<?php
/*
==New BSD License==

Copyright (c) 2013, Colin Mollenhour
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:

    * Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.
    * Redistributions in binary form must reproduce the above copyright
      notice, this list of conditions and the following disclaimer in the
      documentation and/or other materials provided with the distribution.
    * The name of Colin Mollenhour may not be used to endorse or promote products
      derived from this software without specific prior written permission.
    * The class name must remain as Cm_Cache_Backend_Redis.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER BE LIABLE FOR ANY
DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/**
 * Redis adapter for Zend_Cache
 *
 * @copyright  Copyright (c) 2013 Colin Mollenhour (http://colin.mollenhour.com)
 * @license    http://framework.zend.com/license/new-bsd     New BSD License
 * @author     Colin Mollenhour (http://colin.mollenhour.com)
 */
class Cm_Cache_Backend_Redis extends Zend_Cache_Backend implements Zend_Cache_Backend_ExtendedInterface
{

    const SET_IDS         = 'zc:ids';
    const SET_TAGS        = 'zc:tags';

    const PREFIX_KEY      = 'zc:k:';
    const PREFIX_TAG_IDS  = 'zc:ti:';

    const FIELD_DATA      = 'd';
    const FIELD_MTIME     = 'm';
    const FIELD_TAGS      = 't';
    const FIELD_INF       = 'i';

    const MAX_LIFETIME    = 2592000; /* Redis backend limit */
    const COMPRESS_PREFIX = ":\x1f\x8b";
    const DEFAULT_CONNECT_TIMEOUT = 2.5;
    const DEFAULT_CONNECT_RETRIES = 1;

    const LUA_SAVE_SH1 = '1617c9fb2bda7d790bb1aaa320c1099d81825e64';
    const LUA_CLEAN_SH1 = '42ab2fe548aee5ff540123687a2c39a38b54e4a2';
    const LUA_GC_SH1 = 'c00416b970f1aa6363b44965d4cf60ee99a6f065';

    /** @var Credis_Client */
    protected $_redis;

    /** @var bool */
    protected $_notMatchingTags = FALSE;

    /** @var int */
    protected $_lifetimelimit = self::MAX_LIFETIME; /* Redis backend limit */

    /** @var int */
    protected $_compressTags = 1;

    /** @var int */
    protected $_compressData = 1;

    /** @var int */
    protected $_compressThreshold = 20480;

    /** @var string */
    protected $_compressionLib;

    /** @var bool */
    protected $_useLua = false;

    /**
     * Lua's unpack() has a limit on the size of the table imposed by
     * the number of Lua stack slots that a C function can use.
     * This value is defined by LUAI_MAXCSTACK in luaconf.h and for Redis it is set to 8000.
     *
     * @see https://github.com/antirez/redis/blob/b903145/deps/lua/src/luaconf.h#L439
     * @var int
     */
    protected $_luaMaxCStack = 5000;

    /**
     * Contruct Zend_Cache Redis backend
     * @param array $options
     * @return \Cm_Cache_Backend_Redis
     */
    public function __construct($options = array())
    {
        if ( empty($options['server']) ) {
            Zend_Cache::throwException('Redis \'server\' not specified.');
        }

        if ( empty($options['port']) && substr($options['server'],0,1) != '/' ) {
            Zend_Cache::throwException('Redis \'port\' not specified.');
        }

        $port = isset($options['port']) ? $options['port'] : NULL;
        $timeout = isset($options['timeout']) ? $options['timeout'] : self::DEFAULT_CONNECT_TIMEOUT;
        $persistent = isset($options['persistent']) ? $options['persistent'] : '';
        $this->_redis = new Credis_Client($options['server'], $port, $timeout, $persistent);

        if ( isset($options['force_standalone']) && $options['force_standalone']) {
            $this->_redis->forceStandalone();
        }

        $connectRetries = isset($options['connect_retries']) ? (int)$options['connect_retries'] : self::DEFAULT_CONNECT_RETRIES;
        $this->_redis->setMaxConnectRetries($connectRetries);

        if ( ! empty($options['read_timeout']) && $options['read_timeout'] > 0) {
            $this->_redis->setReadTimeout((float) $options['read_timeout']);
        }

        if ( ! empty($options['password'])) {
            $this->_redis->auth($options['password']) || Zend_Cache::throwException('Unable to authenticate with the redis server.');
        }

        // Always select database on startup in case persistent connection is re-used by other code
        if (empty($options['database'])) {
            $options['database'] = 0;
        }
        $this->_redis->select( (int) $options['database']) || Zend_Cache::throwException('The redis database could not be selected.');

        if ( isset($options['notMatchingTags']) ) {
            $this->_notMatchingTags = (bool) $options['notMatchingTags'];
        }

        if ( isset($options['compress_tags'])) {
            $this->_compressTags = (int) $options['compress_tags'];
        }

        if ( isset($options['compress_data'])) {
            $this->_compressData = (int) $options['compress_data'];
        }

        if ( isset($options['lifetimelimit'])) {
            $this->_lifetimelimit = (int) min($options['lifetimelimit'], self::MAX_LIFETIME);
        }

        if ( isset($options['compress_threshold'])) {
            $this->_compressThreshold = (int) $options['compress_threshold'];
        }

        if ( isset($options['automatic_cleaning_factor']) ) {
            $this->_options['automatic_cleaning_factor'] = (int) $options['automatic_cleaning_factor'];
        } else {
            $this->_options['automatic_cleaning_factor'] = 0;
        }

        if ( isset($options['compression_lib']) ) {
            $this->_compressionLib = (string) $options['compression_lib'];
        }
        else if ( function_exists('snappy_compress') ) {
            $this->_compressionLib = 'snappy';
        }
        else if ( function_exists('lz4_compress')) {
            $this->_compressionLib = 'l4z';
        }
        else if ( function_exists('lzf_compress') ) {
            $this->_compressionLib = 'lzf';
        }
        else {
            $this->_compressionLib = 'gzip';
        }
        $this->_compressPrefix = substr($this->_compressionLib,0,2).self::COMPRESS_PREFIX;

        if (isset($options['use_lua'])) {
            $this->_useLua = (bool) $options['use_lua'];
        }

        if (isset($options['lua_max_c_stack'])) {
            $this->_luaMaxCStack = (int) $options['lua_max_c_stack'];
        }
    }

    /**
     * Load value with given id from cache
     *
     * @param  string  $id                     Cache id
     * @param  boolean $doNotTestCacheValidity If set to true, the cache validity won't be tested
     * @return bool|string
     */
    public function load($id, $doNotTestCacheValidity = false)
    {
        $data = $this->_redis->hGet(self::PREFIX_KEY.$id, self::FIELD_DATA);
        if ($data === NULL) {
            return FALSE;
        }
        return $this->_decodeData($data);
    }

    /**
     * Test if a cache is available or not (for the given id)
     *
     * @param  string $id Cache id
     * @return bool|int False if record is not available or "last modified" timestamp of the available cache record
     */
    public function test($id)
    {
        $mtime = $this->_redis->hGet(self::PREFIX_KEY.$id, self::FIELD_MTIME);
        return ($mtime ? $mtime : FALSE);
    }

    /**
     * Save some string datas into a cache record
     *
     * Note : $data is always "string" (serialization is done by the
     * core not by the backend)
     *
     * @param  string $data             Datas to cache
     * @param  string $id               Cache id
     * @param  array  $tags             Array of strings, the cache record will be tagged by each string entry
     * @param  bool|int $specificLifetime If != false, set a specific lifetime for this cache record (null => infinite lifetime)
     * @throws CredisException
     * @return boolean True if no problem
     */
    public function save($data, $id, $tags = array(), $specificLifetime = false)
    {
        if(!is_array($tags))
            $tags = $tags ? array($tags) : array();
        else
            $tags = array_flip(array_flip($tags));

        $lifetime = $this->getLifetime($specificLifetime);

        if ($this->_useLua) {
            $sArgs = array(
                self::PREFIX_KEY,
                self::FIELD_DATA,
                self::FIELD_TAGS,
                self::FIELD_MTIME,
                self::FIELD_INF,
                self::SET_TAGS,
                self::PREFIX_TAG_IDS,
                self::SET_IDS,
                $id,
                $this->_encodeData($data, $this->_compressData),
                $this->_encodeData(implode(',',$tags), $this->_compressTags),
                time(),
                $lifetime ? 0 : 1,
                min($lifetime, self::MAX_LIFETIME),
                $this->_notMatchingTags ? 1 : 0
            );

            $res = $this->_redis->evalSha(self::LUA_SAVE_SH1, $tags, $sArgs);
            if (is_null($res)) {
                $script =
                    "local oldTags = redis.call('HGET', ARGV[1]..ARGV[9], ARGV[3]) ".
                    "redis.call('HMSET', ARGV[1]..ARGV[9], ARGV[2], ARGV[10], ARGV[3], ARGV[11], ARGV[4], ARGV[12], ARGV[5], ARGV[13]) ".
                    "if (ARGV[13] == '0') then ".
                        "redis.call('EXPIRE', ARGV[1]..ARGV[9], ARGV[14]) ".
                    "end ".
                    "if next(KEYS) ~= nil then ".
                        "redis.call('SADD', ARGV[6], unpack(KEYS)) ".
                        "for _, tagname in ipairs(KEYS) do ".
                            "redis.call('SADD', ARGV[7]..tagname, ARGV[9]) ".
                        "end ".
                    "end ".
                    "if (ARGV[15] == '1') then ".
                        "redis.call('SADD', ARGV[8], ARGV[9]) ".
                    "end ".
                    "if (oldTags ~= false) then ".
                        "return oldTags ".
                    "else ".
                        "return '' ".
                    "end";
                $res = $this->_redis->eval($script, $tags, $sArgs);
            }

            // Process removed tags if cache entry already existed
            if ($res) {
                $oldTags = explode(',', $this->_decodeData($res));
                if ($remTags = ($oldTags ? array_diff($oldTags, $tags) : FALSE))
                {
                    // Update the id list for each tag
                    foreach($remTags as $tag)
                    {
                        $this->_redis->sRem(self::PREFIX_TAG_IDS . $tag, $id);
                    }
                }
            }

            return TRUE;
        }

        // Get list of tags previously assigned
        $oldTags = $this->_decodeData($this->_redis->hGet(self::PREFIX_KEY.$id, self::FIELD_TAGS));
        $oldTags = $oldTags ? explode(',', $oldTags) : array();

        $this->_redis->pipeline()->multi();

        // Set the data
        $result = $this->_redis->hMSet(self::PREFIX_KEY.$id, array(
          self::FIELD_DATA => $this->_encodeData($data, $this->_compressData),
          self::FIELD_TAGS => $this->_encodeData(implode(',',$tags), $this->_compressTags),
          self::FIELD_MTIME => time(),
          self::FIELD_INF => $lifetime ? 0 : 1,
        ));
        if( ! $result) {
            throw new CredisException("Could not set cache key $id");
        }

        // Set expiration if specified
        if ($lifetime) {
          $this->_redis->expire(self::PREFIX_KEY.$id, min($lifetime, self::MAX_LIFETIME));
        }

        // Process added tags
        if ($tags)
        {
            // Update the list with all the tags
            $this->_redis->sAdd( self::SET_TAGS, $tags);

            // Update the id list for each tag
            foreach($tags as $tag)
            {
                $this->_redis->sAdd(self::PREFIX_TAG_IDS . $tag, $id);
            }
        }

        // Process removed tags
        if ($remTags = ($oldTags ? array_diff($oldTags, $tags) : FALSE))
        {
            // Update the id list for each tag
            foreach($remTags as $tag)
            {
                $this->_redis->sRem(self::PREFIX_TAG_IDS . $tag, $id);
            }
        }

        // Update the list with all the ids
        if($this->_notMatchingTags) {
            $this->_redis->sAdd(self::SET_IDS, $id);
        }

        $this->_redis->exec();

        return TRUE;
    }

    /**
     * Remove a cache record
     *
     * @param  string $id Cache id
     * @return boolean True if no problem
     */
    public function remove($id)
    {
        // Get list of tags for this id
        $tags = explode(',', $this->_decodeData($this->_redis->hGet(self::PREFIX_KEY.$id, self::FIELD_TAGS)));

        $this->_redis->pipeline()->multi();

        // Remove data
        $this->_redis->del(self::PREFIX_KEY.$id);

        // Remove id from list of all ids
        if($this->_notMatchingTags) {
            $this->_redis->sRem( self::SET_IDS, $id );
        }

        // Update the id list for each tag
        foreach($tags as $tag) {
            $this->_redis->sRem(self::PREFIX_TAG_IDS . $tag, $id);
        }

        $result = $this->_redis->exec();

        return (bool) $result[0];
    }

    /**
     * @param array $tags
     */
    protected function _removeByNotMatchingTags($tags)
    {
        $ids = $this->getIdsNotMatchingTags($tags);
        if($ids)
        {
            $this->_redis->pipeline()->multi();

            // Remove data
            $this->_redis->del( $this->_preprocessIds($ids));

            // Remove ids from list of all ids
            if($this->_notMatchingTags) {
                $this->_redis->sRem( self::SET_IDS, $ids);
            }

            $this->_redis->exec();
        }
    }

    /**
     * @param array $tags
     */
    protected function _removeByMatchingTags($tags)
    {
        $ids = $this->getIdsMatchingTags($tags);
        if($ids)
        {
            $this->_redis->pipeline()->multi();

            // Remove data
            $this->_redis->del( $this->_preprocessIds($ids));

            // Remove ids from list of all ids
            if($this->_notMatchingTags) {
                $this->_redis->sRem( self::SET_IDS, $ids);
            }

            $this->_redis->exec();
        }
    }

    /**
     * @param array $tags
     */
    protected function _removeByMatchingAnyTags($tags)
    {
        if ($this->_useLua) {
            $pTags = $this->_preprocessTagIds($tags);
            $sArgs = array(self::PREFIX_KEY, self::SET_TAGS, self::SET_IDS, ($this->_notMatchingTags ? 1 : 0), (int) $this->_luaMaxCStack);
            if ( ! $this->_redis->evalSha(self::LUA_CLEAN_SH1, $pTags, $sArgs)) {
                $script =
                    "for i = 1, #KEYS, ARGV[5] do ".
                        "local keysToDel = redis.call('SUNION', unpack(KEYS, i, math.min(#KEYS, i + ARGV[5] - 1))) ".
                        "for _, keyname in ipairs(keysToDel) do ".
                            "redis.call('DEL', ARGV[1]..keyname) ".
                            "if (ARGV[4] == '1') then ".
                                "redis.call('SREM', ARGV[3], keyname) ".
                            "end ".
                        "end ".
                        "redis.call('DEL', unpack(KEYS, i, math.min(#KEYS, i + ARGV[5] - 1))) ".
                        "redis.call('SREM', ARGV[2], unpack(KEYS, i, math.min(#KEYS, i + ARGV[5] - 1))) ".
                    "end ".
                    "return true";
                $this->_redis->eval($script, $pTags, $sArgs);
            }
            return;
        }

        $ids = $this->getIdsMatchingAnyTags($tags);

        $this->_redis->pipeline()->multi();

        if($ids)
        {
            // Remove data
            $this->_redis->del( $this->_preprocessIds($ids));

            // Remove ids from list of all ids
            if($this->_notMatchingTags) {
                $this->_redis->sRem( self::SET_IDS, $ids);
            }
        }

        // Remove tag id lists
        $this->_redis->del( $this->_preprocessTagIds($tags));

        // Remove tags from list of tags
        $this->_redis->sRem( self::SET_TAGS, $tags);

        $this->_redis->exec();
    }

    /**
     * Clean up tag id lists since as keys expire the ids remain in the tag id lists
     */
    protected function _collectGarbage()
    {
        // Clean up expired keys from tag id set and global id set

        if ($this->_useLua) {
            $sArgs = array(self::PREFIX_KEY, self::SET_TAGS, self::SET_IDS, self::PREFIX_TAG_IDS, ($this->_notMatchingTags ? 1 : 0));
            $allTags = (array) $this->_redis->sMembers(self::SET_TAGS);
            $tagsCount = count($allTags);
            $counter = 0;
            $tagsBatch = array();
            foreach ($allTags as $tag) {
                $tagsBatch[] = $tag;
                $counter++;
                if (count($tagsBatch) == 10 || $counter == $tagsCount ) {
                    if ( ! $this->_redis->evalSha(self::LUA_GC_SH1, $tagsBatch, $sArgs)) {
                        $script =
                            "local tagKeys = {} ".
                            "local expired = {} ".
                            "local expiredCount = 0 ".
                            "local notExpiredCount = 0 ".
                            "for _, tagName in ipairs(KEYS) do ".
                                "tagKeys = redis.call('SMEMBERS', ARGV[4]..tagName) ".
                                "for __, keyName in ipairs(tagKeys) do ".
                                    "if (redis.call('EXISTS', ARGV[1]..keyName) == 0) then ".
                                        "expiredCount = expiredCount + 1 ".
                                        "expired[expiredCount] = keyName ".
                                        /* Redis Lua scripts have a hard limit of 8000 parameters per command */
                                        "if (expiredCount == 7990) then ".
                                            "redis.call('SREM', ARGV[4]..tagName, unpack(expired)) ".
                                            "if (ARGV[5] == '1') then ".
                                                "redis.call('SREM', ARGV[3], unpack(expired)) ".
                                            "end ".
                                            "expiredCount = 0 ".
                                            "expired = {} ".
                                        "end ".
                                    "else ".
                                        "notExpiredCount = notExpiredCount + 1 ".
                                    "end ".
                                "end ".
                                "if (expiredCount > 0) then ".
                                    "redis.call('SREM', ARGV[4]..tagName, unpack(expired)) ".
                                    "if (ARGV[5] == '1') then ".
                                        "redis.call('SREM', ARGV[3], unpack(expired)) ".
                                    "end ".
                                "end ".
                                "if (notExpiredCount == 0) then ".
                                    "redis.call ('DEL', ARGV[4]..tagName) ".
                                    "redis.call ('SREM', ARGV[2], tagName) ".
                                "end ".
                                "expired = {} ".
                                "expiredCount = 0 ".
                                "notExpiredCount = 0 ".
                            "end ".
                            "return true";
                        $this->_redis->eval($script, $tagsBatch, $sArgs);
                    }
                    $tagsBatch = array();
                    /* Give Redis some time to handle other requests */
                    usleep(20000);
                }
            }
            return;
        }

        $exists = array();
        $tags = (array) $this->_redis->sMembers(self::SET_TAGS);
        foreach($tags as $tag)
        {
            // Get list of expired ids for each tag
            $tagMembers = $this->_redis->sMembers(self::PREFIX_TAG_IDS . $tag);
            $numTagMembers = count($tagMembers);
            $expired = array();
            $numExpired = $numNotExpired = 0;
            if($numTagMembers) {
                while ($id = array_pop($tagMembers)) {
                    if( ! isset($exists[$id])) {
                        $exists[$id] = $this->_redis->exists(self::PREFIX_KEY.$id);
                    }
                    if ($exists[$id]) {
                        $numNotExpired++;
                    }
                    else {
                        $numExpired++;
                        $expired[] = $id;

                        // Remove incrementally to reduce memory usage
                        if (count($expired) % 100 == 0 && $numNotExpired > 0) {
                            $this->_redis->sRem( self::PREFIX_TAG_IDS . $tag, $expired);
                            if($this->_notMatchingTags) { // Clean up expired ids from ids set
                                $this->_redis->sRem( self::SET_IDS, $expired);
                            }
                            $expired = array();
                        }
                    }
                }
                if( ! count($expired)) continue;
            }

            // Remove empty tags or completely expired tags
            if ($numExpired == $numTagMembers) {
                $this->_redis->del(self::PREFIX_TAG_IDS . $tag);
                $this->_redis->sRem(self::SET_TAGS, $tag);
            }
            // Clean up expired ids from tag ids set
            else if (count($expired)) {
                $this->_redis->sRem( self::PREFIX_TAG_IDS . $tag, $expired);
                if($this->_notMatchingTags) { // Clean up expired ids from ids set
                    $this->_redis->sRem( self::SET_IDS, $expired);
                }
            }
            unset($expired);
        }

        // Clean up global list of ids for ids with no tag
        if($this->_notMatchingTags) {
            // TODO
        }
    }

    /**
     * Clean some cache records
     *
     * Available modes are :
     * 'all' (default)  => remove all cache entries ($tags is not used)
     * 'old'            => runs _collectGarbage()
     * 'matchingTag'    => supported
     * 'notMatchingTag' => supported
     * 'matchingAnyTag' => supported
     *
     * @param  string $mode Clean mode
     * @param  array  $tags Array of tags
     * @throws Zend_Cache_Exception
     * @return boolean True if no problem
     */
    public function clean($mode = Zend_Cache::CLEANING_MODE_ALL, $tags = array())
    {
        if( $tags && ! is_array($tags)) {
            $tags = array($tags);
        }

        try {
            if ($mode == Zend_Cache::CLEANING_MODE_ALL) {
                return $this->_redis->flushDb();
            }
            if ($mode == Zend_Cache::CLEANING_MODE_OLD) {
                $this->_collectGarbage();
                return TRUE;
            }
            if ( ! count($tags)) {
                return TRUE;
            }
            switch ($mode)
            {
                case Zend_Cache::CLEANING_MODE_MATCHING_TAG:

                    $this->_removeByMatchingTags($tags);
                    break;

                case Zend_Cache::CLEANING_MODE_NOT_MATCHING_TAG:

                    $this->_removeByNotMatchingTags($tags);
                    break;

                case Zend_Cache::CLEANING_MODE_MATCHING_ANY_TAG:

                    $this->_removeByMatchingAnyTags($tags);
                    break;

                default:
                    Zend_Cache::throwException('Invalid mode for clean() method: '.$mode);
            }
        } catch (CredisException $e) {
            Zend_Cache::throwException('Error cleaning cache by mode '.$mode.': '.$e->getMessage(), $e);
        }
        return TRUE;
    }

    /**
     * Return true if the automatic cleaning is available for the backend
     *
     * @return boolean
     */
    public function isAutomaticCleaningAvailable()
    {
        return TRUE;
    }

    /**
     * Set the frontend directives
     *
     * @param  array $directives Assoc of directives
     * @throws Zend_Cache_Exception
     * @return void
     */
    public function setDirectives($directives)
    {
        parent::setDirectives($directives);
        $lifetime = $this->getLifetime(false);
        if ($lifetime > self::MAX_LIFETIME) {
            Zend_Cache::throwException('Redis backend has a limit of 30 days (2592000 seconds) for the lifetime');
        }
    }

    /**
     * Return an array of stored cache ids
     *
     * @return array array of stored cache ids (string)
     */
    public function getIds()
    {
        if($this->_notMatchingTags) {
            return (array) $this->_redis->sMembers(self::SET_IDS);
        } else {
            $keys = $this->_redis->keys(self::PREFIX_KEY . '*');
            $prefixLen = strlen(self::PREFIX_KEY);
            foreach($keys as $index => $key) {
                $keys[$index] = substr($key, $prefixLen);
            }
            return $keys;
        }
    }

    /**
     * Return an array of stored tags
     *
     * @return array array of stored tags (string)
     */
    public function getTags()
    {
        return (array) $this->_redis->sMembers(self::SET_TAGS);
    }

    /**
     * Return an array of stored cache ids which match given tags
     *
     * In case of multiple tags, a logical AND is made between tags
     *
     * @param array $tags array of tags
     * @return array array of matching cache ids (string)
     */
    public function getIdsMatchingTags($tags = array())
    {
        if ($tags) {
            return (array) $this->_redis->sInter( $this->_preprocessTagIds($tags) );
        }
        return array();
    }

    /**
     * Return an array of stored cache ids which don't match given tags
     *
     * In case of multiple tags, a negated logical AND is made between tags
     *
     * @param array $tags array of tags
     * @return array array of not matching cache ids (string)
     */
    public function getIdsNotMatchingTags($tags = array())
    {
        if( ! $this->_notMatchingTags) {
            Zend_Cache::throwException("notMatchingTags is currently disabled.");
        }
        if ($tags) {
            return (array) $this->_redis->sDiff( self::SET_IDS, $this->_preprocessTagIds($tags) );
        }
        return (array) $this->_redis->sMembers( self::SET_IDS );
    }

    /**
     * Return an array of stored cache ids which match any given tags
     *
     * In case of multiple tags, a logical OR is made between tags
     *
     * @param array $tags array of tags
     * @return array array of any matching cache ids (string)
     */
    public function getIdsMatchingAnyTags($tags = array())
    {
        if ($tags) {
            return (array) $this->_redis->sUnion( $this->_preprocessTagIds($tags));
        }
        return array();
    }

    /**
     * Return the filling percentage of the backend storage
     *
     * @throws Zend_Cache_Exception
     * @return int integer between 0 and 100
     */
    public function getFillingPercentage()
    {
        $maxMem = $this->_redis->config('GET','maxmemory');
        if (0 == (int) $maxMem['maxmemory']) {
            return 1;
        }
        $info = $this->_redis->info();
        return round(
            ($info['used_memory']/$maxMem['maxmemory']*100)
            ,0
            ,PHP_ROUND_HALF_UP
        );
    }

    /**
     * Return an array of metadatas for the given cache id
     *
     * The array must include these keys :
     * - expire : the expire timestamp
     * - tags : a string array of tags
     * - mtime : timestamp of last modification time
     *
     * @param string $id cache id
     * @return array array of metadatas (false if the cache id is not found)
     */
    public function getMetadatas($id)
    {
        list($tags, $mtime, $inf) = $this->_redis->hMGet(self::PREFIX_KEY.$id, array(self::FIELD_TAGS, self::FIELD_MTIME, self::FIELD_INF));
        if( ! $mtime) {
          return FALSE;
        }
        $tags = explode(',', $this->_decodeData($tags));
        $expire = $inf === '1' ? FALSE : time() + $this->_redis->ttl(self::PREFIX_KEY.$id);

        return array(
            'expire' => $expire,
            'tags'   => $tags,
            'mtime'  => $mtime,
        );
    }

    /**
     * Give (if possible) an extra lifetime to the given cache id
     *
     * @param string $id cache id
     * @param int $extraLifetime
     * @return boolean true if ok
     */
    public function touch($id, $extraLifetime)
    {
        list($inf) = $this->_redis->hGet(self::PREFIX_KEY.$id, self::FIELD_INF);
        if ($inf === '0') {
            $expireAt = time() + $this->_redis->ttl(self::PREFIX_KEY.$id) + $extraLifetime;
            return (bool) $this->_redis->expireAt(self::PREFIX_KEY.$id, $expireAt);
        }
        return false;
    }

    /**
     * Return an associative array of capabilities (booleans) of the backend
     *
     * The array must include these keys :
     * - automatic_cleaning (is automating cleaning necessary)
     * - tags (are tags supported)
     * - expired_read (is it possible to read expired cache records
     *                 (for doNotTestCacheValidity option for example))
     * - priority does the backend deal with priority when saving
     * - infinite_lifetime (is infinite lifetime can work with this backend)
     * - get_list (is it possible to get the list of cache ids and the complete list of tags)
     *
     * @return array associative of with capabilities
     */
    public function getCapabilities()
    {
        return array(
            'automatic_cleaning' => ($this->_options['automatic_cleaning_factor'] > 0),
            'tags'               => true,
            'expired_read'       => false,
            'priority'           => false,
            'infinite_lifetime'  => true,
            'get_list'           => true,
        );
    }

    /**
     * @param string $data
     * @param int $level
     * @throws CredisException
     * @return string
     */
    protected function _encodeData($data, $level)
    {
        if ($this->_compressionLib && $level && strlen($data) >= $this->_compressThreshold) {
            switch($this->_compressionLib) {
                case 'snappy': $data = snappy_compress($data); break;
                case 'lzf':    $data = lzf_compress($data); break;
                case 'l4z':    $data = lz4_compress($data,($level > 1 ? true : false)); break;
                case 'gzip':   $data = gzcompress($data, $level); break;
                default:       throw new CredisException("Unrecognized 'compression_lib'.");
            }
            if( ! $data) {
                throw new CredisException("Could not compress cache data.");
            }
            return $this->_compressPrefix.$data;
        }
        return $data;
    }

    /**
     * @param bool|string $data
     * @return string
     */
    protected function _decodeData($data)
    {
        if (substr($data,2,3) == self::COMPRESS_PREFIX) {
            switch(substr($data,0,2)) {
                case 'sn': return snappy_uncompress(substr($data,5));
                case 'lz': return lzf_decompress(substr($data,5));
                case 'l4': return lz4_uncompress(substr($data,5));
                case 'gz': case 'zc': return gzuncompress(substr($data,5));
            }
        }
        return $data;
    }

    /**
     * @param $item
     * @param $index
     * @param $prefix
     */
    protected function _preprocess(&$item, $index, $prefix)
    {
        $item = $prefix . $item;
    }

    /**
     * @param $ids
     * @return array
     */
    protected function _preprocessIds($ids)
    {
        array_walk($ids, array($this, '_preprocess'), self::PREFIX_KEY);
        return $ids;
    }

    /**
     * @param $tags
     * @return array
     */
    protected function _preprocessTagIds($tags)
    {
        array_walk($tags, array($this, '_preprocess'), self::PREFIX_TAG_IDS);
        return $tags;
    }

    /**
     * Required to pass unit tests
     *
     * @param  string $id
     * @return void
     */
    public function ___expire($id)
    {
        $this->_redis->del(self::PREFIX_KEY.$id);
    }

}
