wiki.techinc.nl/includes/ContentHandler.php

<?php

/**
 * Exception representing a failure to serialize or unserialize a content object.
 */
class MWContentSerializationException extends MWException {

}

/**
 * A content handler knows how do deal with a specific type of content on a wiki page.
 * Content is stored in the database in a serialized form (using a serialization format aka mime type)
 * and is be unserialized into it's native PHP representation (the content model), which is wrapped in
 * an instance of the appropriate subclass of Content.
 *
 * ContentHandler instances are stateless singletons that serve, among other things, as a factory for
 * Content objects. Generally, there is one subclass of ContentHandler and one subclass of Content
 * for every type of content model.
 *
 * Some content types have a flat model, that is, their native representation is the
 * same as their serialized form. Examples would be JavaScript and CSS code. As of now,
 * this also applies to wikitext (mediawiki's default content type), but wikitext
 * content may be represented by a DOM or AST structure in the future.
 *
 * @since 1.WD
 */
abstract class ContentHandler {

	/**
	 * Convenience function for getting flat text from a Content object. This should only
	 * be used in the context of backwards compatibility with code that is not yet able
	 * to handle Content objects!
	 *
	 * If $content is null, this method returns the empty string.
	 *
	 * If $content is an instance of TextContent, this method returns the flat text as returned by $content->getNativeData().
	 *
	 * If $content is not a TextContent object, the behavior of this method depends on the global $wgContentHandlerTextFallback:
	 * * If $wgContentHandlerTextFallback is 'fail' and $content is not a TextContent object, an MWException is thrown.
	 * * If $wgContentHandlerTextFallback is 'serialize' and $content is not a TextContent object, $content->serialize()
	 * is called to get a string form of the content.
	 * * If $wgContentHandlerTextFallback is 'ignore' and $content is not a TextContent object, this method returns null.
	 * * otherwise, the behaviour is undefined.
	 *
	 * @since WD.1
	 *
	 * @static
	 * @param Content|null $content
	 * @return null|string the textual form of $content, if available
	 * @throws MWException if $content is not an instance of TextContent and $wgContentHandlerTextFallback was set to 'fail'.
	 */
	public static function getContentText( Content $content = null ) {
		global $wgContentHandlerTextFallback;

		if ( is_null( $content ) ) {
			return '';
		}

		if ( $content instanceof TextContent ) {
			return $content->getNativeData();
		}

		if ( $wgContentHandlerTextFallback == 'fail' ) {
			throw new MWException( "Attempt to get text from Content with model " . $content->getModel() );
		}

		if ( $wgContentHandlerTextFallback == 'serialize' ) {
			return $content->serialize();
		}

		return null;
	}

	/**
	 * Convenience function for creating a Content object from a given textual representation.
	 *
	 * $text will be deserialized into a Content object of the model specified by $modelId (or,
	 * if that is not given, $title->getContentModel()) using the given format.
	 *
	 * @since WD.1
	 *
	 * @static
	 * @param string $text the textual representation, will be unserialized to create the Content object
	 * @param Title $title the title of the page this text belongs to, required as a context for deserialization
	 * @param null|String $modelId the model to deserialize to. If not provided, $title->getContentModel() is used.
	 * @param null|String $format the format to use for deserialization. If not given, the model's default format is used.
	 *
	 * @return Content a Content object representing $text
	 * @throw MWException if $model or $format is not supported or if $text can not be unserialized using $format.
	 */
	public static function makeContent( $text, Title $title, $modelId = null, $format = null ) {

		if ( is_null( $modelId ) ) {
			$modelId = $title->getContentModel();
		}

		$handler = ContentHandler::getForModelID( $modelId );
		return $handler->unserializeContent( $text, $format );
	}

	/**
	 * Returns the name of the default content model to be used for the page with the given title.
	 *
	 * Note: There should rarely be need to call this method directly.
	 * To determine the actual content model for a given page, use Title::getContentModel().
	 *
	 * Which model is to be used per default for the page is determined based on several factors:
	 * * The global setting $wgNamespaceContentModels specifies a content model per namespace.
	 * * The hook DefaultModelFor may be used to override the page's default model.
	 * * Pages in NS_MEDIAWIKI and NS_USER default to the CSS or JavaScript model if they end in .js or .css, respectively.
	 * * Pages in NS_MEDIAWIKI default to the wikitext model otherwise.
	 * * The hook TitleIsCssOrJsPage may be used to force a page to use the CSS or JavaScript model if they end in .js or .css, respectively.
	 * * The hook TitleIsWikitextPage may be used to force a page to use the wikitext model.
	 *
	 * If none of the above applies, the wikitext model is used.
	 *
	 * Note: this is used by, and may thus not use, Title::getContentModel()
	 *
	 * @since WD.1
	 *
	 * @static
	 * @param Title $title
	 * @return null|string default model name for the page given by $title
	 */
	public static function getDefaultModelFor( Title $title ) {
		global $wgNamespaceContentModels;

		// NOTE: this method must not rely on $title->getContentModel() directly or indirectly,
		//       because it is used to initialized the mContentModel member.

		$ns = $title->getNamespace();

		$ext = false;
		$m = null;
		$model = null;

		if ( !empty( $wgNamespaceContentModels[ $ns ] ) ) {
			$model = $wgNamespaceContentModels[ $ns ];
		}

		// hook can determin default model
		if ( !wfRunHooks( 'ContentHandlerDefaultModelFor', array( $title, &$model ) ) ) {
			if ( !is_null( $model ) ) {
				return $model;
			}
		}

		// Could this page contain custom CSS or JavaScript, based on the title?
		$isCssOrJsPage = NS_MEDIAWIKI == $ns && preg_match( '!\.(css|js)$!u', $title->getText(), $m );
		if ( $isCssOrJsPage ) {
			$ext = $m[1];
		}

		// hook can force js/css
		wfRunHooks( 'TitleIsCssOrJsPage', array( $title, &$isCssOrJsPage ) );

		// Is this a .css subpage of a user page?
		$isJsCssSubpage = NS_USER == $ns && !$isCssOrJsPage && preg_match( "/\\/.*\\.(js|css)$/", $title->getText(), $m );
		if ( $isJsCssSubpage ) {
			$ext = $m[1];
		}

		// is this wikitext, according to $wgNamespaceContentModels or the DefaultModelFor hook?
		$isWikitext = is_null( $model ) || $model == CONTENT_MODEL_WIKITEXT;
		$isWikitext = $isWikitext && !$isCssOrJsPage && !$isJsCssSubpage;

		// hook can override $isWikitext
		wfRunHooks( 'TitleIsWikitextPage', array( $title, &$isWikitext ) );

		if ( !$isWikitext ) {
			switch ( $ext ) {
				case 'js':
					return CONTENT_MODEL_JAVASCRIPT;
				case 'css':
					return CONTENT_MODEL_CSS;
				default:
					return is_null( $model ) ? CONTENT_MODEL_TEXT : $model;
			}
		}

		// we established that is must be wikitext

		return CONTENT_MODEL_WIKITEXT;
	}

	/**
	 * returns the appropriate ContentHandler singleton for the given title
	 *
	 * @since WD.1
	 *
	 * @static
	 * @param Title $title
	 * @return ContentHandler
	 */
	public static function getForTitle( Title $title ) {
		$modelId = $title->getContentModel();
		return ContentHandler::getForModelID( $modelId );
	}

	/**
	 * returns the appropriate ContentHandler singleton for the given Content object
	 *
	 * @since WD.1
	 *
	 * @static
	 * @param Content $content
	 * @return ContentHandler
	 */
	public static function getForContent( Content $content ) {
		$modelId = $content->getModel();
		return ContentHandler::getForModelID( $modelId );
	}

	/**
	 * returns the ContentHandler singleton for the given model id. Use the CONTENT_MODEL_XXX constants to
	 * identify the desired content model.
	 *
	 * ContentHandler singletons are take from the global $wgContentHandlers array. Keys in that array are
	 * model names, the values are either ContentHandler singleton objects, or strings specifying the appropriate
	 * subclass of ContentHandler.
	 *
	 * If a class name in encountered when looking up the singleton for a given model name, the class is
	 * instantiated and the class name is replaced by te resulting singleton in $wgContentHandlers.
	 *
	 * If no ContentHandler is defined for the desired $modelId, the ContentHandler may be provided by the
	 * a ContentHandlerForModelID hook. if no ContentHandler can be determined, an MWException is raised.
	 *
	 * @since WD.1
	 *
	 * @static
	 * @param $modelId int the id of the content model for which to get a handler. Use CONTENT_MODEL_XXX constants.
	 * @return ContentHandler the ContentHandler singleton for handling the model given by $modelId
	 * @throws MWException if no handler is known for $modelId.
	 */
	public static function getForModelID( $modelId ) {
		global $wgContentHandlers;

		if ( empty( $wgContentHandlers[$modelId] ) ) {
			$handler = null;

			wfRunHooks( 'ContentHandlerForModelID', array( $modelId, &$handler ) );

			if ( $handler ) { // NOTE: may be a string or an object, either is fine!
				$wgContentHandlers[$modelId] = $handler;
			} else {
				throw new MWException( "No handler for model #$modelId registered in \$wgContentHandlers" );
			}
		}

		if ( is_string( $wgContentHandlers[$modelId] ) ) {
			$class = $wgContentHandlers[$modelId];
			$wgContentHandlers[$modelId] = new $class( $modelId );
		}

		return $wgContentHandlers[$modelId];
	}

	/**
	 * Returns the appropriate mime type for a given content format,
	 * or null if no mime type is known for this format.
	 *
	 * Mime types can be registered in the global array $wgContentFormatMimeTypes.
	 *
	 * @static
	 * @param int $id the content format id, as given by a CONTENT_FORMAT_XXX constant
	 *        or returned by Revision::getContentFormat().
	 *
	 * @return String|null the content format's mime type.
	 */
	public static function getContentFormatMimeType( $id ) {
		global $wgContentFormatMimeTypes;

		if ( !isset( $wgContentFormatMimeTypes[ $id ] ) ) {
			return null;
		}

		return $wgContentFormatMimeTypes[ $id ];
	}

	/**
	 * Returns the content format if for a given mime type,
	 * or null if no format id if known for this mime type.
	 *
	 * Mime types can be registered in the global array $wgContentFormatMimeTypes.
	 *
	 * @static
	 * @param String $mime the mime type
	 *
	 * @return int|null the format id, as defined by a CONTENT_FORMAT_XXX constant
	 */
	public static function getContentFormatID( $mime ) {
		global $wgContentFormatMimeTypes;

		static $format_ids = null;

		if ( $format_ids === null ) {
			$format_ids = array_flip( $wgContentFormatMimeTypes );
		}

		if ( !isset( $format_ids[ $mime ] ) ) {
			return null;
		}

		return $format_ids[ $mime ];
	}

	/**
	 * Returns the localized name for a given content model,
	 * or null of no mime type is known.
	 *
	 * Model names are localized using system messages. Message keys
	 * have the form content-model-$id.
	 *
	 * @static
	 * @param int $id the content model id, as given by a CONTENT_MODEL_XXX constant
	 *        or returned by Revision::getContentModel().
	 *
	 * @return String|null the content format's mime type.
	 */
	public static function getContentModelName( $id ) {
		$key = "content-model-$id";

		if ( wfEmptyMsg( $key ) ) return null;
		else return wfMsg( $key );
	}

	// ----------------------------------------------------------------------------------------------------------

	protected $mModelID;
	protected $mSupportedFormats;

	/**
	 * Constructor, initializing the ContentHandler instance with it's model id and a list of supported formats.
	 * Values for the parameters are typically provided as literals by subclasses' constructors.
	 *
	 * @param int $modelId (use CONTENT_MODEL_XXX constants).
	 * @param array $formats list for supported serialization formats (typically as MIME types)
	 */
	public function __construct( $modelId, $formats ) {
		$this->mModelID = $modelId;
		$this->mSupportedFormats = $formats;
	}


	/**
	 * Serializes Content object of the type supported by this ContentHandler.
	 *
	 * @since WD.1
	 *
	 * @abstract
	 * @param Content $content the Content object to serialize
	 * @param null $format the desired serialization format
	 * @return String serialized form of the content
	 */
	public abstract function serializeContent( Content $content, $format = null );

	/**
	 * Unserializes a Content object of the type supported by this ContentHandler.
	 *
	 * @since WD.1
	 *
	 * @abstract
	 * @param $blob String serialized form of the content
	 * @param null $format the format used for serialization
	 * @return Content the Content object created by deserializing $blob
	 */
	public abstract function unserializeContent( $blob, $format = null );

	/**
	 * Creates an empty Content object of the type supported by this ContentHandler.
	 *
	 * @since WD.1
	 *
	 * @return Content
	 */
	public abstract function makeEmptyContent();

	/**
	 * Returns the model id that identifies the content model this ContentHandler can handle.
	 * Use with the CONTENT_MODEL_XXX constants.
	 *
	 * @since WD.1
	 *
	 * @return int the model id
	 */
	public function getModelID() {
		return $this->mModelID;
	}

	/**
	 * Throws an MWException if $model_id is not the id of the content model
	 * supported by this ContentHandler.
	 *
	 * @since WD.1
	 *
	 * @param int $model_id the model to check
	 *
	 * @throws MWException
	 */
	protected function checkModelID( $model_id ) {
		if ( $model_id !== $this->mModelID ) {
			$model_name = ContentHandler::getContentModelName( $model_id );
			$own_model_name = ContentHandler::getContentModelName( $this->mModelID );

			throw new MWException( "Bad content model: expected {$this->mModelID} ($own_model_name) but got found $model_id ($model_name)." );
		}
	}

	/**
	 * Returns a list of serialization formats supported by the serializeContent() and unserializeContent() methods of
	 * this ContentHandler.
	 *
	 * @since WD.1
	 *
	 * @return array of serialization formats as MIME type like strings
	 */
	public function getSupportedFormats() {
		return $this->mSupportedFormats;
	}

	/**
	 * The format used for serialization/deserialization per default by this ContentHandler.
	 *
	 * This default implementation will return the first element of the array of formats
	 * that was passed to the constructor.
	 *
	 * @since WD.1
	 *
	 * @return String the name of the default serialization format as a MIME type
	 */
	public function getDefaultFormat() {
		return $this->mSupportedFormats[0];
	}

	/**
	 * Returns true if $format is a serialization format supported by this ContentHandler,
	 * and false otherwise.
	 *
	 * Note that if $format is null, this method always returns true, because null
	 * means "use the default format".
	 *
	 * @since WD.1
	 *
	 * @param String $format the serialization format to check
	 * @return bool
	 */
	public function isSupportedFormat( $format ) {

		if ( !$format ) {
			return true; // this means "use the default"
		}

		return in_array( $format, $this->mSupportedFormats );
	}

	/**
	 * Throws an MWException if isSupportedFormat( $format ) is not true. Convenient
	 * for checking whether a format provided as a parameter is actually supported.
	 *
	 * @param String $format the serialization format to check
	 *
	 * @throws MWException
	 */
	protected function checkFormat( $format ) {
		if ( !$this->isSupportedFormat( $format ) ) {
			throw new MWException( "Format $format is not supported for content model " . $this->getModelID() );
		}
	}

	/**
	 * Returns if the content is consistent with the database, that is if saving it to the database would not violate any
	 * global constraints.
	 *
	 * Content needs to be valid using this method before it can be saved.
	 *
	 * This default implementation always returns true.
	 *
	 * @since WD.1
	 *
	 * @param \Content $content
	 *
	 * @return boolean
	 */
	public function isConsistentWithDatabase( Content $content ) {
		return true;
	}

	/**
	 * Returns overrides for action handlers.
	 * Classes listed here will be used instead of the default one when
	 * (and only when) $wgActions[$action] === true. This allows subclasses
	 * to override the default action handlers.
	 *
	 * @since WD.1
	 *
	 * @return Array
	 */
	public function getActionOverrides() {
		return array();
	}

	/**
	 * Factory creating an appropriate DifferenceEngine for this content model.
	 *
	 * @since WD.1
	 *
	 * @param            $context      IContextSource context to use, anything else will be ignored
	 * @param            $old          Integer old ID we want to show and diff with.
	 * @param int|String $new          String either 'prev' or 'next'.
	 * @param            $rcid         Integer ??? FIXME (default 0)
	 * @param            $refreshCache boolean If set, refreshes the diff cache
	 * @param            $unhide       boolean If set, allow viewing deleted revs
	 *
	 * @return DifferenceEngine
	 */
	public function createDifferenceEngine( IContextSource $context, $old = 0, $new = 0, $rcid = 0, #FIMXE: use everywhere!
										 $refreshCache = false, $unhide = false ) {

		$this->checkModelID( $context->getTitle()->getContentModel() );

		$diffEngineClass = $this->getDiffEngineClass();

		return new $diffEngineClass( $context, $old, $new, $rcid, $refreshCache, $unhide );
	}

	/**
	 * Returns the name of the diff engine to use.
	 *
	 * @since WD.1
	 *
	 * @return string
	 */
	protected function getDiffEngineClass() {
		return 'DifferenceEngine';
	}

	/**
	 * attempts to merge differences between three versions.
	 * Returns a new Content object for a clean merge and false for failure or a conflict.
	 *
	 * This default implementation always returns false.
	 *
	 * @since WD.1
	 *
	 * @param Content|String $oldContent  String
	 * @param Content|String $myContent   String
	 * @param Content|String $yourContent String
	 *
	 * @return Content|Bool
	 */
	public function merge3( Content $oldContent, Content $myContent, Content $yourContent ) {
		return false;
	}

	/**
	 * Return an applicable auto-summary if one exists for the given edit.
	 *
	 * @since WD.1
	 *
	 * @param $oldContent Content|null: the previous text of the page.
	 * @param $newContent Content|null: The submitted text of the page.
	 * @param $flags Int bit mask: a bit mask of flags submitted for the edit.
	 *
	 * @return string An appropriate auto-summary, or an empty string.
	 */
	public function getAutosummary( Content $oldContent = null, Content $newContent = null, $flags ) {
		global $wgContLang;

		// Decide what kind of auto-summary is needed.

		// Redirect auto-summaries

		/**
		 * @var $ot Title
		 * @var $rt Title
		 */

		$ot = !is_null( $oldContent ) ? $oldContent->getRedirectTarget() : null;
		$rt = !is_null( $newContent ) ? $newContent->getRedirectTarget() : null;

		if ( is_object( $rt ) && ( !is_object( $ot ) || !$rt->equals( $ot ) || $ot->getFragment() != $rt->getFragment() ) ) {

			$truncatedtext = $newContent->getTextForSummary(
				250
					- strlen( wfMsgForContent( 'autoredircomment' ) )
					- strlen( $rt->getFullText() ) );

			return wfMsgForContent( 'autoredircomment', $rt->getFullText(), $truncatedtext );
		}

		// New page auto-summaries
		if ( $flags & EDIT_NEW && $newContent->getSize() > 0 ) {
			// If they're making a new article, give its text, truncated, in the summary.

			$truncatedtext = $newContent->getTextForSummary(
				200 - strlen( wfMsgForContent( 'autosumm-new' ) ) );

			return wfMsgForContent( 'autosumm-new', $truncatedtext );
		}

		// Blanking auto-summaries
		if ( !empty( $oldContent ) && $oldContent->getSize() > 0 && $newContent->getSize() == 0 ) {
			return wfMsgForContent( 'autosumm-blank' );
		} elseif ( !empty( $oldContent ) && $oldContent->getSize() > 10 * $newContent->getSize() && $newContent->getSize() < 500 ) {
			// Removing more than 90% of the article

			$truncatedtext = $newContent->getTextForSummary(
				200 - strlen( wfMsgForContent( 'autosumm-replace' ) ) );

			return wfMsgForContent( 'autosumm-replace', $truncatedtext );
		}

		// If we reach this point, there's no applicable auto-summary for our case, so our
		// auto-summary is empty.

		return '';
	}

	/**
	 * Auto-generates a deletion reason
	 *
	 * @since WD.1
	 *
	 * @param $title Title: the page's title
	 * @param &$hasHistory Boolean: whether the page has a history
	 * @return mixed String containing deletion reason or empty string, or boolean false
	 *    if no revision occurred
	 *
	 * @XXX &$hasHistory is extremely ugly, it's here because WikiPage::getAutoDeleteReason() and Article::getReason() have it / want it.
	 */
	public function getAutoDeleteReason( Title $title, &$hasHistory ) {
		$dbw = wfGetDB( DB_MASTER );

		// Get the last revision
		$rev = Revision::newFromTitle( $title );

		if ( is_null( $rev ) ) {
			return false;
		}

		// Get the article's contents
		$content = $rev->getContent();
		$blank = false;

		// If the page is blank, use the text from the previous revision,
		// which can only be blank if there's a move/import/protect dummy revision involved
		if ( $content->getSize() == 0 ) {
			$prev = $rev->getPrevious();

			if ( $prev )	{
				$content = $prev->getContent();
				$blank = true;
			}
		}

		// Find out if there was only one contributor
		// Only scan the last 20 revisions
		$res = $dbw->select( 'revision', 'rev_user_text',
			array( 'rev_page' => $title->getArticleID(), $dbw->bitAnd( 'rev_deleted', Revision::DELETED_USER ) . ' = 0' ),
			__METHOD__,
			array( 'LIMIT' => 20 )
		);

		if ( $res === false ) {
			// This page has no revisions, which is very weird
			return false;
		}

		$hasHistory = ( $res->numRows() > 1 );
		$row = $dbw->fetchObject( $res );

		if ( $row ) { // $row is false if the only contributor is hidden
			$onlyAuthor = $row->rev_user_text;
			// Try to find a second contributor
			foreach ( $res as $row ) {
				if ( $row->rev_user_text != $onlyAuthor ) { // Bug 22999
					$onlyAuthor = false;
					break;
				}
			}
		} else {
			$onlyAuthor = false;
		}

		// Generate the summary with a '$1' placeholder
		if ( $blank ) {
			// The current revision is blank and the one before is also
			// blank. It's just not our lucky day
			$reason = wfMsgForContent( 'exbeforeblank', '$1' );
		} else {
			if ( $onlyAuthor ) {
				$reason = wfMsgForContent( 'excontentauthor', '$1', $onlyAuthor );
			} else {
				$reason = wfMsgForContent( 'excontent', '$1' );
			}
		}

		if ( $reason == '-' ) {
			// Allow these UI messages to be blanked out cleanly
			return '';
		}

		// Max content length = max comment length - length of the comment (excl. $1)
		$text = $content->getTextForSummary( 255 - ( strlen( $reason ) - 2 ) );

		// Now replace the '$1' placeholder
		$reason = str_replace( '$1', $text, $reason );

		return $reason;
	}

	/**
	 * Parse the Content object and generate a ParserObject from the result. $result->getText() can
	 * be used to obtain the generated HTML. If no HTML is needed,  $generateHtml can be set to false;
	 * in that case, $result->getText() may return null.
	 *
	 * @param Content $content the content to render
	 * @param Title $title the page title to use as a context for rendering
	 * @param null|int $revId the revision being rendered (optional)
	 * @param null|ParserOptions $options any parser options
	 * @param Boolean $generateHtml whether to generate Html (default: true). If false,
	 *        the result of calling getText() on the ParserOutput object returned by
	 *        this method is undefined.
	 *
	 * @since WD.1
	 *
	 * @return ParserOutput
	 */
	public abstract function getParserOutput( Content $content, Title $title, $revId = null, ParserOptions $options = null, $generateHtml = true );
	#TODO: make RenderOutput and RenderOptions base classes

	/**
	 * Returns a list of DataUpdate objects for recording information about this Content in some secondary
	 * data store. If the optional second argument, $old, is given, the updates may model only the changes that
	 * need to be made to replace information about the old content with information about the new content.
	 *
	 * This default implementation calls $this->getParserOutput( $title, null, null, false ), and then
	 * calls getSecondaryDataUpdates( $title, $recursive ) on the resulting ParserOutput object.
	 *
	 * Subclasses may implement this to determine the necessary updates more efficiently, or make use of information
	 * about the old content.
	 *
	 * @param Title $title the context for determining the necessary updates
	 * @param Content|null $old a Content object representing the previous content, i.e. the content being
	 *                     replaced by this Content object.
	 * @param bool $recursive whether to include recursive updates (default: false).
	 *
	 * @return Array. A list of DataUpdate objects for putting information about this content object somewhere.
	 *
	 * @since WD.1
	 */
	public function getSecondaryDataUpdates( Content $content, Title $title, Content $old = null, $recursive = false ) {
		$po = $this->getParserOutput( $content, $title, null, null, false );
		return $po->getSecondaryDataUpdates( $title, $recursive );
	}


	/**
	 * Get the Content object that needs to be saved in order to undo all revisions
	 * between $undo and $undoafter. Revisions must belong to the same page,
	 * must exist and must not be deleted
	 *
	 * @since WD.1
	 *
	 * @param $current Revision the current text
	 * @param $undo Revision the revision to undo
	 * @param $undoafter Revision Must be an earlier revision than $undo
	 *
	 * @return mixed string on success, false on failure
	 */
	public function getUndoContent( Revision $current, Revision $undo, Revision $undoafter ) {
		$cur_content = $current->getContent();

		if ( empty( $cur_content ) ) {
			return false; // no page
		}

		$undo_content = $undo->getContent();
		$undoafter_content = $undoafter->getContent();

		if ( $cur_content->equals( $undo_content ) ) {
			// No use doing a merge if it's just a straight revert.
			return $undoafter_content;
		}

		$undone_content = $this->merge3( $undo_content, $undoafter_content, $cur_content );

		return $undone_content;
	}

	/**
	 * Returns true for content models that support caching using the ParserCache mechanism.
	 * See WikiPage::isParserCacheUser().
	 *
	 * @since WD.1
	 *
	 * @return bool
	 */
	public function isParserCacheSupported() {
		return true;
	}

	/**
	 * Returns a lost of updates to perform when the given content is deleted.
	 * The necessary updates may be taken from the Content object, or depend on the current state of the database.
	 *
	 * @since WD.1
	 *
	 * @param \Content           $content the Content object for deletion
	 * @param \Title             $title   the title of the deleted page
	 * @param null|\ParserOutput $parserOutput optional parser output object for efficient access to meta-information
	 *                           about the content object. Provide if you have one handy.
	 *
	 * @return array a list of DataUpdate instances that will clean up the database ofter deletion.
	 */
	public function getDeletionUpdates( Content $content, Title $title, ParserOutput $parserOutput = null ) {
		return array(
			new LinksDeletionUpdate( $title ),
		);
	}
}

/**
 * @since WD.1
 */
abstract class TextContentHandler extends ContentHandler {

	public function __construct( $modelId, $formats ) {
		parent::__construct( $modelId, $formats );
	}

	/**
	 * Returns the content's text as-is.
	 *
	 * @param Content $content
	 * @param String|null $format
	 * @return mixed
	 */
	public function serializeContent( Content $content, $format = null ) {
		$this->checkFormat( $format );
		return $content->getNativeData();
	}

	/**
	 * attempts to merge differences between three versions.
	 * Returns a new Content object for a clean merge and false for failure or a conflict.
	 *
	 * All three Content objects passed as parameters must have the same content model.
	 *
	 * This text-based implementation uses wfMerge().
	 *
	 * @param \Content|String $oldContent  String
	 * @param \Content|String $myContent   String
	 * @param \Content|String $yourContent String
	 *
	 * @return Content|Bool
	 */
	public function merge3( Content $oldContent, Content $myContent, Content $yourContent ) {
		$this->checkModelID( $oldContent->getModel() );
		$this->checkModelID( $myContent->getModel() );
		$this->checkModelID( $yourContent->getModel() );

		$format = $this->getDefaultFormat();

		$old = $this->serializeContent( $oldContent, $format );
		$mine = $this->serializeContent( $myContent, $format );
		$yours = $this->serializeContent( $yourContent, $format );

		$ok = wfMerge( $old, $mine, $yours, $result );

		if ( !$ok ) {
			return false;
		}

		if ( !$result ) {
			return $this->makeEmptyContent();
		}

		$mergedContent = $this->unserializeContent( $result, $format );
		return $mergedContent;
	}

	/**
	 * Returns a generic ParserOutput object, wrapping the HTML returned by getHtml().
	 *
	 * @param Content $content the content to render
	 * @param Title              $title context title for parsing
	 * @param int|null           $revId revision id (the parser wants that for some reason)
	 * @param ParserOptions|null $options parser options
	 * @param bool               $generateHtml whether or not to generate HTML
	 *
	 * @return ParserOutput representing the HTML form of the text
	 */
	public function getParserOutput( Content $content, Title $title, $revId = null, ParserOptions $options = null, $generateHtml = true ) {
		# generic implementation, relying on $this->getHtml()

		if ( $generateHtml ) $html = $this->getHtml( $content );
		else $html = '';

		$po = new ParserOutput( $html );
		return $po;
	}

	/**
	 * Generates an HTML version of the content, for display.
	 * Used by getParserOutput() to construct a ParserOutput object
	 *
	 * @param Content $content the content to render
	 *
	 * @return String
	 */
	protected abstract function getHtml( Content $content );


}

/**
 * @since WD.1
 */
class WikitextContentHandler extends TextContentHandler {

	public function __construct( $modelId = CONTENT_MODEL_WIKITEXT ) {
		parent::__construct( $modelId, array( CONTENT_FORMAT_WIKITEXT ) );
	}

	public function unserializeContent( $text, $format = null ) {
		$this->checkFormat( $format );

		return new WikitextContent( $text );
	}

	public function makeEmptyContent() {
		return new WikitextContent( '' );
	}

	/**
	 * Returns a ParserOutput object resulting from parsing the content's text using $wgParser.
	 *
	 * @since    WD.1
	 *
	 * @param Content $content the content to render
	 * @param \Title             $title
	 * @param null               $revId
	 * @param null|ParserOptions $options
	 * @param bool               $generateHtml
	 *
	 * @internal param \IContextSource|null $context
	 * @return ParserOutput representing the HTML form of the text
	 */
	public function getParserOutput( Content $content, Title $title, $revId = null, ParserOptions $options = null, $generateHtml = true ) {
		global $wgParser;

		if ( !$options ) {
			$options = new ParserOptions();
		}

		$po = $wgParser->parse( $content->getNativeData(), $title, $options, true, true, $revId );
		return $po;
	}

	protected function getHtml( Content $content ) {
		throw new MWException( "getHtml() not implemented for wikitext. Use getParserOutput()->getText()." );
	}

}

#XXX: make ScriptContentHandler base class with plugin interface for syntax highlighting?

/**
 * @since WD.1
 */
class JavaScriptContentHandler extends TextContentHandler {

	public function __construct( $modelId = CONTENT_MODEL_JAVASCRIPT ) {
		parent::__construct( $modelId, array( CONTENT_FORMAT_JAVASCRIPT ) );
	}

	public function unserializeContent( $text, $format = null ) {
		$this->checkFormat( $format );

		return new JavaScriptContent( $text );
	}

	public function makeEmptyContent() {
		return new JavaScriptContent( '' );
	}

	protected function getHtml( Content $content ) {
		$html = "";
		$html .= "<pre class=\"mw-code mw-js\" dir=\"ltr\">\n";
		$html .= htmlspecialchars( $content->getNativeData() );
		$html .= "\n</pre>\n";

		return $html;
	}
}

/**
 * @since WD.1
 */
class CssContentHandler extends TextContentHandler {

	public function __construct( $modelId = CONTENT_MODEL_CSS ) {
		parent::__construct( $modelId, array( CONTENT_FORMAT_CSS ) );
	}

	public function unserializeContent( $text, $format = null ) {
		$this->checkFormat( $format );

		return new CssContent( $text );
	}

	public function makeEmptyContent() {
		return new CssContent( '' );
	}


	protected function getHtml( Content $content ) {
		$html = "";
		$html .= "<pre class=\"mw-code mw-css\" dir=\"ltr\">\n";
		$html .= htmlspecialchars( $content->getNativeData() );
		$html .= "\n</pre>\n";

		return $html;
	}
}