev->revText();
            if ($this->mNewtext === false) {
                return false;
            }
        }
        return true;
    }

    public function loadNewText()
    {
        if ($this->mTextLoaded >= 1) {
            return true;
        } else {
            $this->mTextLoaded = 1;
        }
        if (!$this->loadRevisionData()) {
            return false;
        }
        $this->mNewtext = $this->mNewRev->getText();
        return true;
    }
}

// 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'));

class _DiffOp
{
    public $type;
    public $orig;
    public $closing;

    public function reverse()
    {
        trigger_error('pure virtual', E_USER_ERROR);
    }

    public function norig()
    {
        return $this->orig ? sizeof($this->orig) : 0;
    }

    public function nclosing()
    {
        return $this->closing ? sizeof($this->closing) : 0;
    }
}

class _DiffOp_Copy extends _DiffOp
{
    public $type = 'copy';

    public function __construct($orig, $closing = false)
    {
        if (!is_array($closing)) {
            $closing = $orig;
        }
        $this->orig = $orig;
        $this->closing = $closing;
    }

    public function reverse()
    {
        return new _DiffOp_Copy($this->closing, $this->orig);
    }
}

class _DiffOp_Delete extends _DiffOp
{
    public $type = 'delete';

    public function __construct($lines)
    {
        $this->orig = $lines;
        $this->closing = false;
    }

    public function reverse()
    {
        return new _DiffOp_Add($this->orig);
    }
}

class _DiffOp_Add extends _DiffOp
{
    public $type = 'add';

    public function __construct($lines)
    {
        $this->closing = $lines;
        $this->orig = false;
    }

    public function reverse()
    {
        return new _DiffOp_Delete($this->closing);
    }
}

class _DiffOp_Change extends _DiffOp
{
    public $type = 'change';

    public function __construct($orig, $closing)
    {
        $this->orig = $orig;
        $this->closing = $closing;
    }

    public function reverse()
    {
        return new _DiffOp_Change($this->closing, $this->orig);
    }
}


class _DiffEngine
{
    const MAX_XREF_LENGTH = 10000;

    public function diff($from_lines, $to_lines)
    {
        $fname = '_DiffEngine::diff';
        //wfProfileIn( $fname );

        $n_from = sizeof($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));

        // Merge edits when possible
        $this->_shift_boundaries($from_lines, $this->xchanged, $this->ychanged);
        $this->_shift_boundaries($to_lines, $this->ychanged, $this->xchanged);

        // Compute the edit operations.
        $edits = 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);
            }

            // 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( $fname );
        return $edits;
    }

    public 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.
     */
    public function _diag($xoff, $xlim, $yoff, $ylim, $nchunks)
    {
        $fname = '_DiffEngine::_diag';
        //wfProfileIn( $fname );
        $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; $chunk++) {
            //wfProfileIn( "$fname-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;
                    } elseif (empty($this->in_seq[$y])) {
                        $k = $this->_lcs_pos($y);
                        USE_ASSERTS && assert($k > 0);
                        $ymids[$k] = $ymids[$k - 1];
                    }
                }
            }
            //wfProfileOut( "$fname-chunk" );
        }

        $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);

        //wfProfileOut( $fname );
        return array($this->lcs, $seps);
    }

    public function _lcs_pos($ypos)
    {
        $fname = '_DiffEngine::_lcs_pos';
        //wfProfileIn( $fname );

        $end = $this->lcs;
        if ($end == 0 || $ypos > $this->seq[$end]) {
            $this->seq[++$this->lcs] = $ypos;
            $this->in_seq[$ypos] = 1;
            //wfProfileOut( $fname );
            return $this->lcs;
        }

        $beg = 1;
        while ($beg < $end) {
            $mid = (int) (($beg + $end) / 2);
            if ($ypos > $this->seq[$mid]) {
                $beg = $mid + 1;
            } else {
                $end = $mid;
            }
        }

        USE_ASSERTS && assert($ypos != $this->seq[$end]);

        $this->in_seq[$this->seq[$end]] = false;
        $this->seq[$end] = $ypos;
        $this->in_seq[$ypos] = 1;
        //wfProfileOut( $fname );
        return $end;
    }

    /* Find LCS of two sequences.
     *
     * The results are recorded in the vectors $this->{x,y}changed[], by
     * storing a 1 in the element for each line that is an insertion
     * or deletion (ie. is not in the LCS).
     *
     * The subsequence of file 0 is [XOFF, XLIM) and likewise for file 1.
     *
     * Note that XLIM, YLIM are exclusive bounds.
     * All line numbers are origin-0 and discarded lines are not counted.
     */
    public function _compareseq($xoff, $xlim, $yoff, $ylim)
    {
        $fname = '_DiffEngine::_compareseq';
        //wfProfileIn( $fname );

        // 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;
            }
        }
        //wfProfileOut( $fname );
    }

    /* 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).
     */
    public function _shift_boundaries($lines, &$changed, $other_changed)
    {
        $fname = '_DiffEngine::_shift_boundaries';
        //wfProfileIn( $fname );
        $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]);
            }
        }
        //wfProfileOut( $fname );
    }
}

class Diff
{
    public $edits;

    public function __construct($from_lines, $to_lines)
    {
        $eng = new _DiffEngine;
        $this->edits = $eng->diff($from_lines, $to_lines);
        //$this->_check($from_lines, $to_lines);
    }

    public function reverse()
    {
        $rev = $this;
        $rev->edits = array();
        foreach ($this->edits as $edit) {
            $rev->edits[] = $edit->reverse();
        }
        return $rev;
    }

    public function isEmpty()
    {
        foreach ($this->edits as $edit) {
            if ($edit->type != 'copy') {
                return false;
            }
        }
        return true;
    }

    public function lcs()
    {
        $lcs = 0;
        foreach ($this->edits as $edit) {
            if ($edit->type == 'copy') {
                $lcs += sizeof($edit->orig);
            }
        }
        return $lcs;
    }

    public function orig()
    {
        $lines = array();

        foreach ($this->edits as $edit) {
            if ($edit->orig) {
                array_splice($lines, sizeof($lines), 0, $edit->orig);
            }
        }
        return $lines;
    }

    public function closing()
    {
        $lines = array();

        foreach ($this->edits as $edit) {
            if ($edit->closing) {
                array_splice($lines, sizeof($lines), 0, $edit->closing);
            }
        }
        return $lines;
    }

    public function _check($from_lines, $to_lines)
    {
        $fname = 'Diff::_check';
        //wfProfileIn( $fname );
        if (serialize($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( $fname );
    }
}

class MappedDiff extends Diff
{
    public function __construct(
        $from_lines,
        $to_lines,
        $mapped_from_lines,
        $mapped_to_lines
    ) {
        $fname = 'MappedDiff::MappedDiff';
        //wfProfileIn( $fname );

        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( $fname );
    }
}

class DiffFormatter
{
    public $leading_context_lines = 0;

    public $trailing_context_lines = 0;

    public function format($diff)
    {
        $fname = 'DiffFormatter::format';
        //wfProfileIn( $fname );

        $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( $fname );
        return $end;
    }

    public function _block($xbeg, $xlen, $ybeg, $ylen, &$edits)
    {
        $fname = 'DiffFormatter::_block';
        //wfProfileIn( $fname );
        $this->_start_block($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( $fname );
    }

    public function _start_diff()
    {
        ob_start();
    }

    public function _end_diff()
    {
        $val = ob_get_contents();
        ob_end_clean();
        return $val;
    }

    public 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;
    }

    public function _start_block($header)
    {
        echo $header;
    }

    public function _end_block()
    {
    }

    public function _lines($lines, $prefix = ' ')
    {
        foreach ($lines as $line) {
            echo "$prefix $line\n";
        }
    }

    public function _context($lines)
    {
        $this->_lines($lines);
    }

    public function _added($lines)
    {
        $this->_lines($lines, '>');
    }
    public function _deleted($lines)
    {
        $this->_lines($lines, '<');
    }

    public function _changed($orig, $closing)
    {
        $this->_deleted($orig);
        echo "---\n";
        $this->_added($closing);
    }
}


define('NBSP', '&#160;');                       // iso-8859-x non-breaking space.

class _HWLDF_WordAccumulator
{
    public function __construct()
    {
        $this->_lines = array();
        $this->_line = '';
        $this->_group = '';
        $this->_tag = '';
    }

    public function _flushGroup($new_tag)
    {
        if ($this->_group !== '') {
            if ($this->_tag == 'ins') {
                $this->_line .= '[ilDiffInsStart]' .
                    htmlspecialchars($this->_group) . '[ilDiffInsEnd]';
            } elseif ($this->_tag == 'del') {
                $this->_line .= '[ilDiffDelStart]' .
                    htmlspecialchars($this->_group) . '[ilDiffDelEnd]';
            } else {
                $this->_line .= htmlspecialchars($this->_group);
            }
        }
        $this->_group = '';
        $this->_tag = $new_tag;
    }

    public 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 = '';
    }

    public 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;
        }
    }

    public function getLines()
    {
        $this->_flushLine('~done');
        return $this->_lines;
    }
}

class WordLevelDiff extends MappedDiff
{
    const MAX_LINE_LENGTH = 10000;

    public function __construct($orig_lines, $closing_lines)
    {
        $fname = 'WordLevelDiff::WordLevelDiff';
        //wfProfileIn( $fname );

        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( $fname );
    }

    public function _split($lines)
    {
        $fname = 'WordLevelDiff::_split';
        //wfProfileIn( $fname );

        $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( $fname );
        return array($words, $stripped);
    }

    public function orig()
    {
        $fname = 'WordLevelDiff::orig';
        //wfProfileIn( $fname );
        $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( $fname );
        return $lines;
    }

    public function closing()
    {
        $fname = 'WordLevelDiff::closing';
        //wfProfileIn( $fname );
        $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( $fname );
        return $lines;
    }
}

class TableDiffFormatter extends DiffFormatter
{
    public function __construct()
    {
        $this->leading_context_lines = 2;
        $this->trailing_context_lines = 2;
    }

    public function _block_header($xbeg, $xlen, $ybeg, $ylen)
    {
        $r = '<tr><td colspan="2" align="left"><strong><!--LINE ' . $xbeg . "--></strong></td>\n" .
          '<td colspan="2" align="left"><strong><!--LINE ' . $ybeg . "--></strong></td></tr>\n";
        return $r;
    }

    public function _start_block($header)
    {
        echo $header;
    }

    public function _end_block()
    {
    }

    public function _lines($lines, $prefix = ' ', $color = 'white')
    {
    }

    # HTML-escape parameter before calling this
    public function addedLine($line)
    {
        return "<td>+</td><td class='diff-addedline'>{$line}</td>";
    }

    # HTML-escape parameter before calling this
    public function deletedLine($line)
    {
        return "<td>-</td><td class='diff-deletedline'>{$line}</td>";
    }

    # HTML-escape parameter before calling this
    public function contextLine($line)
    {
        return "<td> </td><td class='diff-context'>{$line}</td>";
    }

    public function emptyLine()
    {
        return '<td colspan="2">&nbsp;</td>';
    }

    public function _added($lines)
    {
        foreach ($lines as $line) {
            echo '<tr>' . $this->emptyLine() .
                $this->addedLine(htmlspecialchars($line)) . "</tr>\n";
        }
    }

    public function _deleted($lines)
    {
        foreach ($lines as $line) {
            echo '<tr>' . $this->deletedLine(htmlspecialchars($line)) .
              $this->emptyLine() . "</tr>\n";
        }
    }

    public function _context($lines)
    {
        foreach ($lines as $line) {
            echo '<tr>' .
                $this->contextLine(htmlspecialchars($line)) .
                $this->contextLine(htmlspecialchars($line)) . "</tr>\n";
        }
    }

    public function _changed($orig, $closing)
    {
        $fname = 'TableDiffFormatter::_changed';
        //wfProfileIn( $fname );

        $diff = new WordLevelDiff($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( $fname );
    }
}