class DrupalDatabaseCache

class DrupalDatabaseCache implements DrupalCacheInterface {
    protected $bin;
    
    /**
     * Constructs a DrupalDatabaseCache object.
     *
     * @param $bin
     *   The cache bin for which the object is created.
     */
    function __construct($bin) {
        $this->bin = $bin;
    }
    
    /**
     * Implements DrupalCacheInterface::get().
     */
    function get($cid) {
        $cids = array(
            $cid,
        );
        $cache = $this->getMultiple($cids);
        return reset($cache);
    }
    
    /**
     * Implements DrupalCacheInterface::getMultiple().
     */
    function getMultiple(&$cids) {
        try {
            // Garbage collection necessary when enforcing a minimum cache lifetime.
            $this->garbageCollection($this->bin);
            // When serving cached pages, the overhead of using db_select() was found
            // to add around 30% overhead to the request. Since $this->bin is a
            // variable, this means the call to db_query() here uses a concatenated
            // string. This is highly discouraged under any other circumstances, and
            // is used here only due to the performance overhead we would incur
            // otherwise. When serving an uncached page, the overhead of using
            // db_select() is a much smaller proportion of the request.
            $result = db_query('SELECT cid, data, created, expire, serialized FROM {' . db_escape_table($this->bin) . '} WHERE cid IN (:cids)', array(
                ':cids' => $cids,
            ));
            $cache = array();
            foreach ($result as $item) {
                $item = $this->prepareItem($item);
                if ($item) {
                    $cache[$item->cid] = $item;
                }
            }
            $cids = array_diff($cids, array_keys($cache));
            return $cache;
        } catch (Exception $e) {
            // If the database is never going to be available, cache requests should
            // return FALSE in order to allow exception handling to occur.
            return array();
        }
    }
    
    /**
     * Garbage collection for get() and getMultiple().
     *
     * @param $bin
     *   The bin being requested.
     */
    protected function garbageCollection() {
        $cache_lifetime = variable_get('cache_lifetime', 0);
        // Clean-up the per-user cache expiration session data, so that the session
        // handler can properly clean-up the session data for anonymous users.
        if (isset($_SESSION['cache_expiration'])) {
            $expire = REQUEST_TIME - $cache_lifetime;
            foreach ($_SESSION['cache_expiration'] as $bin => $timestamp) {
                if ($timestamp < $expire) {
                    unset($_SESSION['cache_expiration'][$bin]);
                }
            }
            if (!$_SESSION['cache_expiration']) {
                unset($_SESSION['cache_expiration']);
            }
        }
        // Garbage collection of temporary items is only necessary when enforcing
        // a minimum cache lifetime.
        if (!$cache_lifetime) {
            return;
        }
        // When cache lifetime is in force, avoid running garbage collection too
        // often since this will remove temporary cache items indiscriminately.
        $cache_flush = variable_get('cache_flush_' . $this->bin, 0);
        if ($cache_flush && $cache_flush + $cache_lifetime <= REQUEST_TIME) {
            // Reset the variable immediately to prevent a meltdown in heavy load situations.
            variable_set('cache_flush_' . $this->bin, 0);
            // Time to flush old cache data
            db_delete($this->bin)
                ->condition('expire', CACHE_PERMANENT, '<>')
                ->condition('expire', $cache_flush, '<=')
                ->execute();
        }
    }
    
    /**
     * Prepares a cached item.
     *
     * Checks that items are either permanent or did not expire, and unserializes
     * data as appropriate.
     *
     * @param $cache
     *   An item loaded from cache_get() or cache_get_multiple().
     *
     * @return
     *   The item with data unserialized as appropriate or FALSE if there is no
     *   valid item to load.
     */
    protected function prepareItem($cache) {
        global $user;
        if (!isset($cache->data)) {
            return FALSE;
        }
        // If the cached data is temporary and subject to a per-user minimum
        // lifetime, compare the cache entry timestamp with the user session
        // cache_expiration timestamp. If the cache entry is too old, ignore it.
        if ($cache->expire != CACHE_PERMANENT && variable_get('cache_lifetime', 0) && isset($_SESSION['cache_expiration'][$this->bin]) && $_SESSION['cache_expiration'][$this->bin] > $cache->created) {
            // Ignore cache data that is too old and thus not valid for this user.
            return FALSE;
        }
        // If the data is permanent or not subject to a minimum cache lifetime,
        // unserialize and return the cached data.
        if ($cache->serialized) {
            $cache->data = unserialize($cache->data);
        }
        return $cache;
    }
    
    /**
     * Implements DrupalCacheInterface::set().
     */
    function set($cid, $data, $expire = CACHE_PERMANENT) {
        $fields = array(
            'serialized' => 0,
            'created' => REQUEST_TIME,
            'expire' => $expire,
        );
        if (!is_string($data)) {
            $fields['data'] = serialize($data);
            $fields['serialized'] = 1;
        }
        else {
            $fields['data'] = $data;
            $fields['serialized'] = 0;
        }
        try {
            db_merge($this->bin)
                ->key(array(
                'cid' => $cid,
            ))
                ->fields($fields)
                ->execute();
        } catch (Exception $e) {
            // The database may not be available, so we'll ignore cache_set requests.
        }
    }
    
    /**
     * Implements DrupalCacheInterface::clear().
     */
    function clear($cid = NULL, $wildcard = FALSE) {
        global $user;
        if (empty($cid)) {
            if (variable_get('cache_lifetime', 0)) {
                // We store the time in the current user's session. We then simulate
                // that the cache was flushed for this user by not returning cached
                // data that was cached before the timestamp.
                $_SESSION['cache_expiration'][$this->bin] = REQUEST_TIME;
                $cache_flush = variable_get('cache_flush_' . $this->bin, 0);
                if ($cache_flush == 0) {
                    // This is the first request to clear the cache, start a timer.
                    variable_set('cache_flush_' . $this->bin, REQUEST_TIME);
                }
                elseif (REQUEST_TIME > $cache_flush + variable_get('cache_lifetime', 0)) {
                    // Clear the cache for everyone, cache_lifetime seconds have
                    // passed since the first request to clear the cache.
                    db_delete($this->bin)
                        ->condition('expire', CACHE_PERMANENT, '<>')
                        ->condition('expire', REQUEST_TIME, '<')
                        ->execute();
                    variable_set('cache_flush_' . $this->bin, 0);
                }
            }
            else {
                // No minimum cache lifetime, flush all temporary cache entries now.
                db_delete($this->bin)
                    ->condition('expire', CACHE_PERMANENT, '<>')
                    ->condition('expire', REQUEST_TIME, '<')
                    ->execute();
            }
        }
        else {
            if ($wildcard) {
                if ($cid == '*') {
                    // Check if $this->bin is a cache table before truncating. Other
                    // cache_clear_all() operations throw a PDO error in this situation,
                    // so we don't need to verify them first. This ensures that non-cache
                    // tables cannot be truncated accidentally.
                    if ($this->isValidBin()) {
                        db_truncate($this->bin)
                            ->execute();
                    }
                    else {
                        throw new Exception(t('Invalid or missing cache bin specified: %bin', array(
                            '%bin' => $this->bin,
                        )));
                    }
                }
                else {
                    db_delete($this->bin)
                        ->condition('cid', db_like($cid) . '%', 'LIKE')
                        ->execute();
                }
            }
            elseif (is_array($cid)) {
                // Delete in chunks when a large array is passed.
                do {
                    db_delete($this->bin)
                        ->condition('cid', array_splice($cid, 0, 1000), 'IN')
                        ->execute();
                } while (count($cid));
            }
            else {
                db_delete($this->bin)
                    ->condition('cid', $cid)
                    ->execute();
            }
        }
    }
    
    /**
     * Implements DrupalCacheInterface::isEmpty().
     */
    function isEmpty() {
        $this->garbageCollection();
        $query = db_select($this->bin);
        $query->addExpression('1');
        $result = $query->range(0, 1)
            ->execute()
            ->fetchField();
        return empty($result);
    }
    
    /**
     * Checks if $this->bin represents a valid cache table.
     *
     * This check is required to ensure that non-cache tables are not truncated
     * accidentally when calling cache_clear_all().
     *
     * @return boolean
     */
    function isValidBin() {
        if ($this->bin == 'cache' || substr($this->bin, 0, 6) == 'cache_') {
            // Skip schema check for bins with standard table names.
            return TRUE;
        }
        // These fields are required for any cache table.
        $fields = array(
            'cid',
            'data',
            'expire',
            'created',
            'serialized',
        );
        // Load the table schema.
        $schema = drupal_get_schema($this->bin);
        // Confirm that all fields are present.
        return isset($schema['fields']) && !array_diff($fields, array_keys($schema['fields']));
    }

}