Most Discussed Posts Feed

// Feed name: the feed lives at /feed/popular/ (or ?feed=popular with plain permalinks).
$wpide_popular_feed_name = 'popular';
// Only count posts published in the last number of days (0 = all time) and how many to list.
$wpide_popular_feed_days  = 90;
$wpide_popular_feed_items = 10;

add_action( 'init', function () use ( $wpide_popular_feed_name, $wpide_popular_feed_days, $wpide_popular_feed_items ) {
    add_feed( $wpide_popular_feed_name, function () use ( $wpide_popular_feed_days, $wpide_popular_feed_items ) {
        $cache_key = 'wpide_popular_feed_' . md5( $wpide_popular_feed_days . '|' . $wpide_popular_feed_items );
        $ids       = get_transient( $cache_key );

        if ( ! is_array( $ids ) ) {
            $args = array(
                'post_type'           => 'post',
                'post_status'         => 'publish',
                'posts_per_page'      => max( 1, (int) $wpide_popular_feed_items ),
                'orderby'             => array( 'comment_count' => 'DESC', 'date' => 'DESC' ),
                'has_password'        => false,
                'ignore_sticky_posts' => true,
                'no_found_rows'       => true,
                'fields'              => 'ids',
            );

            if ( $wpide_popular_feed_days > 0 ) {
                $args['date_query'] = array( array( 'after' => absint( $wpide_popular_feed_days ) . ' days ago' ) );
            }

            $ids = get_posts( $args );
            set_transient( $cache_key, $ids, HOUR_IN_SECONDS );
        }

        $GLOBALS['wp_query'] = new WP_Query( array(
            'post_type'           => 'post',
            'post__in'            => $ids ? $ids : array( 0 ),
            'orderby'             => 'post__in',
            'posts_per_page'      => count( $ids ) ? count( $ids ) : 1,
            'ignore_sticky_posts' => true,
            'no_found_rows'       => true,
        ) );

        add_filter( 'get_wp_title_rss', function ( $title ) {
            return $title . ' – Most discussed';
        } );

        load_template( ABSPATH . WPINC . '/feed-rss2.php' );
    } );
} );