class ApcuBackend

class ApcuBackend implements CacheBackendInterface {
    
    /**
     * The name of the cache bin to use.
     *
     * @var string
     */
    protected $bin;
    
    /**
     * Prefix for all keys in the storage that belong to this site.
     *
     * @var string
     */
    protected $sitePrefix;
    
    /**
     * Prefix for all keys in this cache bin.
     *
     * Includes the site-specific prefix in $sitePrefix.
     *
     * @var string
     */
    protected $binPrefix;
    
    /**
     * The cache tags checksum provider.
     *
     * @var \Drupal\Core\Cache\CacheTagsChecksumInterface
     */
    protected $checksumProvider;
    
    /**
     * Constructs a new ApcuBackend instance.
     *
     * @param string $bin
     *   The name of the cache bin.
     * @param string $site_prefix
     *   The prefix to use for all keys in the storage that belong to this site.
     * @param \Drupal\Core\Cache\CacheTagsChecksumInterface $checksum_provider
     *   The cache tags checksum provider.
     */
    public function __construct($bin, $site_prefix, CacheTagsChecksumInterface $checksum_provider) {
        $this->bin = $bin;
        $this->sitePrefix = $site_prefix;
        $this->checksumProvider = $checksum_provider;
        $this->binPrefix = $this->sitePrefix . '::' . $this->bin . '::';
    }
    
    /**
     * Prepends the APCu user variable prefix for this bin to a cache item ID.
     *
     * @param string $cid
     *   The cache item ID to prefix.
     *
     * @return string
     *   The APCu key for the cache item ID.
     */
    public function getApcuKey($cid) {
        return $this->binPrefix . $cid;
    }
    
    /**
     * {@inheritdoc}
     */
    public function get($cid, $allow_invalid = FALSE) {
        $cache = apcu_fetch($this->getApcuKey($cid));
        return $this->prepareItem($cache, $allow_invalid);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getMultiple(&$cids, $allow_invalid = FALSE) {
        // Translate the requested cache item IDs to APCu keys.
        $map = [];
        foreach ($cids as $cid) {
            $map[$this->getApcuKey($cid)] = $cid;
        }
        $result = apcu_fetch(array_keys($map));
        $cache = [];
        if ($result) {
            foreach ($result as $key => $item) {
                $item = $this->prepareItem($item, $allow_invalid);
                if ($item) {
                    $cache[$map[$key]] = $item;
                }
            }
        }
        unset($result);
        $cids = array_diff($cids, array_keys($cache));
        return $cache;
    }
    
    /**
     * Returns all cached items, optionally limited by a cache ID prefix.
     *
     * APCu is a memory cache, shared across all server processes. To prevent
     * cache item clashes with other applications/installations, every cache item
     * is prefixed with a unique string for this site. Therefore, functions like
     * apcu_clear_cache() cannot be used, and instead, a list of all cache items
     * belonging to this application need to be retrieved through this method
     * instead.
     *
     * @param string $prefix
     *   (optional) A cache ID prefix to limit the result to.
     *
     * @return \APCUIterator
     *   An APCUIterator containing matched items.
     */
    protected function getAll($prefix = '') {
        return $this->getIterator('/^' . preg_quote($this->getApcuKey($prefix), '/') . '/');
    }
    
    /**
     * Prepares a cached item.
     *
     * Checks that the item is either permanent or did not expire.
     *
     * @param object $cache
     *   An item loaded from self::get() or self::getMultiple().
     * @param bool $allow_invalid
     *   If TRUE, a cache item may be returned even if it is expired or has been
     *   invalidated. See ::get().
     *
     * @return mixed
     *   The cache item or FALSE if the item expired.
     */
    protected function prepareItem($cache, $allow_invalid) {
        if (!isset($cache->data)) {
            return FALSE;
        }
        $cache->tags = $cache->tags ? explode(' ', $cache->tags) : [];
        // Check expire time.
        $cache->valid = $cache->expire == Cache::PERMANENT || $cache->expire >= REQUEST_TIME;
        // Check if invalidateTags() has been called with any of the entry's tags.
        if (!$this->checksumProvider
            ->isValid($cache->checksum, $cache->tags)) {
            $cache->valid = FALSE;
        }
        if (!$allow_invalid && !$cache->valid) {
            return FALSE;
        }
        return $cache;
    }
    
    /**
     * {@inheritdoc}
     */
    public function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, array $tags = []) {
        assert(Inspector::assertAllStrings($tags), 'Cache tags must be strings.');
        $tags = array_unique($tags);
        $cache = new \stdClass();
        $cache->cid = $cid;
        $cache->created = round(microtime(TRUE), 3);
        $cache->expire = $expire;
        $cache->tags = implode(' ', $tags);
        $cache->checksum = $this->checksumProvider
            ->getCurrentChecksum($tags);
        // APCu serializes/unserializes any structure itself.
        $cache->serialized = 0;
        $cache->data = $data;
        // Expiration is handled by our own prepareItem(), not APCu.
        apcu_store($this->getApcuKey($cid), $cache);
    }
    
    /**
     * {@inheritdoc}
     */
    public function setMultiple(array $items = []) {
        foreach ($items as $cid => $item) {
            $this->set($cid, $item['data'], $item['expire'] ?? CacheBackendInterface::CACHE_PERMANENT, $item['tags'] ?? []);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function delete($cid) {
        apcu_delete($this->getApcuKey($cid));
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteMultiple(array $cids) {
        apcu_delete(array_map([
            $this,
            'getApcuKey',
        ], $cids));
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteAll() {
        apcu_delete($this->getIterator('/^' . preg_quote($this->binPrefix, '/') . '/'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function garbageCollection() {
        // APCu performs garbage collection automatically.
    }
    
    /**
     * {@inheritdoc}
     */
    public function removeBin() {
        apcu_delete($this->getIterator('/^' . preg_quote($this->binPrefix, '/') . '/'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function invalidate($cid) {
        $this->invalidateMultiple([
            $cid,
        ]);
    }
    
    /**
     * {@inheritdoc}
     */
    public function invalidateMultiple(array $cids) {
        foreach ($this->getMultiple($cids) as $cache) {
            $this->set($cache->cid, $cache, REQUEST_TIME - 1);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function invalidateAll() {
        foreach ($this->getAll() as $data) {
            $cid = str_replace($this->binPrefix, '', $data['key']);
            $this->set($cid, $data['value'], REQUEST_TIME - 1);
        }
    }
    
    /**
     * Instantiates and returns the APCUIterator class.
     *
     * @param mixed $search
     *   A PCRE regular expression that matches against APC key names, either as a
     *   string for a single regular expression, or as an array of regular
     *   expressions. Or, optionally pass in NULL to skip the search.
     * @param int $format
     *   The desired format, as configured with one or more of the APC_ITER_*
     *   constants.
     * @param int $chunk_size
     *   The chunk size. Must be a value greater than 0. The default value is 100.
     * @param int $list
     *   The type to list. Either pass in APC_LIST_ACTIVE or APC_LIST_DELETED.
     *
     * @return \APCUIterator
     */
    protected function getIterator($search = NULL, $format = APC_ITER_ALL, $chunk_size = 100, $list = APC_LIST_ACTIVE) {
        return new \APCUIterator($search, $format, $chunk_size, $list);
    }

}