public function build_root_map() {

    $links            = [];
    $entries_per_page = $this->get_entries_per_page();

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

    
/**
 * Filter the sitemap links array before the index sitemap is built.
 *
 * @param array  $links Array of sitemap links
 */
    $links = apply_filters( 'wpseo_sitemap_index_links', $links );

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

      return;
    }

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