diff -r 3d4e9c994f10 -r a86126ab1dd4 wp/wp-includes/sitemaps/class-wp-sitemaps-index.php
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/wp/wp-includes/sitemaps/class-wp-sitemaps-index.php	Tue Dec 15 13:49:49 2020 +0100
@@ -0,0 +1,95 @@
+<?php
+/**
+ * Sitemaps: WP_Sitemaps_Index class.
+ *
+ * Generates the sitemap index.
+ *
+ * @package WordPress
+ * @subpackage Sitemaps
+ * @since 5.5.0
+ */
+
+/**
+ * Class WP_Sitemaps_Index.
+ * Builds the sitemap index page that lists the links to all of the sitemaps.
+ *
+ * @since 5.5.0
+ */
+class WP_Sitemaps_Index {
+	/**
+	 * The main registry of supported sitemaps.
+	 *
+	 * @since 5.5.0
+	 * @var WP_Sitemaps_Registry
+	 */
+	protected $registry;
+
+	/**
+	 * Maximum number of sitemaps to include in an index.
+	 *
+	 * @sincee 5.5.0
+	 *
+	 * @var int Maximum number of sitemaps.
+	 */
+	private $max_sitemaps = 50000;
+
+	/**
+	 * WP_Sitemaps_Index constructor.
+	 *
+	 * @since 5.5.0
+	 *
+	 * @param WP_Sitemaps_Registry $registry Sitemap provider registry.
+	 */
+	public function __construct( WP_Sitemaps_Registry $registry ) {
+		$this->registry = $registry;
+	}
+
+	/**
+	 * Gets a sitemap list for the index.
+	 *
+	 * @since 5.5.0
+	 *
+	 * @return array[] Array of all sitemaps.
+	 */
+	public function get_sitemap_list() {
+		$sitemaps = array();
+
+		$providers = $this->registry->get_providers();
+		/* @var WP_Sitemaps_Provider $provider */
+		foreach ( $providers as $name => $provider ) {
+			$sitemap_entries = $provider->get_sitemap_entries();
+
+			// Prevent issues with array_push and empty arrays on PHP < 7.3.
+			if ( ! $sitemap_entries ) {
+				continue;
+			}
+
+			// Using array_push is more efficient than array_merge in a loop.
+			array_push( $sitemaps, ...$sitemap_entries );
+			if ( count( $sitemaps ) >= $this->max_sitemaps ) {
+				break;
+			}
+		}
+
+		return array_slice( $sitemaps, 0, $this->max_sitemaps, true );
+	}
+
+	/**
+	 * Builds the URL for the sitemap index.
+	 *
+	 * @since 5.5.0
+	 *
+	 * @global WP_Rewrite $wp_rewrite WordPress rewrite component.
+	 *
+	 * @return string The sitemap index URL.
+	 */
+	public function get_index_url() {
+		global $wp_rewrite;
+
+		if ( ! $wp_rewrite->using_permalinks() ) {
+			return home_url( '/?sitemap=index' );
+		}
+
+		return home_url( '/wp-sitemap.xml' );
+	}
+}