protected function build_thread_data($data, &$depth, &$children, $level = 0)
    {
        foreach ((array)$data as $key => $val) {
            $empty          = empty($val) || !is_array($val);
            $children[$key] = !$empty;
            $depth[$key]    = $level;
            if (!$empty) {
                $this->build_thread_data($val, $depth, $children, $level + 1);
            }
        }
    }