public function getTimestamp($resource)
    {
        $path = $this->getSourcePath($resource);

        if (!file_exists($path)) {
            throw new \RuntimeException('There is no cached value for '.$resource);
        }

        if (false === $mtime = @filemtime($path)) {
            // @codeCoverageIgnoreStart
            throw new \RuntimeException('Unable to determine file mtime for '.$path);
            // @codeCoverageIgnoreEnd
        }

        return $mtime;
    }