e 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;
		}

		$entries_per_page = $this->get_entries_per_page();

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

			try {
				$links = $provider->get_sitemap_links( $type, $entries_per_page, $this->current_page );
			} catch ( OutOfBoundsException $exception ) {
				$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();
		$entries_per_page = $this->get_entries_per_page();

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

		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' );

		readfile( WPSEO_PATH . 'css/main-sitemap.xsl' );
	}

	/**
	 * Spit out the generated sitemap.
	 */
	public function output() {
		$this->send_headers();
		echo $this->renderer->get_output( $this->sitemap, $this->transient );
	}

	/**
	 * Makes a request to the sitemap index to cache it before the arrival of the search engines.
	 *
	 * @return void
	 */
	public function hit_sitemap_index() {
		if ( ! $this->cache->is_enabled() ) {
			return;
		}

		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 ) ) {
				$post_statuses = array_map( 'esc_sql', self::get_post_statuses() );

				$sql = "
					SELECT post_type, MAX(post_modified_gmt) AS date
					FROM $wpdb->posts
					WHERE post_status IN ('" . implode( "','", $post_statuses ) . "')
						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( 'https://www.google.com/ping?sitemap=' . $url, array( 'blocking' => false ) );
		wp_remote_get( 'https://www.bing.com/ping?sitemap=' . $url, array( 'blocking' => false ) );
	}

	/**
	 * Get the maximum number of entries per XML sitemap.
	 *
	 * @return int The maximum number of entries.
	 */
	protected function get_entries_per_page() {
		/**
		 * Filter the maximum number of entries per XML sitemap.
		 *
		 * After changing the output of the filter, make sure that you disable and enable the
		 * sitemaps to make sure the value is picked up for the sitemap cache.
		 *
		 * @param int $entries The maximum number of entries per XML sitemap.
		 */
		$entries = (int) apply_filters( 'wpseo_sitemap_entries_per_page', 1000 );

		return $entries;
	}

	/**
	 * Get post statuses for post_type or the root sitemap.
	 *
	 * @param string $type Provide a type for a post_type sitemap, SITEMAP_INDEX_TYPE for the root sitemap.
	 *
	 * @since 10.2
	 *
	 * @return array List of post statuses.
	 */
	public static function get_post_statuses( $type = self::SITEMAP_INDEX_TYPE ) {
		/**
		 * Filter post status list for sitemap query for the post type.
		 *
		 * @param array  $post_statuses Post status list, defaults to array( 'publish' ).
		 * @param string $type          Post type or SITEMAP_INDEX_TYPE.
		 */
		$post_statuses = apply_filters( 'wpseo_sitemap_post_statuses', array( 'publish' ), $type );

		if ( ! is_array( $post_statuses ) || empty( $post_statuses ) ) {
			$post_statuses = array( 'publish' );
		}

		if ( ( $type === self::SITEMAP_INDEX_TYPE || $type === 'attachment' )
			&& ! in_array( 'inherit', $post_statuses, true )
		) {
			$post_statuses[] = 'inherit';
		}

		return $post_statuses;
	}

	/**
	 * Sends all the required HTTP Headers.
	 */
	private function send_headers() {
		if ( headers_sent() ) {
			return;
		}

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

		/**
		 * Filter the HTTP headers we send before an XML sitemap.
		 *
		 * @param array  $headers The HTTP headers we're going to send out.
		 */
		$headers = apply_filters( 'wpseo_sitemap_http_headers', $headers );

		foreach ( $headers as $header => $status ) {
			if ( is_numeric( $status ) ) {
				header( $header, true, $status );
				continue;
			}
			header( $header, true );
		}
	}
}