public function getFileDirPrefix($filename)
    {
        if (empty($filename)) {
            throw new InvalidArgumentException('Invalid filename argument');
        }

        $path = array();
        $parts = explode('.', $filename);

        for ($i = 0; $i < min(strlen($parts[0]), $this->prefixSize); $i++) {
            $path[] = $filename[$i];
        }

        $path = $this->getPath($path);

        return $path;
    }