ite = '' ) {
		add_action( 'wpseo_xsl_' . $name, $function );
		if ( ! empty( $rewrite ) ) {
			add_rewrite_rule( $rewrite, 'index.php?xsl=' . $name, 'top' );
		}
	}

	/**
	 * Set the sitemap current page to allow creating partial sitemaps with wp-cli
	 * in a one-off process.
	 *
	 * @param integer $current_page The part that should be generated.
	 */
	public function set_n( $current_page ) {
		if ( is_scalar( $current_page ) && intval( $current_page ) > 0 ) {
			$this->current_page = intval( $current_page );
		}
	}

	/**
	 * Set the sitemap content to display after you have generated it.
	 *
	 * @param string $sitemap The generated sitemap to output.
	 */
	public function set_sitemap( $sitemap ) {
		$this->sitemap = $sitemap;
	}

	/**
	 * Set as true to make the request 404. Used stop the display of empty sitemaps or invalid requests.
	 *
	 * @param bool $bool Is this a bad request. True or false.
	 */
	public function set_bad_sitemap( $bool ) {
		$this->bad_sitemap = (bool) $bool;
	}

	/**
	 * Prevent stupid plugins from running shutdown scripts when we're obviously not outputting HTML.
	 *
	 * @since 1.4.16
	 */
	public function sitemap_close() {
		remove_all_actions( 'wp_footer' );
		die();
	}

	/**
	 * Hijack requests for potential sitemaps and XSL files.
	 *
	 * @param \WP_Query $query Main query instance.
	 */
	public function redirect( $query ) {

		if ( ! $query->is_main_query() ) {
			return;
		}

		$xsl = get_query_var( 'xsl' );

		if ( ! empty( $xsl ) ) {
			$this->xsl_output( $xsl );
			$this->sitemap_close();

			return;
		}

		$type = get_query_var( 'sitemap' );

		if ( empty( $type ) ) {
			return;
		}

		$this->set_n( get_query_var( 'sitemap_n' ) );

		if ( ! $this->get_sitemap_from_cache( $type, $this->current_page ) ) {
			$this->build_sitemap( $type );
		}

		if ( $this->bad_sitemap ) {
			$query->set_404();
			status_header( 404 );

			return;
		}

		$this->output();
		$this->sitemap_close();
	}

	/**
	 * Try to get the sitemap from cache
	 *
	 * @param string $type        Sitemap type.
	 * @param int    $page_number The page number to retrieve.
	 *
	 * @return bool If the sitemap has been retrieved from cache.
	 */
	private function get_sitemap_from_cache( $type, $page_number ) {

		$this->transient = false;

		if ( true !== $this->cache->is_enabled() ) {
			return false;
		}

		/**
		 * Fires before the attempt to retrieve XML sitemap from the transient cache.
		 *
		 * @param WPSEO_Sitemaps $sitemaps Sitemaps object.
		 */
		do_action( 'wpseo_sitemap_stylesheet_cache_' . $type, $this );

		$sitemap_cache_data = $this->cache->get_sitemap_data( $type, $page_number );

		// No cache was found, refresh it because cache is enabled.
		if ( empty( $sitemap_cache_data ) ) {
			return $this->refresh_sitemap_cache( $type, $page_number );
		}

		// Cache object was found, parse information.
		$this->transient = true;

		$this->sitemap     = $sitemap_cache_data->get_sitemap();
		$this->bad_sitemap = ! $sitemap_cache_data->is_usable();

		return true;
	}

	/**
	 * Build and save sitemap to cache.
	 *
	 * @param string $type        Sitemap type.
	 * @param int    $page_number The page number to save to.
	 *
	 * @return bool
	 */
	private function refresh_sitemap_cache( $type, $page_number ) {
		$this->set_n( $page_number );
		$this->build_sitemap( $type );

		return $this->cache->store_sitemap( $type, $page_number, $this->sitemap, ! $this->bad_sitemap );
	}

	/**
	 * Attempts to build the requested sitemap.
	 *
	 * Sets $bad_sitemap if this isn't for the root sitemap, a post type or taxonomy.
	 *
	 * @param string $type The requested sitemap's identifier.
	 */
	public function build_sitemap( $type ) {

		/**
		 * Filter the type of sitemap to build.
		 *
		 * @param string $type Sitemap type, determined by the request.
		 */
		$type = apply_filters( 'wpseo_build_sitemap_post_type', $type );

		if ( $type === '1' ) {
			$this->build_root_map();

			return;
		}

		foreach ( $this->providers as $provider ) {
			if ( ! $provider->handles_type( $type ) ) {
				continue;
			}

			$links = $provider->get_sitemap_links( $type, $this->max_entries, $this->current_page );

			if ( empty( $links ) ) {
				$this->bad_sitemap = true;

				return;
			}

			$this->sitemap = $this->renderer->get_sitemap( $links, $type, $this->current_page );

			return;
		}

		if ( has_action( 'wpseo_do_sitemap_' . $type ) ) {
			/**
			 * Fires custom handler, if hooked to generate sitemap for the type.
			 */
			do_action( 'wpseo_do_sitemap_' . $type );

			return;
		}

		$this->bad_sitemap = true;
	}

	/**
	 * Build the root sitemap (example.com/sitemap_index.xml) which lists sub-sitemaps for other content types.
	 */
	public function build_root_map() {

		$links = array();

		foreach ( $this->providers as $provider ) {
			$links = array_merge( $links, $provider->get_index_links( $this->max_entries ) );
		}

		if ( empty( $links ) ) {
			$this->bad_sitemap = true;
			$this->sitemap     = '';

			return;
		}

		$this->sitemap = $this->renderer->get_index( $links );
	}

	/**
	 * Spits out the XSL for the XML sitemap.
	 *
	 * @param string $type Type to output.
	 *
	 * @since 1.4.13
	 */
	public function xsl_output( $type ) {

		if ( $type !== 'main' ) {

			/**
			 * Fires for the output of XSL for XML sitemaps, other than type "main".
			 */
			do_action( 'wpseo_xsl_' . $type );

			return;
		}

		header( $this->http_protocol . ' 200 OK', true, 200 );
		// Prevent the search engines from indexing the XML Sitemap.
		header( 'X-Robots-Tag: noindex, follow', true );
		header( 'Content-Type: text/xml' );

		// Make the browser cache this file properly.
		$expires = YEAR_IN_SECONDS;
		header( 'Pragma: public' );
		header( 'Cache-Control: maxage=' . $expires );
		header( 'Expires: ' . gmdate( 'D, d M Y H:i:s', ( time() + $expires ) ) . ' GMT' );

		require_once WPSEO_PATH . 'css/xml-sitemap-xsl.php';
	}

	/**
	 * Spit out the generated sitemap and relevant headers and encoding information.
	 */
	public function output() {

		if ( ! headers_sent() ) {
			header( $this->http_protocol . ' 200 OK', true, 200 );
			// Prevent the search engines from indexing the XML Sitemap.
			header( 'X-Robots-Tag: noindex, follow', true );
			header( 'Content-Type: text/xml; charset=' . esc_attr( $this->renderer->get_output_charset() ) );
		}

		echo $this->renderer->get_output( $this->sitemap, $this->transient );
	}

	/**
	 * Make a request for the sitemap index so as to cache it before the arrival of the search engines.
	 */
	public function hit_sitemap_index() {
		wp_remote_get( WPSEO_Sitemaps_Router::get_base_url( 'sitemap_index.xml' ) );
	}

	/**
	 * Get the GMT modification date for the last modified post in the post type.
	 *
	 * @since 3.2
	 *
	 * @param string|array $post_types Post type or array of types.
	 * @param bool         $return_all Flag to return array of values.
	 *
	 * @return string|array|false
	 */
	public static function get_last_modified_gmt( $post_types, $return_all = false ) {

		global $wpdb;

		static $post_type_dates = null;

		if ( ! is_array( $post_types ) ) {
			$post_types = array( $post_types );
		}

		foreach ( $post_types as $post_type ) {
			if ( ! isset( $post_type_dates[ $post_type ] ) ) { // If we hadn't seen post type before. R.
				$post_type_dates = null;
				break;
			}
		}

		if ( is_null( $post_type_dates ) ) {

			$post_type_dates = array();

			// Consider using WPSEO_Post_Type::get_accessible_post_types() to filter out any `no-index` post-types.
			$post_type_names = get_post_types( array( 'public' => true ) );

			if ( ! empty( $post_type_names ) ) {
				$sql = "
				SELECT post_type, MAX(post_modified_gmt) AS date
				FROM $wpdb->posts
				WHERE post_status IN ('publish','inherit')
					AND post_type IN ('" . implode( "','", $post_type_names ) . "')
				GROUP BY post_type
				ORDER BY post_modified_gmt DESC
			";

				foreach ( $wpdb->get_results( $sql ) as $obj ) {
					$post_type_dates[ $obj->post_type ] = $obj->date;
				}
			}
		}

		$dates = array_intersect_key( $post_type_dates, array_flip( $post_types ) );

		if ( count( $dates ) > 0 ) {
			if ( $return_all ) {
				return $dates;
			}

			return max( $dates );
		}

		return false;
	}

	/**
	 * Get the modification date for the last modified post in the post type.
	 *
	 * @param array $post_types Post types to get the last modification date for.
	 *
	 * @return string
	 */
	public function get_last_modified( $post_types ) {

		return $this->timezone->format_date( self::get_last_modified_gmt( $post_types ) );
	}

	/**
	 * Notify search engines of the updated sitemap.
	 *
	 * @param string|null $url Optional URL to make the ping for.
	 */
	public static function ping_search_engines( $url = null ) {

		/**
		 * Filter: 'wpseo_allow_xml_sitemap_ping' - Check if pinging is not allowed (allowed by default)
		 *
		 * @api boolean $allow_ping The boolean that is set to true by default.
		 */
		if ( apply_filters( 'wpseo_allow_xml_sitemap_ping', true ) === false ) {
			return;
		}

		if ( '0' === get_option( 'blog_public' ) ) { // Don't ping if blog is not public.
			return;
		}

		if ( empty( $url ) ) {
			$url = urlencode( WPSEO_Sitemaps_Router::get_base_url( 'sitemap_index.xml' ) );
		}

		// Ping Google and Bing.
		wp_remote_get( 'http://www.google.com/webmasters/tools/ping?sitemap=' . $url, array( 'blocking' => false ) );
		wp_remote_get( 'http://www.bing.com/ping?sitemap=' . $url, array( 'blocking' => false ) );
	}

	/**
	 * Build the `<url>` tag for a given URL.
	 *
	 * @deprecated 3.2
	 * @see WPSEO_Sitemaps_Renderer::sitemap_url()
	 *
	 * @param array $url Array of parts that make up this entry.
	 *
	 * @return string
	 */
	public function sitemap_url( $url ) {
		_deprecated_function( __METHOD__, 'WPSEO 3.2', 'WPSEO_Sitemaps_Renderer::sitemap_url()' );
		return $this->renderer->sitemap_url( $url );
	}

	/**
	 * Set a custom stylesheet for this sitemap. Set to empty to just remove the default stylesheet.
	 *
	 * @deprecated 3.2
	 * @see WPSEO_Sitemaps_Renderer::set_stylesheet()
	 *
	 * @param string $stylesheet Full xml-stylesheet declaration.
	 */
	public function set_stylesheet( $stylesheet ) {
		_deprecated_function( __METHOD__, 'WPSEO 3.2', 'WPSEO_Sitemaps_Renderer::set_stylesheet()' );
		$this->renderer->set_stylesheet( $stylesheet );
	}

	/**
	 * Function to dynamically filter the change frequency.
	 *
	 * @deprecated 3.5 Change frequency data dropped from sitemaps.
	 *
	 * @param string $filter  Expands to wpseo_sitemap_$filter_change_freq, allowing for a change of the frequency for
	 *                        numerous specific URLs.
	 * @param string $default The default value for the frequency.
	 * @param string $url     The URL of the current entry.
	 *
	 * @return mixed|void
	 */
	public static function filter_frequency( $filter, $default, $url ) {
		_deprecated_function( __METHOD__, 'WPSEO 3.5' );

		/**
		 * Filter the specific change frequency
		 *
		 * @param string $default The default change frequency.
		 * @param string $url     URL to filter frequency for.
		 */
		$change_freq = apply_filters( 'wpseo_sitemap_' . $filter . '_change_freq', $default, $url );

		if ( ! in_array( $change_freq, array(
			'always',
			'hourly',
			'daily',
			'weekly',
			'monthly',
			'yearly',
			'never',
		), true )
		) {
			$change_freq = $default;
		}

		return $change_freq;
	}
}