private function getCache($file)
    {
        if (null === $this->cache) {
            return null;
        }

        $key = md5($file);

        // File already present
        if ($this->cache->has($key)) {
            list($mtime, $path) = unserialize($this->cache->get($key));

            if ($mtime === filemtime($file)) {
                return $path;
            }

            $delete = $this->directory.'/'.$path;
            if (file_exists($delete) && false === @unlink($delete)) {
                throw new \RuntimeException('Unable to remove file '.$delete);
            }
        }

        return null;
    }