protected function getMappedKeyIndex($mappedKey, $key)
    {
        $startSize = strlen($this->getStartMapDelimiter()) - 1;
        $key = substr($key, $startSize, strrpos($key, $this->getEndMapDelimiter())-$startSize);
        return substr($mappedKey, strlen($key . self::MAPPED_KEY_SEPARATOR));
    }