protected function generateIndexKey($cacheKey)
    {
        $cacheKeyStack = explode('.', $cacheKey);

        return implode('.', [
            $this->sanitizeCacheKeyPart($this->options['global_prefix']),
            $this->sanitizeCacheKeyPart($this->options['prefix']),
            $this->sanitizeCacheKeyPart($this->options['index_key']),
            $this->sanitizeCacheKeyPart($cacheKeyStack[2]), // filter
        ]);
    }