<?php
/**
 * Create the WordPress rewrite rule to handle third level slugs for videos.
 *
 */
function video_cpt_generating_rule( $wp_rewrite ) {
    $rules = [];
    $terms = get_terms( [
        'taxonomy'   => 'series',
        'hide_empty' => false,
    ] );

    $post_type = 'video';

    foreach ( $terms as $term ) {

        $rules['video-series/' . $term->slug . '/([^/]*)$'] = 'index.php?post_type=' . $post_type. '&series=$matches[1]&name=$matches[1]';

    }

    // merge with global rules
    $wp_rewrite->rules = $rules + $wp_rewrite->rules;
}
add_filter( 'generate_rewrite_rules', 'video_cpt_generating_rule' );

/**
 * Change series slug to show rewrite rule.
 *
 */
function video_change_link( $permalink, $post ) {

    if ( $post->post_type == 'video' ) {
        $series_terms = get_the_terms( $post, 'series' );
        $term_slug    = '';
        if ( ! empty( $series_terms ) ) {
            foreach ( $series_terms as $term ) {

                $term_slug = $term->slug;
                break;
            }
        }
        $permalink = get_home_url() ."/video-series/" . $term_slug . '/' . $post->post_name;
    }
    return $permalink;

}
add_filter( 'post_type_link', 'video_change_link', 10, 2 );