Brent Shepherd's Rewrite Rule generator

/**
 * Generates all the rewrite rules for a given post type.
 *
 * The rewrite rules allow a post type to be filtered by all possible combinations & permutations
 * of taxonomies that apply to the specified post type and additional query_vars specified with
 * the $query_vars parameter.
 *
 * Must be called from a function hooked to the 'generate_rewrite_rules' action so that the global
 * $wp_rewrite->preg_index function returns the correct value.
 *
 * @param string|object $post_type The post type for which you wish to create the rewrite rules
 * @param array $query_vars optional Non-taxonomy query vars you wish to create rewrite rules for. Rules will be created to capture any single string for the query_var, that is, a rule of the form '/query_var/(.+)/'
 *
 * @author Brent Shepherd <me@brentshepherd.com>
 * @since 1.0
 */
function eg_generate_rewrite_rules( $post_type, $query_vars = array() ) {
    global $wp_rewrite;

    if( ! is_object( $post_type ) )
        $post_type = get_post_type_object( $post_type );

    $new_rewrite_rules = array();

    $taxonomies = get_object_taxonomies( $post_type->name, 'objects' );

    // Add taxonomy filters to the query vars array
    foreach( $taxonomies as $taxonomy )
        $query_vars[] = $taxonomy->query_var;

    // Loop over all the possible combinations of the query vars
    for( $i = 1; $i <= count( $query_vars );  $i++ ) {

        $new_rewrite_rule =  $post_type->rewrite['slug'] . '/';
        $new_query_string = 'index.php?post_type=' . $post_type->name;

        // Prepend the rewrites & queries
        for( $n = 1; $n <= $i; $n++ ) {
            $new_rewrite_rule .= '(' . implode( '|', $query_vars ) . ')/(.+?)/';
            $new_query_string .= '&' . $wp_rewrite->preg_index( $n * 2 - 1 ) . '=' . $wp_rewrite->preg_index( $n * 2 );
        }

        // Allow paging of filtered post type - WordPress expects 'page' in the URL but uses 'paged' in the query string so paging doesn't fit into our regex
        $new_paged_rewrite_rule = $new_rewrite_rule . 'page/([0-9]{1,})/';
        $new_paged_query_string = $new_query_string . '&paged=' . $wp_rewrite->preg_index( $i * 2 + 1 );

        // Make the trailing backslash optional
        $new_paged_rewrite_rule = $new_paged_rewrite_rule . '?$';
        $new_rewrite_rule = $new_rewrite_rule . '?$';

        // Add the new rewrites
        $new_rewrite_rules = array( $new_paged_rewrite_rule => $new_paged_query_string,
                                    $new_rewrite_rule       => $new_query_string )
                             + $new_rewrite_rules;
    }

    return $new_rewrite_rules;
}



// add the rules
function mv_add_rewrite_rule($wp_rewrite){
    $new_rules = eg_generate_rewrite_rules('product');
    $wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
}
add_action('generate_rewrite_rules','mv_add_rewrite_rule',99);