DifferenceEngine.php

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<?php
define( 'MW_DIFF_VERSION', '1.11a' );

class DifferenceEngine extends ContextSource {
    var $mOldid, $mNewid;
    var $mOldTags, $mNewTags;
    var $mOldContent, $mNewContent;
    protected $mDiffLang;

    var $mOldPage, $mNewPage;

    var $mOldRev, $mNewRev;
    private $mRevisionsIdsLoaded = false; // Have the revisions IDs been loaded
    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?

    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;

    // Link to action=markpatrolled
    protected $mMarkPatrolledLink = null;

    protected $unhide = false; # show rev_deleted content if allowed
    function __construct( $context = null, $old = 0, $new = 0, $rcid = 0,
        $refreshCache = false, $unhide = false )
    {
        if ( $context instanceof IContextSource ) {
            $this->setContext( $context );
        }

        wfDebug( "DifferenceEngine old '$old' new '$new' rcid '$rcid'\n" );

        $this->mOldid = $old;
        $this->mNewid = $new;
        $this->mRefreshCache = $refreshCache;
        $this->unhide = $unhide;
    }

    function setReducedLineNumbers( $value = true ) {
        $this->mReducedLineNumbers = $value;
    }

    function getDiffLang() {
        if ( $this->mDiffLang === null ) {
            # Default language in which the diff text is written.
            $this->mDiffLang = $this->getTitle()->getPageLanguage();
        }
        return $this->mDiffLang;
    }

    function wasCacheHit() {
        return $this->mCacheHit;
    }

    function getOldid() {
        $this->loadRevisionIds();
        return $this->mOldid;
    }

    function getNewid() {
        $this->loadRevisionIds();
        return $this->mNewid;
    }

    function deletedLink( $id ) {
        if ( $this->getUser()->isAllowed( 'deletedhistory' ) ) {
            $dbr = wfGetDB( DB_SLAVE );
            $row = $dbr->selectRow( 'archive', '*',
                array( 'ar_rev_id' => $id ),
                __METHOD__ );
            if ( $row ) {
                $rev = Revision::newFromArchiveRow( $row );
                $title = Title::makeTitleSafe( $row->ar_namespace, $row->ar_title );
                return SpecialPage::getTitleFor( 'Undelete' )->getFullURL( array(
                    'target' => $title->getPrefixedText(),
                    'timestamp' => $rev->getTimestamp()
                ));
            }
        }
        return false;
    }

    function deletedIdMarker( $id ) {
        $link = $this->deletedLink( $id );
        if ( $link ) {
            return "[$link $id]";
        } else {
            return $id;
        }
    }

    private function showMissingRevision() {
        $out = $this->getOutput();

        $missing = array();
        if ( $this->mOldRev === null ||
            ( $this->mOldRev && $this->mOldContent === null )
        ) {
            $missing[] = $this->deletedIdMarker( $this->mOldid );
        }
        if ( $this->mNewRev === null ||
            ( $this->mNewRev && $this->mNewContent === null )
        ) {
            $missing[] = $this->deletedIdMarker( $this->mNewid );
        }

        $out->setPageTitle( $this->msg( 'errorpagetitle' ) );
        $out->addWikiMsg( 'difference-missing-revision',
            $this->getLanguage()->listToText( $missing ), count( $missing ) );
    }

    function showDiffPage( $diffOnly = false ) {
        wfProfileIn( __METHOD__ );

        # Allow frames except in certain special cases
        $out = $this->getOutput();
        $out->allowClickjacking();
        $out->setRobotPolicy( 'noindex,nofollow' );

        if ( !$this->loadRevisionData() ) {
            $this->showMissingRevision();
            wfProfileOut( __METHOD__ );
            return;
        }

        $user = $this->getUser();
        $permErrors = $this->mNewPage->getUserPermissionsErrors( 'read', $user );
        if ( $this->mOldPage ) { # mOldPage might not be set, see below.
            $permErrors = wfMergeErrorArrays( $permErrors,
                $this->mOldPage->getUserPermissionsErrors( 'read', $user ) );
        }
        if ( count( $permErrors ) ) {
            wfProfileOut( __METHOD__ );
            throw new PermissionsError( 'read', $permErrors );
        }

        $rollback = '';
        $undoLink = '';

        $query = array();
        # Carry over 'diffonly' param via navigation links
        if ( $diffOnly != $user->getBoolOption( 'diffonly' ) ) {
            $query['diffonly'] = $diffOnly;
        }
        # Cascade unhide param in links for easy deletion browsing
        if ( $this->unhide ) {
            $query['unhide'] = 1;
        }

        # Check if one of the revisions is deleted/suppressed
        $deleted = $suppressed = false;
        $allowed = $this->mNewRev->userCan( Revision::DELETED_TEXT, $user );

        $revisionTools = array();

        # mOldRev 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->mOldRev === false ) {
            $out->setPageTitle( $this->msg( 'difference-title', $this->mNewPage->getPrefixedText() ) );
            $samePage = true;
            $oldHeader = '';
        } else {
            wfRunHooks( 'DiffViewHeader', array( $this, $this->mOldRev, $this->mNewRev ) );

            if ( $this->mNewPage->equals( $this->mOldPage ) ) {
                $out->setPageTitle( $this->msg( 'difference-title', $this->mNewPage->getPrefixedText() ) );
                $samePage = true;
            } else {
                $out->setPageTitle( $this->msg( 'difference-title-multipage', $this->mOldPage->getPrefixedText(),
                    $this->mNewPage->getPrefixedText() ) );
                $out->addSubtitle( $this->msg( 'difference-multipage' ) );
                $samePage = false;
            }

            if ( $samePage && $this->mNewPage->quickUserCan( 'edit', $user ) ) {
                if ( $this->mNewRev->isCurrent() && $this->mNewPage->userCan( 'rollback', $user ) ) {
                    $rollbackLink = Linker::generateRollback( $this->mNewRev, $this->getContext() );
                    if ( $rollbackLink ) {
                        $out->preventClickjacking();
                        $rollback = '&#160;&#160;&#160;' . $rollbackLink;
                    }
                }
                if ( !$this->mOldRev->isDeleted( Revision::DELETED_TEXT ) && !$this->mNewRev->isDeleted( Revision::DELETED_TEXT ) ) {
                    $undoLink = Html::element( 'a', array(
                            'href' => $this->mNewPage->getLocalURL( array(
                                'action' => 'edit',
                                'undoafter' => $this->mOldid,
                                'undo' => $this->mNewid ) ),
                            'title' => Linker::titleAttrib( 'undo' )
                        ),
                        $this->msg( 'editundo' )->text()
                    );
                    $revisionTools[] = $undoLink;
                }
            }

            # Make "previous revision link"
            if ( $samePage && $this->mOldRev->getPrevious() ) {
                $prevlink = Linker::linkKnown(
                    $this->mOldPage,
                    $this->msg( 'previousdiff' )->escaped(),
                    array( 'id' => 'differences-prevlink' ),
                    array( 'diff' => 'prev', 'oldid' => $this->mOldid ) + $query
                );
            } else {
                $prevlink = '&#160;';
            }

            if ( $this->mOldRev->isMinor() ) {
                $oldminor = ChangesList::flag( 'minor' );
            } else {
                $oldminor = '';
            }

            $ldel = $this->revisionDeleteLink( $this->mOldRev );
            $oldRevisionHeader = $this->getRevisionHeader( $this->mOldRev, 'complete' );
            $oldChangeTags = ChangeTags::formatSummaryRow( $this->mOldTags, 'diff' );

            $oldHeader = '<div id="mw-diff-otitle1"><strong>' . $oldRevisionHeader . '</strong></div>' .
                '<div id="mw-diff-otitle2">' .
                    Linker::revUserTools( $this->mOldRev, !$this->unhide ) . '</div>' .
                '<div id="mw-diff-otitle3">' . $oldminor .
                    Linker::revComment( $this->mOldRev, !$diffOnly, !$this->unhide ) . $ldel . '</div>' .
                '<div id="mw-diff-otitle5">' . $oldChangeTags[0] . '</div>' .
                '<div id="mw-diff-otitle4">' . $prevlink . '</div>';

            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
                }
            }

            # Check if this user can see the revisions
            if ( !$this->mOldRev->userCan( Revision::DELETED_TEXT, $user ) ) {
                $allowed = false;
            }
        }

        # Make "next revision link"
        # Skip next link on the top revision
        if ( $samePage && !$this->mNewRev->isCurrent() ) {
            $nextlink = Linker::linkKnown(
                $this->mNewPage,
                $this->msg( 'nextdiff' )->escaped(),
                array( 'id' => 'differences-nextlink' ),
                array( 'diff' => 'next', 'oldid' => $this->mNewid ) + $query
            );
        } else {
            $nextlink = '&#160;';
        }

        if ( $this->mNewRev->isMinor() ) {
            $newminor = ChangesList::flag( 'minor' );
        } else {
            $newminor = '';
        }

        # Handle RevisionDelete links...
        $rdel = $this->revisionDeleteLink( $this->mNewRev );

        # Allow extensions to define their own revision tools
        wfRunHooks( 'DiffRevisionTools', array( $this->mNewRev, &$revisionTools ) );
        $formattedRevisionTools = array();
        // Put each one in parentheses (poor man's button)
        foreach ( $revisionTools as $tool ) {
            $formattedRevisionTools[] = $this->msg( 'parentheses' )->rawParams( $tool )->escaped();
        }
        $newRevisionHeader = $this->getRevisionHeader( $this->mNewRev, 'complete' ) . ' ' . implode( ' ', $formattedRevisionTools );
        $newChangeTags = ChangeTags::formatSummaryRow( $this->mNewTags, 'diff' );

        $newHeader = '<div id="mw-diff-ntitle1"><strong>' . $newRevisionHeader . '</strong></div>' .
            '<div id="mw-diff-ntitle2">' . Linker::revUserTools( $this->mNewRev, !$this->unhide ) .
                " $rollback</div>" .
            '<div id="mw-diff-ntitle3">' . $newminor .
                Linker::revComment( $this->mNewRev, !$diffOnly, !$this->unhide ) . $rdel . '</div>' .
            '<div id="mw-diff-ntitle5">' . $newChangeTags[0] . '</div>' .
            '<div id="mw-diff-ntitle4">' . $nextlink . $this->markPatrolledLink() . '</div>';

        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();
            $out->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
                $out->wrapWikiMsg( "<div id='mw-$msg' class='mw-warning plainlinks'>\n$1\n</div>\n",
                    array( $msg ) );
            } else {
                # Give explanation and add a link to view the diff...
                $link = $this->getTitle()->getFullURL( $this->getRequest()->appendQueryValue( 'unhide', '1', true ) );
                $msg = $suppressed ? 'rev-suppressed-unhide-diff' : 'rev-deleted-unhide-diff';
                $out->wrapWikiMsg( "<div id='mw-$msg' class='mw-warning plainlinks'>\n$1\n</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';
                $notice = "<div id='mw-$msg' class='mw-warning plainlinks'>\n" . $this->msg( $msg )->parse() . "</div>\n";
            }
            $this->showDiff( $oldHeader, $newHeader, $notice );
            if ( !$diffOnly ) {
                $this->renderNewRevision();
            }
        }
        wfProfileOut( __METHOD__ );
    }

    protected function markPatrolledLink() {
        global $wgUseRCPatrol, $wgEnableAPI, $wgEnableWriteAPI;
        $user = $this->getUser();

        if ( $this->mMarkPatrolledLink === null ) {
            // Prepare a change patrol link, if applicable
            if (
                // Is patrolling enabled and the user allowed to?
                $wgUseRCPatrol && $this->mNewPage->quickUserCan( 'patrol', $user ) &&
                // Only do this if the revision isn't more than 6 hours older
                // than the Max RC age (6h because the RC might not be cleaned out regularly)
                RecentChange::isInRCLifespan( $this->mNewRev->getTimestamp(), 21600 )
            ) {
                // Look for an unpatrolled change corresponding to this diff

                $db = wfGetDB( DB_SLAVE );
                $change = RecentChange::newFromConds(
                    array(
                        'rc_timestamp' => $db->timestamp( $this->mNewRev->getTimestamp() ),
                        'rc_this_oldid' => $this->mNewid,
                        'rc_patrolled' => 0
                    ),
                    __METHOD__,
                    array( 'USE INDEX' => 'rc_timestamp' )
                );

                if ( $change && $change->getPerformer()->getName() !== $user->getName() ) {
                    $rcid = $change->getAttribute( 'rc_id' );
                } else {
                    // None found or the page has been created by the current user.
                    // If the user could patrol this it already would be patrolled
                    $rcid = 0;
                }
                // Build the link
                if ( $rcid ) {
                    $this->getOutput()->preventClickjacking();
                    if ( $wgEnableAPI && $wgEnableWriteAPI
                        && $user->isAllowed( 'writeapi' )
                    ) {
                        $this->getOutput()->addModules( 'mediawiki.page.patrol.ajax' );
                    }

                    $token = $user->getEditToken( $rcid );
                    $this->mMarkPatrolledLink = ' <span class="patrollink">[' . Linker::linkKnown(
                        $this->mNewPage,
                        $this->msg( 'markaspatrolleddiff' )->escaped(),
                        array(),
                        array(
                            'action' => 'markpatrolled',
                            'rcid' => $rcid,
                            'token' => $token,
                        )
                    ) . ']</span>';
                } else {
                    $this->mMarkPatrolledLink = '';
                }
            } else {
                $this->mMarkPatrolledLink = '';
            }
        }

        return $this->mMarkPatrolledLink;
    }

    protected function revisionDeleteLink( $rev ) {
        $link = Linker::getRevDeleteLink( $this->getUser(), $rev, $rev->getTitle() );
        if ( $link !== '' ) {
            $link = '&#160;&#160;&#160;' . $link . ' ';
        }
        return $link;
    }

    function renderNewRevision() {
        wfProfileIn( __METHOD__ );
        $out = $this->getOutput();
        $revHeader = $this->getRevisionHeader( $this->mNewRev );
        # Add "current version as of X" title
        $out->addHTML( "<hr class='diff-hr' />
        <h2 class='diff-currentversion-title'>{$revHeader}</h2>\n" );
        # Page content may be handled by a hooked call instead...
        if ( wfRunHooks( 'ArticleContentOnDiff', array( $this, $out ) ) ) {
            $this->loadNewText();
            $out->setRevisionId( $this->mNewid );
            $out->setRevisionTimestamp( $this->mNewRev->getTimestamp() );
            $out->setArticleFlag( true );

            // NOTE: only needed for B/C: custom rendering of JS/CSS via hook
            if ( $this->mNewPage->isCssJsSubpage() || $this->mNewPage->isCssOrJsPage() ) {
                // Stolen from Article::view --AG 2007-10-11
                // Give hooks a chance to customise the output
                // @todo standardize this crap into one function
                if ( ContentHandler::runLegacyHooks( 'ShowRawCssJs', array( $this->mNewContent, $this->mNewPage, $out ) ) ) {
                    // NOTE: deprecated hook, B/C only
                    // use the content object's own rendering
                    $cnt = $this->mNewRev->getContent();
                    $po = $cnt ? $cnt->getParserOutput( $this->mNewRev->getTitle(), $this->mNewRev->getId() ) : null;
                    $txt = $po ? $po->getText() : '';
                    $out->addHTML( $txt );
                }
            } elseif ( !wfRunHooks( 'ArticleContentViewCustom', array( $this->mNewContent, $this->mNewPage, $out ) ) ) {
                // Handled by extension
            } elseif ( !ContentHandler::runLegacyHooks( 'ArticleViewCustom', array( $this->mNewContent, $this->mNewPage, $out ) ) ) {
                // NOTE: deprecated hook, B/C only
                // Handled by extension
            } else {
                // Normal page
                if ( $this->getTitle()->equals( $this->mNewPage ) ) {
                    // If the Title stored in the context is the same as the one
                    // of the new revision, we can use its associated WikiPage
                    // object.
                    $wikiPage = $this->getWikiPage();
                } else {
                    // Otherwise we need to create our own WikiPage object
                    $wikiPage = WikiPage::factory( $this->mNewPage );
                }

                $parserOutput = $this->getParserOutput( $wikiPage, $this->mNewRev );

                # Also try to load it as a redirect
                $rt = $this->mNewContent ? $this->mNewContent->getRedirectTarget() : null;

                if ( $rt ) {
                    $article = Article::newFromTitle( $this->mNewPage, $this->getContext() );
                    $out->addHTML( $article->viewRedirect( $rt ) );

                    # WikiPage::getParserOutput() should not return false, but just in case
                    if ( $parserOutput ) {
                        # Show categories etc.
                        $out->addParserOutputNoText( $parserOutput );
                    }
                } elseif ( $parserOutput ) {
                    $out->addParserOutput( $parserOutput );
                }
            }
        }
        # Add redundant patrol link on bottom...
        $out->addHTML( $this->markPatrolledLink() );

        wfProfileOut( __METHOD__ );
    }

    protected function getParserOutput( WikiPage $page, Revision $rev ) {
        $parserOptions = $page->makeParserOptions( $this->getContext() );

        if ( !$rev->isCurrent() || !$rev->getTitle()->quickUserCan( "edit" ) ) {
            $parserOptions->setEditSection( false );
        }

        $parserOutput = $page->getParserOutput( $parserOptions, $rev->getId() );
        return $parserOutput;
    }

    function showDiff( $otitle, $ntitle, $notice = '' ) {
        $diff = $this->getDiff( $otitle, $ntitle, $notice );
        if ( $diff === false ) {
            $this->showMissingRevision();
            return false;
        } else {
            $this->showDiffStyle();
            $this->getOutput()->addHTML( $diff );
            return true;
        }
    }

    function showDiffStyle() {
        $this->getOutput()->addModuleStyles( 'mediawiki.action.history.diff' );
    }

    function getDiff( $otitle, $ntitle, $notice = '' ) {
        $body = $this->getDiffBody();
        if ( $body === false ) {
            return false;
        } else {
            $multi = $this->getMultiNotice();
            // Display a message when the diff is empty
            if ( $body === '' ) {
                $notice .= '<div class="mw-diff-empty">' . $this->msg( 'diff-empty' )->parse() . "</div>\n";
            }
            return $this->addHeader( $body, $otitle, $ntitle, $multi, $notice );
        }
    }

    public function getDiffBody() {
        global $wgMemc;
        wfProfileIn( __METHOD__ );
        $this->mCacheHit = true;
        // Check if the diff should be hidden from this user
        if ( !$this->loadRevisionData() ) {
            wfProfileOut( __METHOD__ );
            return false;
        } elseif ( $this->mOldRev && !$this->mOldRev->userCan( Revision::DELETED_TEXT, $this->getUser() ) ) {
            wfProfileOut( __METHOD__ );
            return false;
        } elseif ( $this->mNewRev && !$this->mNewRev->userCan( Revision::DELETED_TEXT, $this->getUser() ) ) {
            wfProfileOut( __METHOD__ );
            return false;
        }
        // Short-circuit
        if ( $this->mOldRev === false || ( $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;
                }
            } // 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->generateContentDiffBody( $this->mOldContent, $this->mNewContent );

        // 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 $difftext;
    }

    function generateContentDiffBody( Content $old, Content $new ) {
        if ( !( $old instanceof TextContent ) ) {
            throw new MWException( "Diff not implemented for " . get_class( $old ) . "; "
                    . "override generateContentDiffBody to fix this." );
        }

        if ( !( $new instanceof TextContent ) ) {
            throw new MWException( "Diff not implemented for " . get_class( $new ) . "; "
                . "override generateContentDiffBody to fix this." );
        }

        $otext = $old->serialize();
        $ntext = $new->serialize();

        return $this->generateTextDiffBody( $otext, $ntext );
    }

    function generateDiffBody( $otext, $ntext ) {
        ContentHandler::deprecated( __METHOD__, "1.21" );

        return $this->generateTextDiffBody( $otext, $ntext );
    }

    function generateTextDiffBody( $otext, $ntext ) {
        global $wgExternalDiffEngine, $wgContLang;

        wfProfileIn( __METHOD__ );

        $otext = str_replace( "\r\n", "\n", $otext );
        $ntext = str_replace( "\r\n", "\n", $ntext );

        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 ) );
            wfProfileOut( __METHOD__ );
            return $wgContLang->unsegmentForDiff( 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' );
            wfProfileOut( __METHOD__ );
            return $text;
        }
        if ( $wgExternalDiffEngine != 'wikidiff3' && $wgExternalDiffEngine !== false ) {
            # Diff via the shell
            $tmpDir = wfTempDir();
            $tempName1 = tempnam( $tmpDir, 'diff_' );
            $tempName2 = tempnam( $tmpDir, '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;
        }

        # 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;
    }

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

    function localiseLineNumbersCb( $matches ) {
        if ( $matches[1] === '1' && $this->mReducedLineNumbers ) {
            return '';
        }
        return $this->msg( 'lineno' )->numParams( $matches[1] )->escaped();
    }

    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 '';
        }

        if ( $this->mOldRev->getTimestamp() > $this->mNewRev->getTimestamp() ) {
            $oldRev = $this->mNewRev; // flip
            $newRev = $this->mOldRev; // flip
        } else { // normal case
            $oldRev = $this->mOldRev;
            $newRev = $this->mNewRev;
        }

        $nEdits = $this->mNewPage->countRevisionsBetween( $oldRev, $newRev );
        if ( $nEdits > 0 ) {
            $limit = 100; // use diff-multi-manyusers if too many users
            $numUsers = $this->mNewPage->countAuthorsBetween( $oldRev, $newRev, $limit );
            return self::intermediateEditsMsg( $nEdits, $numUsers, $limit );
        }
        return ''; // nothing
    }

    public static function intermediateEditsMsg( $numEdits, $numUsers, $limit ) {
        if ( $numUsers > $limit ) {
            $msg = 'diff-multi-manyusers';
            $numUsers = $limit;
        } else {
            $msg = 'diff-multi';
        }
        return wfMessage( $msg )->numParams( $numEdits, $numUsers )->parse();
    }

    protected function getRevisionHeader( Revision $rev, $complete = '' ) {
        $lang = $this->getLanguage();
        $user = $this->getUser();
        $revtimestamp = $rev->getTimestamp();
        $timestamp = $lang->userTimeAndDate( $revtimestamp, $user );
        $dateofrev = $lang->userDate( $revtimestamp, $user );
        $timeofrev = $lang->userTime( $revtimestamp, $user );

        $header = $this->msg(
            $rev->isCurrent() ? 'currentrev-asof' : 'revisionasof',
            $timestamp,
            $dateofrev,
            $timeofrev
        )->escaped();

        if ( $complete !== 'complete' ) {
            return $header;
        }

        $title = $rev->getTitle();

        $header = Linker::linkKnown( $title, $header, array(),
            array( 'oldid' => $rev->getID() ) );

        if ( $rev->userCan( Revision::DELETED_TEXT, $user ) ) {
            $editQuery = array( 'action' => 'edit' );
            if ( !$rev->isCurrent() ) {
                $editQuery['oldid'] = $rev->getID();
            }

            $msg = $this->msg( $title->quickUserCan( 'edit', $user ) ? 'editold' : 'viewsourceold' )->escaped();
            $header .= ' ' . $this->msg( 'parentheses' )->rawParams(
                Linker::linkKnown( $title, $msg, array(), $editQuery ) )->plain();
            if ( $rev->isDeleted( Revision::DELETED_TEXT ) ) {
                $header = Html::rawElement( 'span', array( 'class' => 'history-deleted' ), $header );
            }
        } else {
            $header = Html::rawElement( 'span', array( 'class' => 'history-deleted' ), $header );
        }

        return $header;
    }

    function addHeader( $diff, $otitle, $ntitle, $multi = '', $notice = '' ) {
        // shared.css sets diff in interface language/dir, but the actual content
        // is often in a different language, mostly the page content language/dir
        $tableClass = 'diff diff-contentalign-' . htmlspecialchars( $this->getDiffLang()->alignStart() );
        $header = "<table class='$tableClass'>";

        if ( !$diff && !$otitle ) {
            $header .= "
            <tr style='vertical-align: top;'>
            <td class='diff-ntitle'>{$ntitle}</td>
            </tr>";
            $multiColspan = 1;
        } else {
            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;
            }
            if ( $otitle || $ntitle ) {
                $header .= "
                <tr style='vertical-align: top;'>
                <td colspan='$colspan' class='diff-otitle'>{$otitle}</td>
                <td colspan='$colspan' class='diff-ntitle'>{$ntitle}</td>
                </tr>";
            }
        }

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

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

    function setText( $oldText, $newText ) {
        ContentHandler::deprecated( __METHOD__, "1.21" );

        $oldContent = ContentHandler::makeContent( $oldText, $this->getTitle() );
        $newContent = ContentHandler::makeContent( $newText, $this->getTitle() );

        $this->setContent( $oldContent, $newContent );
    }

    function setContent( Content $oldContent, Content $newContent ) {
        $this->mOldContent = $oldContent;
        $this->mNewContent = $newContent;

        $this->mTextLoaded = 2;
        $this->mRevisionsLoaded = true;
    }

    function setTextLanguage( $lang ) {
        $this->mDiffLang = wfGetLangObj( $lang );
    }

    private function loadRevisionIds() {
        if ( $this->mRevisionsIdsLoaded ) {
            return;
        }

        $this->mRevisionsIdsLoaded = true;

        $old = $this->mOldid;
        $new = $this->mNewid;

        if ( $new === 'prev' ) {
            # Show diff between revision $old and the previous one.
            # Get previous one from DB.
            $this->mNewid = intval( $old );
            $this->mOldid = $this->getTitle()->getPreviousRevisionID( $this->mNewid );
        } elseif ( $new === 'next' ) {
            # Show diff between revision $old and the next one.
            # Get next one from DB.
            $this->mOldid = intval( $old );
            $this->mNewid = $this->getTitle()->getNextRevisionID( $this->mOldid );
            if ( $this->mNewid === false ) {
                # 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( $this->getTitle(), &$this->mOldid, &$this->mNewid, $old, $new ) );
        }
    }

    function loadRevisionData() {
        if ( $this->mRevisionsLoaded ) {
            return true;
        }

        // Whether it succeeds or fails, we don't want to try again
        $this->mRevisionsLoaded = true;

        $this->loadRevisionIds();

        // Load the new revision object
        $this->mNewRev = $this->mNewid
            ? Revision::newFromId( $this->mNewid )
            : Revision::newFromTitle( $this->getTitle(), false, Revision::READ_NORMAL );

        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();
        $this->mNewPage = $this->mNewRev->getTitle();

        // 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();
        }

        // Load tags information for both revisions
        $dbr = wfGetDB( DB_SLAVE );
        if ( $this->mOldid !== false ) {
            $this->mOldTags = $dbr->selectField(
                'tag_summary',
                'ts_tags',
                array( 'ts_rev_id' => $this->mOldid ),
                __METHOD__
            );
        } else {
            $this->mOldTags = false;
        }
        $this->mNewTags = $dbr->selectField(
            'tag_summary',
            'ts_tags',
            array( 'ts_rev_id' => $this->mNewid ),
            __METHOD__
        );

        return true;
    }

    function loadText() {
        if ( $this->mTextLoaded == 2 ) {
            return true;
        }

        // Whether it succeeds or fails, we don't want to try again
        $this->mTextLoaded = 2;

        if ( !$this->loadRevisionData() ) {
            return false;
        }

        if ( $this->mOldRev ) {
            $this->mOldContent = $this->mOldRev->getContent( Revision::FOR_THIS_USER, $this->getUser() );
            if ( $this->mOldContent === null ) {
                return false;
            }
        }

        if ( $this->mNewRev ) {
            $this->mNewContent = $this->mNewRev->getContent( Revision::FOR_THIS_USER, $this->getUser() );
            if ( $this->mNewContent === null ) {
                return false;
            }
        }

        return true;
    }

    function loadNewText() {
        if ( $this->mTextLoaded >= 1 ) {
            return true;
        }

        $this->mTextLoaded = 1;

        if ( !$this->loadRevisionData() ) {
            return false;
        }

        $this->mNewContent = $this->mNewRev->getContent( Revision::FOR_THIS_USER, $this->getUser() );

        return true;
    }
}