<?php
/**
 * User interface for the difference engine.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 * @ingroup DifferenceEngine
 */

use MediaWiki\ChangeTags\ChangeTags;
use MediaWiki\CommentFormatter\CommentFormatter;
use MediaWiki\Content\Content;
use MediaWiki\Content\IContentHandlerFactory;
use MediaWiki\Context\ContextSource;
use MediaWiki\Context\IContextSource;
use MediaWiki\Debug\DeprecationHelper;
use MediaWiki\Diff\TextDiffer\ManifoldTextDiffer;
use MediaWiki\Exception\PermissionsError;
use MediaWiki\HookContainer\HookRunner;
use MediaWiki\Html\Html;
use MediaWiki\Language\Language;
use MediaWiki\Linker\Linker;
use MediaWiki\Linker\LinkRenderer;
use MediaWiki\MainConfigNames;
use MediaWiki\MediaWikiServices;
use MediaWiki\Message\Message;
use MediaWiki\Page\ParserOutputAccess;
use MediaWiki\Page\WikiPageFactory;
use MediaWiki\Permissions\Authority;
use MediaWiki\Permissions\PermissionStatus;
use MediaWiki\RecentChanges\ChangesList;
use MediaWiki\RecentChanges\RecentChange;
use MediaWiki\Revision\ArchivedRevisionLookup;
use MediaWiki\Revision\BadRevisionException;
use MediaWiki\Revision\RevisionRecord;
use MediaWiki\Revision\RevisionStore;
use MediaWiki\Revision\SlotRecord;
use MediaWiki\SpecialPage\SpecialPage;
use MediaWiki\Storage\NameTableAccessException;
use MediaWiki\Title\Title;
use MediaWiki\User\Options\UserOptionsLookup;
use MediaWiki\User\UserEditTracker;
use MediaWiki\User\UserGroupManager;
use MediaWiki\User\UserGroupMembership;
use MediaWiki\User\UserIdentity;
use MediaWiki\User\UserIdentityUtils;
use Wikimedia\Rdbms\IConnectionProvider;

/**
 * DifferenceEngine is responsible for rendering the difference between two revisions as HTML.
 * This includes interpreting URL parameters, retrieving revision data, checking access permissions,
 * selecting and invoking the diff generator class for the individual slots, doing post-processing
 * on the generated diff, adding the rest of the HTML (such as headers) and writing the whole thing
 * to OutputPage.
 *
 * DifferenceEngine can be subclassed by extensions, by customizing
 * ContentHandler::createDifferenceEngine; the content handler will be selected based on the
 * content model of the main slot (of the new revision, when the two are different).
 * That might change after PageTypeHandler gets introduced.
 *
 * In the past, the class was also used for slot-level diff generation, and extensions might still
 * subclass it and add such functionality. When that is the case (specifically, when a
 * ContentHandler returns a standard SlotDiffRenderer but a nonstandard DifferenceEngine)
 * DifferenceEngineSlotDiffRenderer will be used to convert the old behavior into the new one.
 *
 * @ingroup DifferenceEngine
 *
 * @todo This class is huge and poorly defined. It should be split into a controller responsible
 * for interpreting query parameters, retrieving data and checking permissions; and a HTML renderer.
 */
class DifferenceEngine extends ContextSource {

	use DeprecationHelper;

	/**
	 * Constant to indicate diff cache compatibility.
	 * Bump this when changing the diff formatting in a way that
	 * fixes important bugs or such to force cached diff views to
	 * clear.
	 */
	private const DIFF_VERSION = '1.41';

	/**
	 * Revision ID for the old revision. 0 for the revision previous to $mNewid, false
	 * if the diff does not have an old revision (e.g. 'oldid=<first revision of page>&diff=prev'),
	 * or the revision does not exist, null if the revision is unsaved.
	 * @var int|false|null
	 */
	protected $mOldid;

	/**
	 * Revision ID for the new revision. 0 for the last revision of the current page
	 * (as defined by the request context), false if the revision does not exist, null
	 * if it is unsaved, or an alias such as 'next'.
	 * @var int|string|false|null
	 */
	protected $mNewid;

	/**
	 * Old revision (left pane).
	 * Allowed to be an unsaved revision, unlikely that's ever needed though.
	 * False when the old revision does not exist; this can happen when using
	 * diff=prev on the first revision. Null when the revision should exist but
	 * doesn't (e.g. load failure); loadRevisionData() will return false in that
	 * case. Also null until lazy-loaded. Ignored completely when isContentOverridden
	 * is set.
	 * @var RevisionRecord|null|false
	 */
	private $mOldRevisionRecord;

	/**
	 * New revision (right pane).
	 * Note that this might be an unsaved revision (e.g. for edit preview).
	 * Null in case of load failure; diff methods will just return an error message in that case,
	 * and loadRevisionData() will return false. Also null until lazy-loaded. Ignored completely
	 * when isContentOverridden is set.
	 * @var RevisionRecord|null
	 */
	private $mNewRevisionRecord;

	/**
	 * Title of old revision or null if the old revision does not exist or does not belong to a page.
	 * @var Title|null
	 */
	protected $mOldPage;

	/**
	 * Title of new revision or null if the new revision does not exist or does not belong to a page.
	 * @var Title|null
	 */
	protected $mNewPage;

	/**
	 * Change tags of old revision or null if it does not exist / is not saved.
	 * @var string|false|null
	 */
	private $mOldTags;

	/**
	 * Change tags of new revision or null if it does not exist / is not saved.
	 * @var string|null
	 */
	private $mNewTags;

	/**
	 * @var Content|null
	 * @deprecated since 1.32, content slots are now handled by the corresponding SlotDiffRenderer.
	 *   This property is set to the content of the main slot, but not actually used for the main diff.
	 */
	private $mOldContent;

	/**
	 * @var Content|null
	 * @deprecated since 1.32, content slots are now handled by the corresponding SlotDiffRenderer.
	 *   This property is set to the content of the main slot, but not actually used for the main diff.
	 */
	private $mNewContent;

	/** @var Language */
	protected $mDiffLang;

	/** @var bool Have the revisions IDs been loaded */
	private $mRevisionsIdsLoaded = false;

	/** @var bool Have the revisions been loaded */
	protected $mRevisionsLoaded = false;

	/** @var int How many text blobs have been loaded, 0, 1 or 2? */
	protected $mTextLoaded = 0;

	/**
	 * Was the content overridden via setContent()?
	 * If the content was overridden, most internal state (e.g. mOldid or mOldRev) should be ignored
	 * and only mOldContent and mNewContent is reliable.
	 * (Note that setRevisions() does not set this flag as in that case all properties are
	 * overridden and remain consistent with each other, so no special handling is needed.)
	 * @var bool
	 */
	protected $isContentOverridden = false;

	/** @var bool Was the diff fetched from cache? */
	protected $mCacheHit = false;

	/** @var string|null Cache key if the diff was fetched from cache */
	private $cacheHitKey = null;

	/**
	 * Set this to true to add debug info to the HTML output.
	 * Warning: this may cause RSS readers to spuriously mark articles as "new"
	 * (T22601)
	 * @var bool
	 */
	public $enableDebugComment = false;

	/** @var bool 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;

	/** @var string Link to action=markpatrolled */
	protected $mMarkPatrolledLink = null;

	/** @var bool Show rev_deleted content if allowed */
	protected $unhide = false;

	/** @var bool Refresh the diff cache */
	protected $mRefreshCache = false;

	/** @var SlotDiffRenderer[]|null DifferenceEngine classes for the slots, keyed by role name. */
	protected $slotDiffRenderers = null;

	/**
	 * Temporary hack for B/C while slot diff related methods of DifferenceEngine are being
	 * deprecated. When true, we are inside a DifferenceEngineSlotDiffRenderer and
	 * $slotDiffRenderers should not be used.
	 * @var bool
	 */
	protected $isSlotDiffRenderer = false;

	/**
	 * A set of options that will be passed to the SlotDiffRenderer upon creation
	 * @var array
	 */
	private $slotDiffOptions = [];

	/**
	 * Extra query parameters to be appended to diff page links
	 * @var array
	 */
	private $extraQueryParams = [];

	/** @var ManifoldTextDiffer|null */
	private $textDiffer;

	protected LinkRenderer $linkRenderer;
	private IContentHandlerFactory $contentHandlerFactory;
	private RevisionStore $revisionStore;
	private ArchivedRevisionLookup $archivedRevisionLookup;
	private HookRunner $hookRunner;
	private WikiPageFactory $wikiPageFactory;
	private UserOptionsLookup $userOptionsLookup;
	private CommentFormatter $commentFormatter;
	private IConnectionProvider $dbProvider;
	private UserGroupManager $userGroupManager;
	private UserEditTracker $userEditTracker;
	private UserIdentityUtils $userIdentityUtils;

	/** @var Message[] */
	private $revisionLoadErrors = [];

	/**
	 * @param IContextSource|null $context Context to use, anything else will be ignored
	 * @param int $old Old ID we want to show and diff with.
	 * @param string|int $new Either revision ID or 'prev' or 'next'. Default: 0.
	 * @param int $rcid Deprecated, no longer used!
	 * @param bool $refreshCache If set, refreshes the diff cache
	 * @param bool $unhide If set, allow viewing deleted revs
	 */
	public 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'" );

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

		$services = MediaWikiServices::getInstance();
		$this->linkRenderer = $services->getLinkRenderer();
		$this->contentHandlerFactory = $services->getContentHandlerFactory();
		$this->revisionStore = $services->getRevisionStore();
		$this->archivedRevisionLookup = $services->getArchivedRevisionLookup();
		$this->hookRunner = new HookRunner( $services->getHookContainer() );
		$this->wikiPageFactory = $services->getWikiPageFactory();
		$this->userOptionsLookup = $services->getUserOptionsLookup();
		$this->commentFormatter = $services->getCommentFormatter();
		$this->dbProvider = $services->getConnectionProvider();
		$this->userGroupManager = $services->getUserGroupManager();
		$this->userEditTracker = $services->getUserEditTracker();
		$this->userIdentityUtils = $services->getUserIdentityUtils();
	}

	/**
	 * @return SlotDiffRenderer[] Diff renderers for each slot, keyed by role name.
	 *   Includes slots only present in one of the revisions. Does not include slots
	 *   for which content is identical in the two revisions.
	 */
	protected function getSlotDiffRenderers() {
		if ( $this->isSlotDiffRenderer ) {
			throw new LogicException( __METHOD__ . ' called in slot diff renderer mode' );
		}

		if ( $this->slotDiffRenderers === null ) {
			if ( !$this->loadRevisionData() ) {
				return [];
			}

			$slotContents = $this->getSlotContents();
			$this->slotDiffRenderers = [];
			foreach ( $slotContents as $role => $contents ) {
				if ( $contents['new'] && $contents['old']
					&& $contents['new']->equals( $contents['old'] )
				) {
					// Do not produce a diff of identical content
					continue;
				}
				if ( !$contents['new'] && !$contents['old'] ) {
					// Nothing to diff (i.e both revisions are corrupted), just ignore
					continue;
				}
				$handler = ( $contents['new'] ?: $contents['old'] )->getContentHandler();
				$this->slotDiffRenderers[$role] = $handler->getSlotDiffRenderer(
					$this->getContext(),
					$this->slotDiffOptions + [
						'contentLanguage' => $this->getDiffLang()->getCode(),
						'textDiffer' => $this->getTextDiffer()
					]
				);
			}
		}

		return $this->slotDiffRenderers;
	}

	/**
	 * Mark this DifferenceEngine as a slot renderer (as opposed to a page renderer).
	 * This is used in legacy mode when the DifferenceEngine is wrapped in a
	 * DifferenceEngineSlotDiffRenderer.
	 * @internal For use by DifferenceEngineSlotDiffRenderer only.
	 */
	public function markAsSlotDiffRenderer() {
		$this->isSlotDiffRenderer = true;
	}

	/**
	 * Get the old and new content objects for all slots.
	 * This method does not do any permission checks.
	 * @return (Content|null)[][] [ role => [ 'old' => Content|null, 'new' => Content|null ], ... ]
	 */
	protected function getSlotContents() {
		if ( $this->isContentOverridden ) {
			return [
				SlotRecord::MAIN => [ 'old' => $this->mOldContent, 'new' => $this->mNewContent ]
			];
		} elseif ( !$this->loadRevisionData() ) {
			return [];
		}

		$newSlots = $this->mNewRevisionRecord->getPrimarySlots()->getSlots();
		$oldSlots = $this->mOldRevisionRecord ?
			$this->mOldRevisionRecord->getPrimarySlots()->getSlots() :
			[];
		// The order here will determine the visual order of the diff. The current logic is
		// slots of the new revision first in natural order, then deleted ones. This is ad hoc
		// and should not be relied on - in the future we may want the ordering to depend
		// on the page type.
		$roles = array_keys( array_merge( $newSlots, $oldSlots ) );

		$slots = [];
		foreach ( $roles as $role ) {
			$slots[$role] = [
				'old' => $this->loadSingleSlot(
					$oldSlots[$role] ?? null,
					'old'
				),
				'new' => $this->loadSingleSlot(
					$newSlots[$role] ?? null,
					'new'
				)
			];
		}
		// move main slot to front
		if ( isset( $slots[SlotRecord::MAIN] ) ) {
			$slots = [ SlotRecord::MAIN => $slots[SlotRecord::MAIN] ] + $slots;
		}
		return $slots;
	}

	/**
	 * Load the content of a single slot record
	 *
	 * @param SlotRecord|null $slot
	 * @param string $which "new" or "old"
	 * @return Content|null
	 */
	private function loadSingleSlot( ?SlotRecord $slot, string $which ) {
		if ( !$slot ) {
			return null;
		}
		try {
			return $slot->getContent();
		} catch ( BadRevisionException $e ) {
			$this->addRevisionLoadError( $which );
			return null;
		}
	}

	/**
	 * Set a message to show as a notice at the top of the page
	 *
	 * @param string $which "new" or "old"
	 */
	private function addRevisionLoadError( $which ) {
		$this->revisionLoadErrors[] = $this->msg( $which === 'new'
			? 'difference-bad-new-revision' : 'difference-bad-old-revision'
		);
	}

	/**
	 * If errors were encountered while loading the revision contents, this
	 * will return an array of Messages describing the errors.
	 *
	 * @return Message[]
	 */
	public function getRevisionLoadErrors() {
		return $this->revisionLoadErrors;
	}

	/**
	 * Determine whether there was an error loading the new revision
	 * @return bool
	 */
	private function hasNewRevisionLoadError() {
		foreach ( $this->revisionLoadErrors as $error ) {
			if ( $error->getKey() === 'difference-bad-new-revision' ) {
				return true;
			}
		}
		return false;
	}

	/** @inheritDoc */
	public function getTitle() {
		// T202454 avoid errors when there is no title
		return parent::getTitle() ?: Title::makeTitle( NS_SPECIAL, 'BadTitle/DifferenceEngine' );
	}

	/**
	 * Set reduced line numbers mode.
	 * When set, line X is not displayed when X is 1, for example to increase readability and
	 * conserve space with many small diffs.
	 * @param bool $value
	 */
	public function setReducedLineNumbers( $value = true ) {
		$this->mReducedLineNumbers = $value;
	}

	/**
	 * Get the language in which the diff text is written
	 *
	 * @return Language
	 */
	public function getDiffLang() {
		# Default language in which the diff text is written.
		$this->mDiffLang ??= $this->getDefaultLanguage();
		return $this->mDiffLang;
	}

	/**
	 * Get the language to use if none has been set by setTextLanguage().
	 * Wikibase overrides this to use the user language.
	 *
	 * @return Language
	 */
	protected function getDefaultLanguage() {
		return $this->getTitle()->getPageLanguage();
	}

	/**
	 * @return bool
	 */
	public function wasCacheHit() {
		return $this->mCacheHit;
	}

	/**
	 * Get the ID of old revision (left pane) of the diff. 0 for the revision
	 * previous to getNewid(), false if the old revision does not exist, null
	 * if it's unsaved.
	 * To get a real revision ID instead of 0, call loadRevisionData() first.
	 * @return int|false|null
	 */
	public function getOldid() {
		$this->loadRevisionIds();

		return $this->mOldid;
	}

	/**
	 * Get the ID of new revision (right pane) of the diff. 0 for the current revision,
	 * false if the new revision does not exist, null if it's unsaved.
	 * To get a real revision ID instead of 0, call loadRevisionData() first.
	 * @return int|false|null
	 */
	public function getNewid() {
		$this->loadRevisionIds();

		return $this->mNewid;
	}

	/**
	 * Get the left side of the diff.
	 * Could be null when the first revision of the page is diffed to 'prev' (or in the case of
	 * load failure).
	 * @return RevisionRecord|null
	 */
	public function getOldRevision() {
		return $this->mOldRevisionRecord ?: null;
	}

	/**
	 * Get the right side of the diff.
	 * Should not be null but can still happen in the case of load failure.
	 * @return RevisionRecord|null
	 */
	public function getNewRevision() {
		return $this->mNewRevisionRecord;
	}

	/**
	 * Look up a special:Undelete link to the given deleted revision id,
	 * as a workaround for being unable to load deleted diffs in currently.
	 *
	 * @param int $id Revision ID
	 *
	 * @return string|bool Link HTML or false
	 */
	public function deletedLink( $id ) {
		if ( $this->getAuthority()->isAllowed( 'deletedhistory' ) ) {
			$revRecord = $this->archivedRevisionLookup->getArchivedRevisionRecord( null, $id );
			if ( $revRecord ) {
				$title = Title::newFromPageIdentity( $revRecord->getPage() );

				return SpecialPage::getTitleFor( 'Undelete' )->getFullURL( [
					'target' => $title->getPrefixedText(),
					'timestamp' => $revRecord->getTimestamp()
				] );
			}
		}

		return false;
	}

	/**
	 * Build a wikitext link toward a deleted revision, if viewable.
	 *
	 * @param int $id Revision ID
	 *
	 * @return string Wikitext fragment
	 */
	public function deletedIdMarker( $id ) {
		$link = $this->deletedLink( $id );
		if ( $link ) {
			return "[$link $id]";
		} else {
			return (string)$id;
		}
	}

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

		$missing = [];
		if ( $this->mOldid && ( !$this->mOldRevisionRecord || !$this->mOldContent ) ) {
			$missing[] = $this->deletedIdMarker( $this->mOldid );
		}
		if ( !$this->mNewRevisionRecord || !$this->mNewContent ) {
			$missing[] = $this->deletedIdMarker( $this->mNewid );
		}

		$out->setPageTitleMsg( $this->msg( 'errorpagetitle' ) );
		$msg = $this->msg( 'difference-missing-revision' )
			->params( $this->getLanguage()->listToText( $missing ) )
			->numParams( count( $missing ) )
			->parseAsBlock();
		$out->addHTML( $msg );
	}

	/**
	 * Checks whether one of the given Revisions was deleted
	 *
	 * @return bool
	 */
	public function hasDeletedRevision() {
		$this->loadRevisionData();
		return (
				$this->mNewRevisionRecord &&
				$this->mNewRevisionRecord->isDeleted( RevisionRecord::DELETED_TEXT )
			) ||
			(
				$this->mOldRevisionRecord &&
				$this->mOldRevisionRecord->isDeleted( RevisionRecord::DELETED_TEXT )
			);
	}

	/**
	 * Get the permission errors associated with the revisions for the current diff.
	 *
	 * @deprecated since 1.44 Use authorizeView() instead
	 * @param Authority $performer
	 * @return array[] Array of arrays of the arguments to wfMessage to explain permissions problems.
	 */
	public function getPermissionErrors( Authority $performer ) {
		wfDeprecated( __METHOD__, '1.44' );
		return $this->authorizeView( $performer )->toLegacyErrorArray();
	}

	/**
	 * Check whether the user can read both of the pages for the current diff.
	 *
	 * This does not check access to deleted revisions, use isUserAllowedToSeeRevisions() for that.
	 *
	 * @since 1.44
	 */
	public function authorizeView( Authority $performer ): PermissionStatus {
		$this->loadRevisionData();
		$permStatus = PermissionStatus::newEmpty();
		if ( $this->mNewPage ) {
			$performer->authorizeRead( 'read', $this->mNewPage, $permStatus );
		}
		if ( $this->mOldPage ) {
			$performer->authorizeRead( 'read', $this->mOldPage, $permStatus );
		}
		return $permStatus;
	}

	/**
	 * Checks whether one of the given Revisions was suppressed
	 *
	 * @return bool
	 */
	public function hasSuppressedRevision() {
		return $this->hasDeletedRevision() && (
			( $this->mOldRevisionRecord &&
				$this->mOldRevisionRecord->isDeleted( RevisionRecord::DELETED_RESTRICTED ) ) ||
			( $this->mNewRevisionRecord &&
				$this->mNewRevisionRecord->isDeleted( RevisionRecord::DELETED_RESTRICTED ) )
		);
	}

	/**
	 * Renders user associated edit count
	 *
	 * @param UserIdentity $user
	 * @return string
	 */
	private function getUserEditCount( $user ): string {
		$editCount = $this->userEditTracker->getUserEditCount( $user );
		if ( $editCount === null ) {
			return '';
		}

		return Html::rawElement( 'div', [
			'class' => 'mw-diff-usereditcount',
		],
			$this->msg(
				'diff-user-edits',
				$this->getLanguage()->formatNum( $editCount )
			)->parse()
		);
	}

	/**
	 * Renders user roles
	 *
	 * @param UserIdentity $user
	 * @return string
	 */
	private function getUserRoles( UserIdentity $user ) {
		if ( !$this->userIdentityUtils->isNamed( $user ) ) {
			return '';
		}
		$userGroups = $this->userGroupManager->getUserGroups( $user );
		$userGroupLinks = [];
		foreach ( $userGroups as $group ) {
			$userGroupLinks[] = UserGroupMembership::getLinkHTML( $group, $this->getContext() );
		}
		return Html::rawElement( 'div', [
			'class' => 'mw-diff-userroles',
		], $this->getLanguage()->commaList( $userGroupLinks ) );
	}

	/**
	 * Renders user associated meta data
	 *
	 * @param UserIdentity|null $user
	 * @return string
	 */
	private function getUserMetaData( ?UserIdentity $user ) {
		if ( !$user ) {
			return '';
		}
		return Html::rawElement( 'div', [
			'class' => 'mw-diff-usermetadata',
		], $this->getUserRoles( $user ) . $this->getUserEditCount( $user ) );
	}

	/**
	 * Checks whether the current user has permission for accessing the revisions of the diff.
	 * Note that this does not check whether the user has permission to view the page, it only
	 * checks revdelete permissions.
	 *
	 * It is the caller's responsibility to call
	 * $this->getUserPermissionErrors or similar checks.
	 *
	 * @param Authority $performer
	 * @return bool
	 */
	public function isUserAllowedToSeeRevisions( Authority $performer ) {
		$this->loadRevisionData();

		if ( $this->mOldRevisionRecord && !$this->mOldRevisionRecord->userCan(
			RevisionRecord::DELETED_TEXT,
			$performer
		) ) {
			return false;
		}

		// $this->mNewRev will only be falsy if a loading error occurred
		// (in which case the user is allowed to see).
		return !$this->mNewRevisionRecord || $this->mNewRevisionRecord->userCan(
			RevisionRecord::DELETED_TEXT,
			$performer
		);
	}

	/**
	 * Checks whether the diff should be hidden from the current user
	 * This is based on whether the user is allowed to see it and has specifically asked to see it.
	 *
	 * @param Authority $performer
	 * @return bool
	 */
	public function shouldBeHiddenFromUser( Authority $performer ) {
		return $this->hasDeletedRevision() && ( !$this->unhide ||
			!$this->isUserAllowedToSeeRevisions( $performer ) );
	}

	/**
	 * @param bool $diffOnly
	 */
	public function showDiffPage( $diffOnly = false ) {
		# Allow frames except in certain special cases
		$out = $this->getOutput();
		$out->getMetadata()->setPreventClickjacking( false );
		$out->setRobotPolicy( 'noindex,nofollow' );

		// Allow extensions to add any extra output here
		$this->hookRunner->onDifferenceEngineShowDiffPage( $out );

		if ( !$this->loadRevisionData() ) {
			if ( $this->hookRunner->onDifferenceEngineShowDiffPageMaybeShowMissingRevision( $this ) ) {
				$this->showMissingRevision();
			}
			return;
		}

		$user = $this->getUser();
		$permStatus = $this->authorizeView( $this->getAuthority() );
		if ( !$permStatus->isGood() ) {
			throw new PermissionsError( 'read', $permStatus );
		}

		$rollback = '';

		$query = $this->extraQueryParams;
		# Carry over 'diffonly' param via navigation links
		if ( $diffOnly != MediaWikiServices::getInstance()
			->getUserOptionsLookup()->getBoolOption( $user, '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 = $this->hasDeletedRevision();
		$suppressed = $this->hasSuppressedRevision();
		$allowed = $this->isUserAllowedToSeeRevisions( $this->getAuthority() );

		$revisionTools = [];
		$breadCrumbs = '';

		# mOldRevisionRecord 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->mOldRevisionRecord === false ) {
			if ( $this->mNewPage ) {
				$out->setPageTitleMsg(
					$this->msg( 'difference-title' )->plaintextParams( $this->mNewPage->getPrefixedText() )
				);
			}
			$samePage = true;
			$oldHeader = '';
			// Allow extensions to change the $oldHeader variable
			$this->hookRunner->onDifferenceEngineOldHeaderNoOldRev( $oldHeader );
		} else {
			$this->hookRunner->onDifferenceEngineViewHeader( $this );

			if ( !$this->mOldPage || !$this->mNewPage ) {
				// XXX say something to the user?
				$samePage = false;
			} elseif ( $this->mNewPage->equals( $this->mOldPage ) ) {
				$out->setPageTitleMsg(
					$this->msg( 'difference-title' )->plaintextParams( $this->mNewPage->getPrefixedText() )
				);
				$samePage = true;
			} else {
				$out->setPageTitleMsg( $this->msg( 'difference-title-multipage' )->plaintextParams(
					$this->mOldPage->getPrefixedText(), $this->mNewPage->getPrefixedText() ) );
				$out->addSubtitle( $this->msg( 'difference-multipage' ) );
				$samePage = false;
			}

			if ( $samePage && $this->mNewPage &&
				$this->getAuthority()->probablyCan( 'edit', $this->mNewPage )
			) {
				if ( $this->mNewRevisionRecord->isCurrent() &&
					$this->getAuthority()->probablyCan( 'rollback', $this->mNewPage )
				) {
					$rollbackLink = Linker::generateRollback(
						$this->mNewRevisionRecord,
						$this->getContext(),
						[ 'noBrackets' ]
					);
					if ( $rollbackLink ) {
						$out->getMetadata()->setPreventClickjacking( true );
						$rollback = "\u{00A0}\u{00A0}\u{00A0}" . $rollbackLink;
					}
				}

				if ( $this->userCanEdit( $this->mOldRevisionRecord ) &&
					$this->userCanEdit( $this->mNewRevisionRecord )
				) {
					$undoLink = $this->linkRenderer->makeKnownLink(
						$this->mNewPage,
						$this->msg( 'editundo' )->text(),
						[ 'title' => Linker::titleAttrib( 'undo' ) ],
						[
							'action' => 'edit',
							'undoafter' => $this->mOldid,
							'undo' => $this->mNewid
						]
					);
					$revisionTools['mw-diff-undo'] = $undoLink;
				}
			}
			# Make "previous revision link"
			$hasPrevious = $samePage && $this->mOldPage &&
				$this->revisionStore->getPreviousRevision( $this->mOldRevisionRecord );
			if ( $hasPrevious ) {
				$prevlinkQuery = [ 'diff' => 'prev', 'oldid' => $this->mOldid ] + $query;
				$prevlink = $this->linkRenderer->makeKnownLink(
					$this->mOldPage,
					$this->msg( 'previousdiff' )->text(),
					[ 'id' => 'differences-prevlink' ],
					$prevlinkQuery
				);
				$breadCrumbs .= $this->linkRenderer->makeKnownLink(
					$this->mOldPage,
					$this->msg( 'previousdiff' )->text(),
					[
						'class' => 'mw-diff-revision-history-link-previous'
					],
					$prevlinkQuery
				);
			} else {
				$prevlink = "\u{00A0}";
			}

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

			$oldRevRecord = $this->mOldRevisionRecord;

			$ldel = $this->revisionDeleteLink( $oldRevRecord );
			$oldRevisionHeader = $this->getRevisionHeader( $oldRevRecord, 'complete' );
			$oldChangeTags = ChangeTags::formatSummaryRow( $this->mOldTags, 'diff', $this->getContext() );
			$oldRevComment = $this->commentFormatter
				->formatRevision(
					$oldRevRecord, $user, !$diffOnly, !$this->unhide, /** disable parentheseses */ false
				);

			if ( $oldRevComment === '' ) {
				$defaultComment = $this->msg( 'changeslist-nocomment' )->escaped();
				$oldRevComment = "<span class=\"comment mw-comment-none\">$defaultComment</span>";
			}

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

			// Allow extensions to change the $oldHeader variable
			$this->hookRunner->onDifferenceEngineOldHeader(
				$this, $oldHeader, $prevlink, $oldminor, $diffOnly, $ldel, $this->unhide );
		}

		$out->addJsConfigVars( [
			'wgDiffOldId' => $this->mOldid,
			'wgDiffNewId' => $this->mNewid,
		] );

		# Make "next revision link"
		# Skip next link on the top revision
		if ( $samePage && $this->mNewPage && !$this->mNewRevisionRecord->isCurrent() ) {
			$nextlinkQuery = [ 'diff' => 'next', 'oldid' => $this->mNewid ] + $query;
			$nextlink = $this->linkRenderer->makeKnownLink(
				$this->mNewPage,
				$this->msg( 'nextdiff' )->text(),
				[ 'id' => 'differences-nextlink' ],
				$nextlinkQuery
			);
			$breadCrumbs .= $this->linkRenderer->makeKnownLink(
				$this->mNewPage,
				$this->msg( 'nextdiff' )->text(),
				[
					'class' => 'mw-diff-revision-history-link-next'
				],
				$nextlinkQuery
			);
		} else {
			$nextlink = "\u{00A0}";
		}

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

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

		# Allow extensions to define their own revision tools
		$this->hookRunner->onDiffTools(
			$this->mNewRevisionRecord,
			$revisionTools,
			$this->mOldRevisionRecord ?: null,
			$user
		);

		$formattedRevisionTools = [];
		// Put each one in parentheses (poor man's button)
		foreach ( $revisionTools as $key => $tool ) {
			$toolClass = is_string( $key ) ? $key : 'mw-diff-tool';
			$element = Html::rawElement(
				'span',
				[ 'class' => $toolClass ],
				$tool
			);
			$formattedRevisionTools[] = $element;
		}

		$newRevRecord = $this->mNewRevisionRecord;

		$newRevisionHeader = $this->getRevisionHeader( $newRevRecord, 'complete' ) .
			' ' . implode( ' ', $formattedRevisionTools );
		$newChangeTags = ChangeTags::formatSummaryRow( $this->mNewTags, 'diff', $this->getContext() );
		$newRevComment = $this->commentFormatter->formatRevision(
			$newRevRecord, $user, !$diffOnly, !$this->unhide, /** disable parentheseses */ false
		);

		if ( $newRevComment === '' ) {
			$defaultComment = $this->msg( 'changeslist-nocomment' )->escaped();
			$newRevComment = "<span class=\"comment mw-comment-none\">$defaultComment</span>";
		}

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

		// Allow extensions to change the $newHeader variable
		$this->hookRunner->onDifferenceEngineNewHeader( $this, $newHeader,
			$formattedRevisionTools, $nextlink, $rollback, $newminor, $diffOnly,
			$rdel, $this->unhide );

		$out->addHTML(
			Html::rawElement( 'div', [
				'class' => 'mw-diff-revision-history-links'
			], $breadCrumbs )
		);
		$addMessageBoxStyles = false;
		# If the diff cannot be shown due to a deleted revision, then output
		# the diff header and links to unhide (if available)...
		if ( $this->shouldBeHiddenFromUser( $this->getAuthority() ) ) {
			$this->showDiffStyle();
			$multi = $this->getMultiNotice();
			$out->addHTML( $this->addHeader( '', $oldHeader, $newHeader, $multi ) );
			if ( !$allowed ) {
				# Give explanation for why revision is not visible
				$msg = [ $suppressed ? 'rev-suppressed-no-diff' : 'rev-deleted-no-diff' ];
			} else {
				# Give explanation and add a link to view the diff...
				$query = $this->getRequest()->appendQueryValue( 'unhide', '1' );
				$msg = [
					$suppressed ? 'rev-suppressed-unhide-diff' : 'rev-deleted-unhide-diff',
					$this->getTitle()->getFullURL( $query )
				];
			}
			$out->addHTML( Html::warningBox( $this->msg( ...$msg )->parse(), 'plainlinks' ) );
			$addMessageBoxStyles = true;
		# 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 = Html::warningBox( $this->msg( $msg )->parse(), 'plainlinks' );
				$addMessageBoxStyles = true;
			}

			# Add an error if the content can't be loaded
			$this->getSlotContents();
			foreach ( $this->getRevisionLoadErrors() as $msg ) {
				$notice .= Html::warningBox( $msg->parse() );
				$addMessageBoxStyles = true;
			}

			// Check if inline switcher will be needed
			if ( $this->getTextDiffer()->hasFormat( 'inline' ) ) {
				$out->enableOOUI();
			}

			$this->showTablePrefixes();
			$this->showDiff( $oldHeader, $newHeader, $notice );
			if ( !$diffOnly ) {
				$this->renderNewRevision();
			}

			// Allow extensions to optionally not show the final patrolled link
			if ( $this->hookRunner->onDifferenceEngineRenderRevisionShowFinalPatrolLink() ) {
				# Add redundant patrol link on bottom...
				$out->addHTML( $this->markPatrolledLink() );
			}
		}
		if ( $addMessageBoxStyles ) {
			$out->addModuleStyles( 'mediawiki.codex.messagebox.styles' );
		}
	}

	/**
	 * Add table prefixes
	 */
	private function showTablePrefixes() {
		$parts = [];
		foreach ( $this->getSlotDiffRenderers() as $slotDiffRenderer ) {
			$parts += $slotDiffRenderer->getTablePrefix( $this->getContext(), $this->mNewPage );
		}
		ksort( $parts );
		if ( count( array_filter( $parts ) ) > 0 ) {
			$language = $this->getLanguage();
			$attrs = [
				'class' => 'mw-diff-table-prefix',
				'dir' => $language->getDir(),
				'lang' => $language->getCode(),
			];
			$this->getOutput()->addHTML(
				Html::rawElement( 'div', $attrs, implode( '', $parts ) ) );
		}
	}

	/**
	 * Build a link to mark a change as patrolled.
	 *
	 * Returns empty string if there's either no revision to patrol or the user is not allowed to.
	 *
	 * Side effect: When the patrol link is build, this method will call
	 * OutputPage::getMetadata()->setPreventClickjacking(true) and load a
	 * JS module.
	 *
	 * @return string HTML or empty string
	 */
	public function markPatrolledLink() {
		if ( $this->mMarkPatrolledLink === null ) {
			$linkInfo = $this->getMarkPatrolledLinkInfo();
			// If false, there is no patrol link needed/allowed
			if ( !$linkInfo || !$this->mNewPage ) {
				$this->mMarkPatrolledLink = '';
			} else {
				$patrolLinkClass = 'patrollink';
				$this->mMarkPatrolledLink = ' <span class="' . $patrolLinkClass . '" data-mw="interface">[' .
					$this->linkRenderer->makeKnownLink(
						$this->mNewPage,
						$this->msg( 'markaspatrolleddiff' )->text(),
						[],
						[
							'action' => 'markpatrolled',
							'rcid' => $linkInfo['rcid'],
						]
					) . ']</span>';
				// Allow extensions to change the markpatrolled link
				$this->hookRunner->onDifferenceEngineMarkPatrolledLink( $this,
					$this->mMarkPatrolledLink, $linkInfo['rcid'] );
			}
		}
		return $this->mMarkPatrolledLink;
	}

	/**
	 * Returns an array of meta data needed to build a "mark as patrolled" link and
	 * adds a JS module to the output.
	 *
	 * @return array|false An array of meta data for a patrol link (rcid only)
	 *  or false if no link is needed
	 */
	protected function getMarkPatrolledLinkInfo() {
		$user = $this->getUser();
		$config = $this->getConfig();

		// Prepare a change patrol link, if applicable
		if (
			// Is patrolling enabled and the user allowed to?
			$config->get( MainConfigNames::UseRCPatrol ) &&
			$this->mNewPage &&
			$this->getAuthority()->probablyCan( 'patrol', $this->mNewPage ) &&
			// 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->mNewRevisionRecord->getTimestamp(), 21600 )
		) {
			// Look for an unpatrolled change corresponding to this diff
			$change = RecentChange::newFromConds(
				[
					'rc_this_oldid' => $this->mNewid,
					'rc_patrolled' => RecentChange::PRC_UNPATROLLED
				],
				__METHOD__
			);

			if ( $change && !$change->getPerformerIdentity()->equals( $user ) ) {
				$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;
			}

			// Allow extensions to possibly change the rcid here
			// For example the rcid might be set to zero due to the user
			// being the same as the performer of the change but an extension
			// might still want to show it under certain conditions
			$this->hookRunner->onDifferenceEngineMarkPatrolledRCID( $rcid, $this, $change, $user );

			// Build the link
			if ( $rcid ) {
				$this->getOutput()->getMetadata()->setPreventClickjacking( true );
				$this->getOutput()->addModules( 'mediawiki.misc-authed-curate' );

				return [ 'rcid' => $rcid ];
			}
		}

		// No mark as patrolled link applicable
		return false;
	}

	/**
	 * @param RevisionRecord $revRecord
	 *
	 * @return string
	 */
	private function revisionDeleteLink( RevisionRecord $revRecord ) {
		$link = Linker::getRevDeleteLink(
			$this->getAuthority(),
			$revRecord,
			$revRecord->getPageAsLinkTarget()
		);
		if ( $link !== '' ) {
			$link = "\u{00A0}\u{00A0}\u{00A0}" . $link . ' ';
		}

		return $link;
	}

	/**
	 * Show the new revision of the page.
	 *
	 * @note Not supported after calling setContent().
	 */
	public function renderNewRevision() {
		if ( $this->isContentOverridden ) {
			// The code below only works with a RevisionRecord object. We could construct a
			// fake RevisionRecord (here or in setContent), but since this does not seem
			// needed at the moment, we'll just fail for now.
			throw new LogicException(
				__METHOD__
				. ' is not supported after calling setContent(). Use setRevisions() instead.'
			);
		}

		$out = $this->getOutput();
		$revHeader = $this->getRevisionHeader( $this->mNewRevisionRecord );
		# Add "current version as of X" title
		$out->addHTML( "<hr class='diff-hr' id='mw-oldid' />
		<h2 class='diff-currentversion-title'>{$revHeader}</h2>\n" );
		# Page content may be handled by a hooked call instead...
		if ( $this->hookRunner->onArticleContentOnDiff( $this, $out ) ) {
			$this->loadNewText();
			if ( !$this->mNewPage ) {
				// New revision is unsaved; bail out.
				// TODO in theory rendering the new revision is a meaningful thing to do
				// even if it's unsaved, but a lot of untangling is required to do it safely.
				return;
			}
			if ( $this->hasNewRevisionLoadError() ) {
				// There was an error loading the new revision
				return;
			}

			$out->setRevisionId( $this->mNewid );
			$out->setRevisionIsCurrent( $this->mNewRevisionRecord->isCurrent() );
			$out->getMetadata()->setRevisionTimestamp( $this->mNewRevisionRecord->getTimestamp() );
			$out->setArticleFlag( true );

			if ( !$this->hookRunner->onArticleRevisionViewCustom(
				$this->mNewRevisionRecord, $this->mNewPage, $this->mOldid, $out )
			) {
				// Handled by extension
				// NOTE: sync with hooks called in Article::view()
			} 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 = $this->wikiPageFactory->newFromTitle( $this->mNewPage );
				}

				$parserOptions = $wikiPage->makeParserOptions( $this->getContext() );
				$parserOptions->setRenderReason( 'diff-page' );

				$parserOutputAccess = MediaWikiServices::getInstance()->getParserOutputAccess();
				$status = $parserOutputAccess->getParserOutput(
					$wikiPage,
					$parserOptions,
					$this->mNewRevisionRecord,
					// we already checked
					ParserOutputAccess::OPT_NO_AUDIENCE_CHECK |
					// Update cascading protection
					ParserOutputAccess::OPT_LINKS_UPDATE
				);
				if ( $status->isOK() ) {
					$parserOutput = $status->getValue();
					// Allow extensions to change parser output here
					if ( $this->hookRunner->onDifferenceEngineRenderRevisionAddParserOutput(
						$this, $out, $parserOutput, $wikiPage )
					) {
						$out->addParserOutput( $parserOutput, $parserOptions, [
							'enableSectionEditLinks' => $this->mNewRevisionRecord->isCurrent()
								&& $this->getAuthority()->probablyCan(
									'edit',
									$this->mNewRevisionRecord->getPage()
								),
							'absoluteURLs' => $this->slotDiffOptions['expand-url'] ?? false
						] );
					}
				} else {
					$out->addModuleStyles( 'mediawiki.codex.messagebox.styles' );
					foreach ( $status->getMessages() as $msg ) {
						$out->addHTML( Html::errorBox(
							$this->msg( $msg )->parse()
						) );
					}
				}
			}
		}
	}

	/**
	 * Get the diff text, send it to the OutputPage object
	 * Returns false if the diff could not be generated, otherwise returns true
	 *
	 * @param string|false $otitle Header for old text or false
	 * @param string|false $ntitle Header for new text or false
	 * @param string $notice HTML between diff header and body
	 *
	 * @return bool
	 */
	public function showDiff( $otitle, $ntitle, $notice = '' ) {
		// Allow extensions to affect the output here
		$this->hookRunner->onDifferenceEngineShowDiff( $this );

		$diff = $this->getDiff( $otitle, $ntitle, $notice );
		if ( $diff === false ) {
			$this->showMissingRevision();
			return false;
		}

		$this->showDiffStyle();
		if ( $this->slotDiffOptions['expand-url'] ?? false ) {
			$diff = Linker::expandLocalLinks( $diff );
		}
		$this->getOutput()->addHTML( $diff );
		return true;
	}

	/**
	 * Add style sheets for diff display.
	 */
	public function showDiffStyle() {
		if ( !$this->isSlotDiffRenderer ) {
			$this->getOutput()->addModules( 'mediawiki.diff' );
			$this->getOutput()->addModuleStyles( [
				'mediawiki.interface.helpers.styles',
				'mediawiki.diff.styles'
			] );
			foreach ( $this->getSlotDiffRenderers() as $slotDiffRenderer ) {
				$slotDiffRenderer->addModules( $this->getOutput() );
			}
		}
	}

	/**
	 * Get complete diff table, including header
	 *
	 * @param string|false $otitle Header for old text or false
	 * @param string|false $ntitle Header for new text or false
	 * @param string $notice HTML between diff header and body
	 *
	 * @return string|false
	 */
	public function getDiff( $otitle, $ntitle, $notice = '' ) {
		$body = $this->getDiffBody();
		if ( $body === false ) {
			return false;
		}

		$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";
		}

		if ( $this->cacheHitKey !== null ) {
			$body .= "\n<!-- diff cache key " . htmlspecialchars( $this->cacheHitKey ) . " -->\n";
		}

		return $this->addHeader( $body, $otitle, $ntitle, $multi, $notice );
	}

	private function incrementStats( string $cacheStatus ): void {
		$stats = MediaWikiServices::getInstance()->getStatsFactory();
		$stats->getCounter( 'diff_cache_total' )
			->setLabel( 'status', $cacheStatus )
			->copyToStatsdAt( 'diff_cache.' . $cacheStatus )
			->increment();
	}

	/**
	 * Get the diff table body, without header
	 *
	 * @return string|false
	 */
	public function getDiffBody() {
		$this->mCacheHit = true;
		// Check if the diff should be hidden from this user
		if ( !$this->isContentOverridden ) {
			if ( !$this->loadRevisionData() ) {
				return false;
			} elseif ( $this->mOldRevisionRecord &&
				!$this->mOldRevisionRecord->userCan(
					RevisionRecord::DELETED_TEXT,
					$this->getAuthority()
				)
			) {
				return false;
			} elseif ( $this->mNewRevisionRecord &&
				!$this->mNewRevisionRecord->userCan(
					RevisionRecord::DELETED_TEXT,
					$this->getAuthority()
				) ) {
				return false;
			}
			// Short-circuit
			if ( $this->mOldRevisionRecord === false || (
				$this->mOldRevisionRecord &&
				$this->mNewRevisionRecord &&
				$this->mOldRevisionRecord->getId() &&
				$this->mOldRevisionRecord->getId() == $this->mNewRevisionRecord->getId()
			) ) {
				if ( $this->hookRunner->onDifferenceEngineShowEmptyOldContent( $this ) ) {
					return '';
				}
			}
		}

		// Cacheable?
		$key = false;
		$services = MediaWikiServices::getInstance();
		$cache = $services->getMainWANObjectCache();
		$stats = $services->getStatsdDataFactory();
		if ( $this->mOldid && $this->mNewid ) {
			$key = $cache->makeKey( ...$this->getDiffBodyCacheKeyParams() );

			// Try cache
			if ( !$this->mRefreshCache ) {
				$difftext = $cache->get( $key );
				if ( is_string( $difftext ) ) {
					$this->incrementStats( 'hit' );
					$difftext = $this->localiseDiff( $difftext );
					$this->cacheHitKey = $key;
					return $difftext;
				}
			} // don't try to load but save the result
		}
		$this->mCacheHit = false;
		$this->cacheHitKey = null;

		// Loadtext is permission safe, this just clears out the diff
		if ( !$this->loadText() ) {
			return false;
		}

		$difftext = '';
		// We've checked for revdelete at the beginning of this method; it's OK to ignore
		// read permissions here.
		$slotContents = $this->getSlotContents();
		foreach ( $this->getSlotDiffRenderers() as $role => $slotDiffRenderer ) {
			try {
				$slotDiff = $slotDiffRenderer->getDiff( $slotContents[$role]['old'],
					$slotContents[$role]['new'] );
			} catch ( IncompatibleDiffTypesException $e ) {
				$slotDiff = $this->getSlotError( $e->getMessageObject()->parse() );
			}
			if ( $slotDiff && $role !== SlotRecord::MAIN ) {
				// FIXME: ask SlotRoleHandler::getSlotNameMessage
				$slotTitle = $role;
				$difftext .= $this->getSlotHeader( $slotTitle );
			}
			$difftext .= $slotDiff;
		}

		// Save to cache for 7 days
		if ( !$this->hookRunner->onAbortDiffCache( $this ) ) {
			$this->incrementStats( 'uncacheable' );
		} elseif ( $key !== false ) {
			$this->incrementStats( 'miss' );
			$cache->set( $key, $difftext, 7 * 86400 );
		} else {
			$this->incrementStats( 'uncacheable' );
		}
		// localise line numbers and title attribute text
		$difftext = $this->localiseDiff( $difftext );

		return $difftext;
	}

	/**
	 * Get the diff table body for one slot, without header
	 *
	 * @param string $role
	 * @return string|false
	 */
	public function getDiffBodyForRole( $role ) {
		$diffRenderers = $this->getSlotDiffRenderers();
		if ( !isset( $diffRenderers[$role] ) ) {
			return false;
		}

		$slotContents = $this->getSlotContents();
		try {
			$slotDiff = $diffRenderers[$role]->getDiff( $slotContents[$role]['old'],
				$slotContents[$role]['new'] );
		} catch ( IncompatibleDiffTypesException $e ) {
			$slotDiff = $this->getSlotError( $e->getMessageObject()->parse() );
		}
		if ( $slotDiff === '' ) {
			return false;
		}

		if ( $role !== SlotRecord::MAIN ) {
			// TODO use human-readable role name at least
			$slotTitle = $role;
			$slotDiff = $this->getSlotHeader( $slotTitle ) . $slotDiff;
		}

		return $this->localiseDiff( $slotDiff );
	}

	/**
	 * Get a slot header for inclusion in a diff body (as a table row).
	 *
	 * @param string $headerText The text of the header
	 * @return string
	 *
	 */
	protected function getSlotHeader( $headerText ) {
		// The old revision is missing on oldid=<first>&diff=prev; only 2 columns in that case.
		$columnCount = $this->mOldRevisionRecord ? 4 : 2;
		$userLang = $this->getLanguage()->getHtmlCode();
		return Html::rawElement( 'tr', [ 'class' => 'mw-diff-slot-header', 'lang' => $userLang ],
			Html::element( 'th', [ 'colspan' => $columnCount ], $headerText ) );
	}

	/**
	 * Get an error message for inclusion in a diff body (as a table row).
	 *
	 * @param string $errorText The HTML of the error
	 * @return string
	 */
	protected function getSlotError( $errorText ) {
		// The old revision is missing on oldid=<first>&diff=prev; only 2 columns in that case.
		$columnCount = $this->mOldRevisionRecord ? 4 : 2;
		$userLang = $this->getLanguage()->getHtmlCode();
		return Html::rawElement( 'tr', [ 'class' => 'mw-diff-slot-error', 'lang' => $userLang ],
			Html::rawElement( 'td', [ 'colspan' => $columnCount ], $errorText ) );
	}

	/**
	 * Get the cache key parameters
	 *
	 * Subclasses can replace the first element in the array to something
	 * more specific to the type of diff (e.g. "inline-diff"), or append
	 * if the cache should vary on more things. Overriding entirely should
	 * be avoided.
	 *
	 * @since 1.31
	 *
	 * @return string[]
	 * @phan-return non-empty-array<string>
	 */
	protected function getDiffBodyCacheKeyParams() {
		if ( !$this->mOldid || !$this->mNewid ) {
			throw new BadMethodCallException( 'mOldid and mNewid must be set to get diff cache key.' );
		}

		$params = [
			'diff',
			self::DIFF_VERSION,
			"old-{$this->mOldid}",
			"rev-{$this->mNewid}"
		];

		$extraKeys = [];
		if ( !$this->isSlotDiffRenderer ) {
			foreach ( $this->getSlotDiffRenderers() as $slotDiffRenderer ) {
				$extraKeys = array_merge( $extraKeys, $slotDiffRenderer->getExtraCacheKeys() );
			}
		}
		ksort( $extraKeys );
		return array_merge( $params, array_values( $extraKeys ) );
	}

	/**
	 * Implements DifferenceEngineSlotDiffRenderer::getExtraCacheKeys(). Only used when
	 * DifferenceEngine is wrapped in DifferenceEngineSlotDiffRenderer.
	 * @return string[]
	 * @internal for use by DifferenceEngineSlotDiffRenderer only
	 * @deprecated
	 */
	public function getExtraCacheKeys() {
		// This method is called when the DifferenceEngine is used for a slot diff. We only care
		// about special things, not the revision IDs, which are added to the cache key by the
		// page-level DifferenceEngine, and which might not have a valid value for this object.
		$this->mOldid = 123456789;
		$this->mNewid = 987654321;

		// This will repeat a bunch of unnecessary key fields for each slot. Not nice but harmless.
		$params = $this->getDiffBodyCacheKeyParams();

		// Try to get rid of the standard keys to keep the cache key human-readable:
		// call the getDiffBodyCacheKeyParams implementation of the base class, and if
		// the child class includes the same keys, drop them.
		// Uses an obscure PHP feature where static calls to non-static methods are allowed
		// as long as we are already in a non-static method of the same class, and the call context
		// ($this) will be inherited.
		// phpcs:ignore Squiz.Classes.SelfMemberReference.NotUsed
		$standardParams = DifferenceEngine::getDiffBodyCacheKeyParams();
		if ( array_slice( $params, 0, count( $standardParams ) ) === $standardParams ) {
			$params = array_slice( $params, count( $standardParams ) );
		}

		return $params;
	}

	/**
	 * @param array $options for the difference engine. Available options:
	 *    - diff-type: The text diff format, e.g. "table" or "inline". If the
	 *      specified format is not supported, the option will be ignored, so
	 *      the site default format (table) will be used.
	 *    - expand-url: If true, put full URLs in href attributes (for action=render)
	 *      FIXME: expand-url is not a slot diff option, it is a DifferenceEngine option.
	 *    - inline-toggle: If true, and the inline format is supported
	 *      a format selector toggle switch will be shown.
	 */
	public function setSlotDiffOptions( $options ) {
		$validatedOptions = [];
		if ( isset( $options['diff-type'] )
			&& $this->getTextDiffer()->hasFormat( $options['diff-type'] )
		) {
			$validatedOptions['diff-type'] = $options['diff-type'];
		}
		if ( !empty( $options['expand-url'] ) ) {
			$validatedOptions['expand-url'] = true;
		}
		if ( !empty( $options['inline-toggle'] ) ) {
			$validatedOptions['inline-toggle'] = true;
		}
		$this->slotDiffOptions = $validatedOptions;
	}

	/**
	 * Set query parameters to append to diff page links
	 *
	 * @since 1.41
	 *
	 * @param array $params
	 */
	public function setExtraQueryParams( $params ) {
		$this->extraQueryParams = $params;
	}

	/**
	 * Generate a diff, no caching.
	 *
	 * @since 1.21
	 *
	 * @param Content $old Old content
	 * @param Content $new New content
	 *
	 * @throws Exception If old or new content is not an instance of TextContent.
	 * @return bool|string
	 *
	 * @deprecated since 1.32, use a SlotDiffRenderer instead.
	 */
	public function generateContentDiffBody( Content $old, Content $new ) {
		$slotDiffRenderer = $new->getContentHandler()->getSlotDiffRenderer( $this->getContext() );
		if (
			$slotDiffRenderer instanceof DifferenceEngineSlotDiffRenderer
			&& $this->isSlotDiffRenderer
		) {
			// Oops, we are just about to enter an infinite loop (the slot-level DifferenceEngine
			// called a DifferenceEngineSlotDiffRenderer that wraps the same DifferenceEngine class).
			// This will happen when a content model has no custom slot diff renderer, it does have
			// a custom difference engine, but that does not override this method.
			throw new LogicException( get_class( $this ) . ': could not maintain backwards compatibility. '
				. 'Please use a SlotDiffRenderer.' );
		}
		return $slotDiffRenderer->getDiff( $old, $new ) . $this->getDebugString();
	}

	/**
	 * Generate a diff, no caching
	 *
	 * @param string $otext Old text, must be already segmented
	 * @param string $ntext New text, must be already segmented
	 *
	 * @throws LogicException If content handling for text content is configured in a way
	 *   that makes maintaining B/C hard.
	 * @return bool|string
	 *
	 * @deprecated since 1.32, use a TextSlotDiffRenderer instead.
	 */
	public function generateTextDiffBody( $otext, $ntext ) {
		$slotDiffRenderer = $this->contentHandlerFactory
			->getContentHandler( CONTENT_MODEL_TEXT )
			->getSlotDiffRenderer( $this->getContext() );
		if ( !( $slotDiffRenderer instanceof TextSlotDiffRenderer ) ) {
			// Someone used the GetSlotDiffRenderer hook to replace the renderer.
			// This is too unlikely to happen to bother handling properly.
			throw new LogicException( 'The slot diff renderer for text content should be a '
				. 'TextSlotDiffRenderer subclass' );
		}
		return $slotDiffRenderer->getTextDiff( $otext, $ntext ) . $this->getDebugString();
	}

	/**
	 * Process DiffEngine config and get a sensible, usable engine
	 *
	 * @return string 'wikidiff2', 'php', or path to an executable
	 * @internal For use by this class and within Core only.
	 */
	public static function getEngine() {
		$differenceEngine = new self;
		$engine = $differenceEngine->getTextDiffer()->getEngineForFormat( 'table' );
		if ( $engine === 'external' ) {
			return MediaWikiServices::getInstance()->getMainConfig()
				->get( MainConfigNames::ExternalDiffEngine );
		} else {
			return $engine;
		}
	}

	/**
	 * Generate a debug comment indicating diff generating time,
	 * server node, and generator backend.
	 *
	 * @param string $generator What diff engine was used
	 *
	 * @return string
	 */
	protected function debug( $generator = "internal" ) {
		if ( !$this->enableDebugComment ) {
			return '';
		}
		$data = [ $generator ];
		if ( $this->getConfig()->get( MainConfigNames::ShowHostnames ) ) {
			$data[] = wfHostname();
		}
		$data[] = wfTimestamp( TS_DB );

		return "<!-- diff generator: " .
			implode( " ", array_map( "htmlspecialchars", $data ) ) .
			" -->\n";
	}

	/**
	 * @return string
	 */
	private function getDebugString() {
		$engine = self::getEngine();
		if ( $engine === 'wikidiff2' ) {
			return $this->debug( 'wikidiff2' );
		} elseif ( $engine === 'php' ) {
			return $this->debug( 'native PHP' );
		} else {
			return $this->debug( "external $engine" );
		}
	}

	/**
	 * Localise diff output
	 *
	 * @param string $text
	 * @return string
	 */
	private function localiseDiff( $text ) {
		return $this->getTextDiffer()->localize( $this->getTextDiffFormat(), $text );
	}

	/**
	 * Replace a common convention for language-independent line numbers with
	 * the text in the user's language.
	 *
	 * @deprecated since 1.41, use BaseTextDiffer::localizeLineNumbers()
	 * @param string $text
	 * @return string
	 */
	public function localiseLineNumbers( $text ) {
		return preg_replace_callback( '/<!--LINE (\d+)-->/',
			function ( array $matches ) {
				if ( $matches[1] === '1' && $this->mReducedLineNumbers ) {
					return '';
				}
				return $this->msg( 'lineno' )->numParams( $matches[1] )->escaped();
			}, $text );
	}

	/**
	 * If there are revisions between the ones being compared, return a note saying so.
	 *
	 * @return string
	 */
	public function getMultiNotice() {
		// The notice only make sense if we are diffing two saved revisions of the same page.
		if (
			!$this->mOldRevisionRecord || !$this->mNewRevisionRecord
			|| !$this->mOldPage || !$this->mNewPage
			|| !$this->mOldPage->equals( $this->mNewPage )
			|| $this->mOldRevisionRecord->getId() === null
			|| $this->mNewRevisionRecord->getId() === null
			// (T237709) Deleted revs might have different page IDs
			|| $this->mNewPage->getArticleID() !== $this->mOldRevisionRecord->getPageId()
			|| $this->mNewPage->getArticleID() !== $this->mNewRevisionRecord->getPageId()
		) {
			return '';
		}

		if ( $this->mOldRevisionRecord->getTimestamp() > $this->mNewRevisionRecord->getTimestamp() ) {
			$oldRevRecord = $this->mNewRevisionRecord; // flip
			$newRevRecord = $this->mOldRevisionRecord; // flip
		} else { // normal case
			$oldRevRecord = $this->mOldRevisionRecord;
			$newRevRecord = $this->mNewRevisionRecord;
		}

		// Don't show the notice if too many rows must be scanned
		// @todo show some special message for that case
		$nEdits = 0;
		$revisionIdList = $this->revisionStore->getRevisionIdsBetween(
			$this->mNewPage->getArticleID(),
			$oldRevRecord,
			$newRevRecord,
			1000
		);
		// only count revisions that are visible
		if ( count( $revisionIdList ) > 0 ) {
			foreach ( $revisionIdList as $revisionId ) {
				$revision = $this->revisionStore->getRevisionById( $revisionId );
				if ( $revision->getUser( RevisionRecord::FOR_THIS_USER, $this->getAuthority() ) ) {
					$nEdits++;
				}
			}
		}
		if ( $nEdits > 0 && $nEdits <= 1000 ) {
			// Use an invalid username to get the wiki's default gender (as fallback)
			$newRevUserForGender = '[HIDDEN]';
			$limit = 100; // use diff-multi-manyusers if too many users
			try {
				$users = $this->revisionStore->getAuthorsBetween(
					$this->mNewPage->getArticleID(),
					$oldRevRecord,
					$newRevRecord,
					null,
					$limit
				);
				$numUsers = count( $users );

				$newRevUser = $newRevRecord->getUser( RevisionRecord::RAW );
				$newRevUserText = $newRevUser ? $newRevUser->getName() : '';
				$newRevUserSafe = $newRevRecord->getUser(
					RevisionRecord::FOR_THIS_USER,
					$this->getAuthority()
				);
				$newRevUserForGender = $newRevUserSafe ? $newRevUserSafe->getName() : '[HIDDEN]';
				if ( $numUsers == 1 && $users[0]->getName() == $newRevUserText ) {
					$numUsers = 0; // special case to say "by the same user" instead of "by one other user"
				}
			} catch ( InvalidArgumentException $e ) {
				$numUsers = 0;
			}

			return self::intermediateEditsMsg( $nEdits, $numUsers, $limit, $newRevUserForGender );
		}

		return '';
	}

	/**
	 * Get a notice about how many intermediate edits and users there are
	 *
	 * @param int $numEdits
	 * @param int $numUsers
	 * @param int $limit
	 * @param string $lastUser
	 *
	 * @return string
	 */
	public static function intermediateEditsMsg( $numEdits, $numUsers, $limit, $lastUser = '[HIDDEN]' ) {
		if ( $numUsers === 0 ) {
			$msg = 'diff-multi-sameuser';
			return wfMessage( $msg )
				->numParams( $numEdits, $numUsers )
				->params( $lastUser )
				->parse();
		} elseif ( $numUsers > $limit ) {
			$msg = 'diff-multi-manyusers';
			$numUsers = $limit;
		} else {
			$msg = 'diff-multi-otherusers';
		}

		return wfMessage( $msg )->numParams( $numEdits, $numUsers )->parse();
	}

	/**
	 * @param RevisionRecord $revRecord
	 * @return bool whether the user can see and edit the revision.
	 */
	private function userCanEdit( RevisionRecord $revRecord ) {
		if ( !$revRecord->userCan( RevisionRecord::DELETED_TEXT, $this->getAuthority() ) ) {
			return false;
		}

		return true;
	}

	/**
	 * Get a header for a specified revision.
	 *
	 * @param RevisionRecord $rev
	 * @param string $complete 'complete' to get the header wrapped depending
	 *        the visibility of the revision and a link to edit the page.
	 *
	 * @return string HTML fragment
	 */
	public function getRevisionHeader( RevisionRecord $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
		);

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

		$title = $rev->getPageAsLinkTarget();

		if ( $this->userCanEdit( $rev ) ) {
			$header = $this->linkRenderer->makeKnownLink(
				$title,
				$header->text(),
				[],
				[ 'oldid' => $rev->getId() ]
			);
			$editQuery = [ 'action' => 'edit' ];
			if ( !$rev->isCurrent() ) {
				$editQuery['oldid'] = $rev->getId();
			}

			$key = $this->getAuthority()->probablyCan( 'edit', $rev->getPage() ) ? 'editold' : 'viewsourceold';
			$msg = $this->msg( $key )->text();
			$editLink = $this->linkRenderer->makeKnownLink( $title, $msg, [], $editQuery );
			$header .= ' ' . Html::rawElement(
				'span',
				[ 'class' => 'mw-diff-edit' ],
				$editLink
			);
		} else {
			$header = $header->escaped();
		}

		// Machine readable information
		$header .= Html::element( 'span',
			[
				'class' => 'mw-diff-timestamp',
				'data-timestamp' => wfTimestamp( TS_ISO_8601, $revtimestamp ),
			], ''
		);

		if ( $rev->isDeleted( RevisionRecord::DELETED_TEXT ) ) {
			return Html::rawElement(
				'span',
				[ 'class' => Linker::getRevisionDeletedClass( $rev ) ],
				$header
			);
		}

		return $header;
	}

	/**
	 * Add the header to a diff body
	 *
	 * @param string $diff Diff body
	 * @param string $otitle Old revision header
	 * @param string $ntitle New revision header
	 * @param string $multi Notice telling user that there are intermediate
	 *   revisions between the ones being compared
	 * @param string $notice Other notices, e.g. that user is viewing deleted content
	 *
	 * @return string
	 */
	public 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
		$header = Html::openElement( 'table', [
			'class' => [
				'diff',
				// The following classes are used here:
				// * diff-type-table
				// * diff-type-inline
				'diff-type-' . $this->getTextDiffFormat(),
				// The following classes are used here:
				// * diff-contentalign-left
				// * diff-contentalign-right
				'diff-contentalign-' . $this->getDiffLang()->alignStart(),
				// The following classes are used here:
				// * diff-editfont-monospace
				// * diff-editfont-sans-serif
				// * diff-editfont-serif
				'diff-editfont-' . $this->userOptionsLookup->getOption(
					$this->getUser(),
					'editfont'
				)
			],
			'data-mw' => 'interface',
		] );
		$userLang = htmlspecialchars( $this->getLanguage()->getHtmlCode() );

		if ( !$diff && !$otitle ) {
			$header .= "
			<tr class=\"diff-title\" lang=\"{$userLang}\">
			<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 ) {
				// FIXME Hardcoding values from TableDiffFormatter.
				$deletedClass = 'diff-side-deleted';
				$addedClass = 'diff-side-added';
				$header .= "
				<tr class=\"diff-title\" lang=\"{$userLang}\">
				<td colspan=\"$colspan\" class=\"diff-otitle {$deletedClass}\">{$otitle}</td>
				<td colspan=\"$colspan\" class=\"diff-ntitle {$addedClass}\">{$ntitle}</td>
				</tr>";
			}
		}

		if ( $multi != '' ) {
			$header .= "<tr><td colspan=\"{$multiColspan}\" " .
				"class=\"diff-multi\" lang=\"{$userLang}\">{$multi}</td></tr>";
		}
		if ( $notice != '' ) {
			$header .= "<tr><td colspan=\"{$multiColspan}\" " .
				"class=\"diff-notice\" lang=\"{$userLang}\">{$notice}</td></tr>";
		}

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

	/**
	 * Use specified text instead of loading from the database
	 * @param Content $oldContent
	 * @param Content $newContent
	 * @since 1.21
	 * @deprecated since 1.32, use setRevisions or ContentHandler::getSlotDiffRenderer.
	 */
	public function setContent( Content $oldContent, Content $newContent ) {
		$this->mOldContent = $oldContent;
		$this->mNewContent = $newContent;

		$this->mTextLoaded = 2;
		$this->mRevisionsLoaded = true;
		$this->isContentOverridden = true;
		$this->slotDiffRenderers = null;
	}

	/**
	 * Use specified text instead of loading from the database.
	 * @param RevisionRecord|null $oldRevision
	 * @param RevisionRecord $newRevision
	 */
	public function setRevisions(
		?RevisionRecord $oldRevision, RevisionRecord $newRevision
	) {
		if ( $oldRevision ) {
			$this->mOldRevisionRecord = $oldRevision;
			$this->mOldid = $oldRevision->getId();
			$this->mOldPage = Title::newFromPageIdentity( $oldRevision->getPage() );
			// This method is meant for edit diffs and such so there is no reason to provide a
			// revision that's not readable to the user, but check it just in case.
			$this->mOldContent = $oldRevision->getContent( SlotRecord::MAIN,
				RevisionRecord::FOR_THIS_USER, $this->getAuthority() );
			if ( !$this->mOldContent ) {
				$this->addRevisionLoadError( 'old' );
			}
		} else {
			$this->mOldPage = null;
			$this->mOldRevisionRecord = $this->mOldid = false;
		}
		$this->mNewRevisionRecord = $newRevision;
		$this->mNewid = $newRevision->getId();
		$this->mNewPage = Title::newFromPageIdentity( $newRevision->getPage() );
		$this->mNewContent = $newRevision->getContent( SlotRecord::MAIN,
			RevisionRecord::FOR_THIS_USER, $this->getAuthority() );
		if ( !$this->mNewContent ) {
			$this->addRevisionLoadError( 'new' );
		}

		$this->mRevisionsIdsLoaded = $this->mRevisionsLoaded = true;
		$this->mTextLoaded = $oldRevision ? 2 : 1;
		$this->isContentOverridden = false;
		$this->slotDiffRenderers = null;
	}

	/**
	 * Set the language in which the diff text is written
	 *
	 * @param Language $lang
	 * @since 1.19
	 */
	public function setTextLanguage( Language $lang ) {
		$this->mDiffLang = $lang;
	}

	/**
	 * Maps a revision pair definition as accepted by DifferenceEngine constructor
	 * to a pair of actual integers representing revision ids.
	 *
	 * @param int $old Revision id, e.g. from URL parameter 'oldid'
	 * @param int|string $new Revision id or strings 'next' or 'prev', e.g. from URL parameter 'diff'
	 *
	 * @return array List of two revision ids, older first, later second.
	 *     Zero signifies invalid argument passed.
	 *     false signifies that there is no previous/next revision ($old is the oldest/newest one).
	 * @phan-return (int|false)[]
	 */
	public function mapDiffPrevNext( $old, $new ) {
		if ( $new === 'prev' ) {
			// Show diff between revision $old and the previous one. Get previous one from DB.
			$newid = intval( $old );
			$oldid = false;
			$newRev = $this->revisionStore->getRevisionById( $newid );
			if ( $newRev ) {
				$oldRev = $this->revisionStore->getPreviousRevision( $newRev );
				if ( $oldRev ) {
					$oldid = $oldRev->getId();
				}
			}
		} elseif ( $new === 'next' ) {
			// Show diff between revision $old and the next one. Get next one from DB.
			$oldid = intval( $old );
			$newid = false;
			$oldRev = $this->revisionStore->getRevisionById( $oldid );
			if ( $oldRev ) {
				$newRev = $this->revisionStore->getNextRevision( $oldRev );
				if ( $newRev ) {
					$newid = $newRev->getId();
				}
			}
		} else {
			$oldid = intval( $old );
			$newid = intval( $new );
		}

		// @phan-suppress-next-line PhanTypeMismatchReturn getId does not return null here
		return [ $oldid, $newid ];
	}

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

		$this->mRevisionsIdsLoaded = true;

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

		[ $this->mOldid, $this->mNewid ] = self::mapDiffPrevNext( $old, $new );
		if ( $new === 'next' && $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;
		}

		$this->hookRunner->onNewDifferenceEngine(
			// @phan-suppress-next-line PhanTypeMismatchArgumentNullable False positive
			$this->getTitle(), $this->mOldid, $this->mNewid, $old, $new );
	}

	/**
	 * Load revision metadata for the specified revisions. If newid is 0, then compare
	 * the old revision in oldid to the current revision of the current page (as defined
	 * by the request context); if oldid is 0, then compare the revision in newid to the
	 * immediately previous one.
	 *
	 * If oldid is false, leave the corresponding RevisionRecord object set to false. This can
	 * happen with 'diff=prev' pointing to a non-existent revision, and is also used directly
	 * by the API.
	 *
	 * @return bool Whether both revisions were loaded successfully. Setting mOldRevisionRecord
	 *   to false counts as successful loading.
	 */
	public function loadRevisionData() {
		if ( $this->mRevisionsLoaded ) {
			return $this->isContentOverridden ||
			( $this->mOldRevisionRecord !== null && $this->mNewRevisionRecord !== null );
		}

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

		$this->loadRevisionIds();

		// Load the new RevisionRecord object
		if ( $this->mNewid ) {
			$this->mNewRevisionRecord = $this->revisionStore->getRevisionById( $this->mNewid );
		} else {
			$this->mNewRevisionRecord = $this->revisionStore->getRevisionByTitle( $this->getTitle() );
		}

		if ( !$this->mNewRevisionRecord instanceof RevisionRecord ) {
			return false;
		}

		// Update the new revision ID in case it was 0 (makes life easier doing UI stuff)
		$this->mNewid = $this->mNewRevisionRecord->getId();
		$this->mNewPage = $this->mNewid ?
			Title::newFromPageIdentity( $this->mNewRevisionRecord->getPage() ) :
			null;

		// Load the old RevisionRecord object
		$this->mOldRevisionRecord = false;
		if ( $this->mOldid ) {
			$this->mOldRevisionRecord = $this->revisionStore->getRevisionById( $this->mOldid );
		} elseif ( $this->mOldid === 0 ) {
			$revRecord = $this->revisionStore->getPreviousRevision( $this->mNewRevisionRecord );
			// No previous revision; mark to show as first-version only.
			$this->mOldid = $revRecord ? $revRecord->getId() : false;
			$this->mOldRevisionRecord = $revRecord ?? false;
		} /* elseif ( $this->mOldid === false ) leave mOldRevisionRecord false; */

		if ( $this->mOldRevisionRecord === null ) {
			return false;
		}

		if ( $this->mOldRevisionRecord && $this->mOldRevisionRecord->getId() ) {
			$this->mOldPage = Title::newFromPageIdentity( $this->mOldRevisionRecord->getPage() );
		} else {
			$this->mOldPage = null;
		}

		// Load tags information for both revisions
		$dbr = $this->dbProvider->getReplicaDatabase();
		$changeTagDefStore = MediaWikiServices::getInstance()->getChangeTagDefStore();
		if ( $this->mOldid !== false ) {
			$tagIds = $dbr->newSelectQueryBuilder()
				->select( 'ct_tag_id' )
				->from( 'change_tag' )
				->where( [ 'ct_rev_id' => $this->mOldid ] )
				->caller( __METHOD__ )->fetchFieldValues();
			$tags = [];
			foreach ( $tagIds as $tagId ) {
				try {
					$tags[] = $changeTagDefStore->getName( (int)$tagId );
				} catch ( NameTableAccessException $exception ) {
					continue;
				}
			}
			$this->mOldTags = implode( ',', $tags );
		} else {
			$this->mOldTags = false;
		}

		$tagIds = $dbr->newSelectQueryBuilder()
			->select( 'ct_tag_id' )
			->from( 'change_tag' )
			->where( [ 'ct_rev_id' => $this->mNewid ] )
			->caller( __METHOD__ )->fetchFieldValues();
		$tags = [];
		foreach ( $tagIds as $tagId ) {
			try {
				$tags[] = $changeTagDefStore->getName( (int)$tagId );
			} catch ( NameTableAccessException $exception ) {
				continue;
			}
		}
		$this->mNewTags = implode( ',', $tags );

		return true;
	}

	/**
	 * Load the text of the revisions, as well as revision data.
	 * When the old revision is missing (mOldRev is false), loading mOldContent is not attempted.
	 *
	 * @return bool Whether the content of both revisions could be loaded successfully.
	 *   (When mOldRev is false, that still counts as a success.)
	 *
	 */
	public function loadText() {
		if ( $this->mTextLoaded == 2 ) {
			return $this->loadRevisionData() &&
				( $this->mOldRevisionRecord === false || $this->mOldContent )
				&& $this->mNewContent;
		}

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

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

		if ( $this->mOldRevisionRecord ) {
			$this->mOldContent = $this->mOldRevisionRecord->getContent(
				SlotRecord::MAIN,
				RevisionRecord::FOR_THIS_USER,
				$this->getAuthority()
			);
			if ( $this->mOldContent === null ) {
				return false;
			}
		}

		$this->mNewContent = $this->mNewRevisionRecord->getContent(
			SlotRecord::MAIN,
			RevisionRecord::FOR_THIS_USER,
			$this->getAuthority()
		);
		$this->hookRunner->onDifferenceEngineLoadTextAfterNewContentIsLoaded( $this );
		if ( $this->mNewContent === null ) {
			return false;
		}

		return true;
	}

	/**
	 * Load the text of the new revision, not the old one
	 *
	 * @return bool Whether the content of the new revision could be loaded successfully.
	 */
	public function loadNewText() {
		if ( $this->mTextLoaded >= 1 ) {
			return $this->loadRevisionData();
		}

		$this->mTextLoaded = 1;

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

		$this->mNewContent = $this->mNewRevisionRecord->getContent(
			SlotRecord::MAIN,
			RevisionRecord::FOR_THIS_USER,
			$this->getAuthority()
		);

		$this->hookRunner->onDifferenceEngineAfterLoadNewText( $this );

		return true;
	}

	/**
	 * Get the TextDiffer which will be used for rendering text
	 *
	 * @return ManifoldTextDiffer
	 */
	protected function getTextDiffer() {
		if ( $this->textDiffer === null ) {
			$this->textDiffer = new ManifoldTextDiffer(
				$this->getContext(),
				$this->getDiffLang(),
				$this->getConfig()->get( MainConfigNames::DiffEngine ),
				$this->getConfig()->get( MainConfigNames::ExternalDiffEngine ),
				$this->getConfig()->get( MainConfigNames::Wikidiff2Options )
			);
		}
		return $this->textDiffer;
	}

	/**
	 * Get the list of supported text diff formats
	 *
	 * @since 1.41
	 * @return array|string[]
	 */
	public function getSupportedFormats() {
		return $this->getTextDiffer()->getFormats();
	}

	/**
	 * Get the selected text diff format
	 *
	 * @since 1.41
	 * @return string
	 */
	public function getTextDiffFormat() {
		return $this->slotDiffOptions['diff-type'] ?? 'table';
	}

}
