private function getCurrentFileHash($filename)
        {
                if (!empty($hash_cache[$filename])) {
                        return $hash_cache[$filename];
                }

                $url = self::HASH_BASE_URL.$filename;

        $hash_response = $this->fetchUrl($url);
                if ($hash_response === false) return false;
                $hash = trim($hash_response);

                if (preg_match("/^[0-9a-f]{40}$/", $hash)) {
                        $hash_cache[$filename] = $hash;

                        return $hash;
                } else {
                        return false;
                }
        }