protected function &getMapCache(): array
    {
        if (!isset($this->map_cache)) {
            $this->map_cache = []; // Define.

            if (is_file($this->map_cache_file)) {
                $this->map_cache = file_get_contents($this->map_cache_file);
                $this->map_cache = json_decode($this->map_cache, true);
                $this->map_cache = is_array($this->map_cache) ? $this->map_cache : [];
            } // Only if file exists, otherwise empty array.
        }
        return $this->map_cache;
    }