Restricting results to the same category as the current post object

Limiting posts, pages, products or any custom post type results to only categories, or any taxonomy terms - where the post type object (post, page, product etc..) belongs to.

What is this, and where do I put this custom code?

add_filter( 'asp_query_args', 'asp_posts_from_same_terms', 10, 2 );
function asp_posts_from_same_terms($args, $search_id) {
	$taxonomies = 'category, post_tag';	// comma separated list of taxonomies
	$allow_missing = false; // allow (empty) items with no connection to any of the taxonomy terms filter
  $search_ids = 'all';    // comma separated list of search IDs, where the code should apply

	// --- DO NOT CHANGE ANYTHING BELOW ---
    $search_ids = explode(',', $search_ids);
    foreach ( $search_ids as &$sid )
        $sid = trim($sid);
	if (
	        !empty($args['_page_id']) &&
            ( in_array('all', $search_ids) || in_array($search_id, $search_ids) )
    ) {
		$taxonomies = explode(',', $taxonomies);
		foreach ( $taxonomies as $taxonomy ) {
			$taxonomy = trim($taxonomy);
			$terms = wp_get_object_terms(
			    $args['_page_id'],
                $taxonomy,
                array('fields' => 'ids')
            );
			if ( !is_wp_error($terms) && count($terms) ) {
				$args['post_tax_filter'][] = array(
				  'taxonomy'  => $taxonomy, // taxonomy name
				  'include'   => $terms,   	// array of taxonomy term IDs to include
				  'exclude'   => array(),
				  'allow_empty' => $allow_missing    // allow (empty) items with no connection to any of the taxonomy terms filter
				);
			}
		}
	}
	return $args;
}

• $taxonomies (string) - comma separated list of taxonomies, where this code should apply to

• $allow_missing (boolean) - in cases, when the resulting objects (posts, pages etc..) does not have the taxonomy assigned - should they be allowed as results. Ex.: Current page belongs to post_tag named Tag1. The search is configured to search posts and products as well, but products can not have assigned items from post_tag taxonomy. When $allow_missing = false; (default), all of the products will be excluded.

• $search_ids (string) - comma separated list of serach IDs, where the code should apply on. Assigning 'all' (default) value will apply on all search bars.