private function build_one_sitemap_index( $number, $from_id, $datetime, $index_type ) {
    $last_sitemap_id   = $from_id;
    $any_sitemaps_left = true;

    // Check the datetime format.
    $datetime = jp_sitemap_datetime( $datetime );

    $sitemap_type = jp_sitemap_child_type_of( $index_type );

    if ( $this->logger ) {
      $index_debug_name = jp_sitemap_filename( $index_type, $number );
      $this->logger->report( "-- Building $index_debug_name" );
    }

    $buffer = new Jetpack_Sitemap_Buffer_Master(
      JP_SITEMAP_MAX_ITEMS,
      JP_SITEMAP_MAX_BYTES,
      $datetime
    );

    // Add pointer to the previous sitemap index (unless we're at the first one).
    if ( 1 !== $number ) {
      $i              = $number - 1;
      $prev_index_url = $this->finder->construct_sitemap_url(
        jp_sitemap_filename( $index_type, $i )
      );

      $item_array = array(
        'sitemap' => array(
          'loc'     => $prev_index_url,
          'lastmod' => $datetime,
        ),
      );

      $buffer->append( $item_array );
    }

    // Add as many items to the buffer as possible.
    while ( false === $buffer->is_full() ) {
      // Retrieve a batch of posts (in order).
      $posts = $this->librarian->query_sitemaps_after_id(
        $sitemap_type,
        $last_sitemap_id,
        JP_SITEMAP_BATCH_SIZE
      );

      // If there were no posts to get, make a note.
      if ( null == $posts ) { // WPCS: loose comparison ok.
        $any_sitemaps_left = false;
        break;
      }

      // Otherwise, loop through each post in the batch.
      foreach ( $posts as $post ) {
        // Generate the sitemap XML for the post.
        $current_item = $this->sitemap_row_to_index_item( (array) $post );

        // Try adding this item to the buffer.
        if ( true === $buffer->append( $current_item['xml'] ) ) {
          $last_sitemap_id = $post['ID'];
          $buffer->view_time( $current_item['last_modified'] );
        } else {
          // Otherwise stop looping through posts.
          break;
        }
      }
    }

    // If no items were added, return false.
    if ( true === $buffer->is_empty() ) {
      return false;
    }

    $this->librarian->store_sitemap_data(
      $number,
      $index_type,
      $buffer->contents(),
      $buffer->last_modified()
    );

    /*
		 * Now report back with the ID of the last sitemap post ID to
		 * be successfully added, whether there are any sitemap posts
		 * left, and the most recent modification time seen.
		 */
    return array(
      'last_id'       => $last_sitemap_id,
      'any_left'      => $any_sitemaps_left,
      'last_modified' => $buffer->last_modified(),
    );
  }