object-cache.php

<?php
/*
Plugin Name: WordPress APCu Object Cache Backend
Plugin URI: https://github.com/l3rady/object-cache-apcu
Description: APCu backend for WordPress' Object Cache
Version: 1.2
Author: Scott Cariss
Author URI: https://cariss.dev
*/

/*  Copyright 2024  Scott Cariss  (email : scott@cariss.dev)

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/

// Stop direct access
defined('ABSPATH') or exit;


/**
 * Adds data to the cache, if the cache key does not already exist.
 *
 * @param int|string $key The cache key to use for retrieval later
 * @param mixed $data The data to add to the cache store
 * @param string $group The group to add the cache to
 * @param int $expire When the cache data should be expired
 *
 * @return bool False if cache key and group already exist, true on success
 */
function wp_cache_add($key, $data, $group = 'default', $expire = 0)
{
    return WP_Object_Cache::instance()->add($key, $data, $group, $expire);
}


/**
 * Adds multiple values to the cache in one call.
 *
 * @param array  $data   Array of keys and values to be set.
 * @param string $group  Optional. Where the cache contents are grouped. Default default.
 * @param int    $expire Optional. When to expire the cache contents, in seconds.
 *                       Default 0 (no expiration).
 * @return bool[] Array of return values, grouped by key. Each value is either
 *                true on success, or false if cache key and group already exist.
 */
function wp_cache_add_multiple(array $data, $group = 'default', $expire = 0)
{
	return WP_Object_Cache::instance()->add_multiple($data, $group, $expire);
}


/**
 * Closes the cache.
 *
 * This function has ceased to do anything since WordPress 2.5. The
 * functionality was removed along with the rest of the persistent cache. This
 * does not mean that plugins can't implement this function when they need to
 * make sure that the cache is cleaned up after WordPress no longer needs it.
 *
 * @return bool Always returns True
 */
function wp_cache_close()
{
    return true;
}


/**
 * Decrement numeric cache item's value
 *
 * @param int|string $key The cache key to increment
 * @param int $offset The amount by which to decrement the item's value. Default is 1.
 * @param string $group The group the key is in.
 *
 * @return false|int False on failure, the item's new value on success.
 */
function wp_cache_decr($key, $offset = 1, $group = 'default')
{
    return WP_Object_Cache::instance()->decr($key, $offset, $group);
}


/**
 * Removes the cache contents matching key and group.
 *
 * @param int|string $key What the contents in the cache are called
 * @param string $group Where the cache contents are grouped
 *
 * @return bool True on successful removal, false on failure
 */
function wp_cache_delete($key, $group = 'default')
{
    return WP_Object_Cache::instance()->delete($key, $group);
}


/**
 * Deletes multiple values from the cache in one call.
 *
 * @param array  $keys  Array of keys under which the cache to deleted.
 * @param string $group Optional. Where the cache contents are grouped. Default default.
 * @return bool[] Array of return values, grouped by key. Each value is either
 *                true on success, or false if the contents were not deleted.
 */
function wp_cache_delete_multiple(array $keys, $group = 'default')
{
	return WP_Object_Cache::instance()->delete_multiple($keys, $group);
}


/**
 * Removes all cache items.
 *
 * @return bool False on failure, true on success
 */
function wp_cache_flush()
{
    return WP_Object_Cache::instance()->flush();
}


/**
 * Removes all cache items from the in-memory runtime cache.
 *
 * @return bool True on success, false on failure.
 */
function wp_cache_flush_runtime()
{
    return WP_Object_Cache::instance()->flush_runtime();
}


/**
 * Retrieves the cache contents from the cache by key and group.
 *
 * @param int|string $key What the contents in the cache are called
 * @param string $group Where the cache contents are grouped
 * @param bool $force Does nothing with APCu object cache
 * @param bool       &$found Whether key was found in the cache. Disambiguates a return of false, a storable value.
 *
 * @return bool|mixed False on failure to retrieve contents or the cache contents on success
 */
function wp_cache_get($key, $group = 'default', $force = false, &$found = null)
{
    return WP_Object_Cache::instance()->get($key, $group, $force, $found);
}


/**
 * Retrieves multiple values from the cache in one call.
 *
 * @param array  $keys  Array of keys under which the cache contents are stored.
 * @param string $group Optional. Where the cache contents are grouped. Default empty.
 * @param bool   $force Optional. Whether to force an update of the local cache
 *                      from the persistent cache. Default false.
 * @return array Array of return values, grouped by key. Each value is either
 *               the cache contents on success, or false on failure.
 */
function wp_cache_get_multiple($keys, $group = 'default', $force = false)
{
    return WP_Object_Cache::instance()->get_multiple($keys, $group, $force);
}


/**
 * Increment numeric cache item's value
 *
 * @param int|string $key The cache key to increment
 * @param int $offset The amount by which to increment the item's value. Default is 1.
 * @param string $group The group the key is in.
 *
 * @return false|int False on failure, the item's new value on success.
 */
function wp_cache_incr($key, $offset = 1, $group = 'default')
{
    return WP_Object_Cache::instance()->incr($key, $offset, $group);
}


/**
 * Sets up Object Cache Global and assigns it.
 *
 * @global WP_Object_Cache $wp_object_cache WordPress Object Cache
 */
function wp_cache_init()
{
    $GLOBALS['wp_object_cache'] = WP_Object_Cache::instance();
}


/**
 * Replaces the contents of the cache with new data.
 *
 * @param int|string $key What to call the contents in the cache
 * @param mixed $data The contents to store in the cache
 * @param string $group Where to group the cache contents
 * @param int $expire When to expire the cache contents
 *
 * @return bool False if not exists, true if contents were replaced
 */
function wp_cache_replace($key, $data, $group = 'default', $expire = 0)
{
    return WP_Object_Cache::instance()->replace($key, $data, $group, $expire);
}


/**
 * Saves the data to the cache.
 *
 * @param int|string $key What to call the contents in the cache
 * @param mixed $data The contents to store in the cache
 * @param string $group Where to group the cache contents
 * @param int $expire When to expire the cache contents
 *
 * @return bool False on failure, true on success
 */
function wp_cache_set($key, $data, $group = 'default', $expire = 0)
{
    return WP_Object_Cache::instance()->set($key, $data, $group, $expire);
}


/**
 * Sets multiple values to the cache in one call.
 *
 * @param array  $data   Array of keys and values to be set.
 * @param string $group  Optional. Where the cache contents are grouped. Default default.
 * @param int    $expire Optional. When to expire the cache contents, in seconds.
 *                       Default 0 (no expiration).
 * @return bool[] Array of return values, grouped by key. Each value is either
 *                true on success, or false on failure.
 */
function wp_cache_set_multiple(array $data, $group = 'default', $expire = 0)
{
    return WP_Object_Cache::instance()->set_multiple($data, $group, $expire);
}


/**
 * Switch the internal blog id.
 *
 * This changes the blog id used to create keys in blog specific groups.
 *
 * @param int $blog_id Blog ID
 */
function wp_cache_switch_to_blog($blog_id)
{
    WP_Object_Cache::instance()->switch_to_blog($blog_id);
}


/**
 * Adds a group or set of groups to the list of global groups.
 *
 * @param string|array $groups A group or an array of groups to add
 */
function wp_cache_add_global_groups($groups)
{
    WP_Object_Cache::instance()->add_global_groups($groups);
}


/**
 * Adds a group or set of groups to the list of non-persistent groups.
 *
 * @param string|array $groups A group or an array of groups to add
 */
function wp_cache_add_non_persistent_groups($groups)
{
    WP_Object_Cache::instance()->add_non_persistent_groups($groups);
}


/**
 * Function was depreciated and now does nothing
 *
 * @return bool Always returns false
 */
function wp_cache_reset()
{
    _deprecated_function(__FUNCTION__, '3.5', 'wp_cache_switch_to_blog()');
    return false;
}


/**
 * Invalidate a site's object cache
 *
 * @param mixed $sites Sites ID's that want flushing.
 *                     Don't pass a site to flush current site
 *
 * @return bool
 */
function wp_cache_flush_site($sites = null)
{
    return WP_Object_Cache::instance()->flush_sites($sites);
}


/**
 * Invalidate a groups object cache
 *
 * @param mixed $groups A group or an array of groups to invalidate
 *
 * @return bool
 */
function wp_cache_flush_group($groups = 'default')
{
    return WP_Object_Cache::instance()->flush_groups($groups);
}


/**
 * Determines whether the object cache implementation supports a particular feature.
 *
 * @since 6.1.0
 *
 * @param string $feature Name of the feature to check for. Possible values include:
 *                        'add_multiple', 'set_multiple', 'get_multiple', 'delete_multiple',
 *                        'flush_runtime', 'flush_group'.
 * @return bool True if the feature is supported, false otherwise.
 */
function wp_cache_supports( $feature ) {
    switch ( $feature ) {
        case 'add_multiple':
        case 'set_multiple':
        case 'get_multiple':
        case 'delete_multiple':
        case 'flush_runtime':
        case 'flush_group':
            return true;

    default:
        return false;
    }
}


/**
 * WordPress APCu Object Cache Backend
 *
 * The WordPress Object Cache is used to save on trips to the database. The
 * APCu Object Cache stores all of the cache data to APCu and makes the cache
 * contents available by using a key, which is used to name and later retrieve
 * the cache contents.
 */
class WP_Object_Cache
{

    /**
     * @var string MD5 hash of the current installation ABSPATH
     */
    private $_absPath;

    /**
     * @var bool Stores if APCu is available.
     */
    private $_apcuAvailable;

    /**
     * @var int The sites current blog ID. This only
     *    differs if running a multi-site installations
     */
    private $_blogPrefix;

    /**
     * @var int Keeps count of how many times the
     *    cache was successfully received from APCu
     */
    public $cache_hits = 0;

    /**
     * @var int Keeps count of how many times the
     *    cache was not successfully received from APCu
     */
    public $cache_misses = 0;

    /**
     * @var array Holds a list of cache groups that are
     *    shared across all sites in a multi-site installation
     */
    private $_globalGroups = [];

    /**
     * @var array Holds an array of versions of the retrieved groups
     */
    private $_groupVersions = [];

    /**
     * @var bool True if the current installation is a multi-site
     */
    private $_multiSite;

    /**
     * @var array Holds cache that is to be non persistent
     */
    private $_nonPersistentCache = [];

    /**
     * @var array Holds a list of cache groups that are not to be saved to APCu
     */
    private $_nonPersistentGroups = [];

    /**
     * @var array
     */
    private $_localCache = [];

    /**
     * @var array Holds an array of versions of the retrieved sites
     */
    private $_siteVersions = [];

    private static $_instance;

    /**
     * Singleton. Return instance of WP_Object_Cache
     *
     * @return WP_Object_Cache
     */
    public static function instance()
    {
        if (self::$_instance === null) {
            self::$_instance = new WP_Object_Cache();
        }

        return self::$_instance;
    }

    /**
     * __clone not allowed
     */
    private function __clone()
    {
    }

    /**
     * Direct access to __construct not allowed.
     */
    private function __construct()
    {
        global $blog_id;

        if (!defined('WP_APCU_KEY_SALT')) {
            /**
             * Set in config if you are using some sort of shared
             * config where ABSPATH is the same on all sites
             */
            define('WP_APCU_KEY_SALT', 'wp');
        }

        /**
         * define('WP_APCU_LOCAL_CACHE', false) to disable local
         * array cache and force all cache to be returned from APCu
         */
        if (!defined('WP_APCU_LOCAL_CACHE')) {
            define('WP_APCU_LOCAL_CACHE', true);
        }

        $this->_absPath = md5(ABSPATH);
        $this->_apcuAvailable = (extension_loaded('apcu') && ini_get('apc.enabled'));
        $this->_multiSite = is_multisite();
        $this->_blogPrefix = $this->_multiSite ? $blog_id : 1;
    }
    
    public function stats()
    {
        echo '<p>';
        echo "<strong>Cache Hits:</strong> {$this->cache_hits}<br />";
        echo "<strong>Cache Misses:</strong> {$this->cache_misses}<br />";
        echo '</p>';
        echo '<ul>';

        foreach ( $this->_localCache as $group => $cache ) {
            echo '<li><strong>Group:</strong> ' . esc_html( $group ) . ' - ( ' . number_format( strlen( serialize( $cache ) ) / KB_IN_BYTES, 2 ) . 'k )</li>';
        }
        echo '</ul>';
    }

    /**
     * Adds data to the cache, if the cache key does not already exist.
     *
     * @param int|string $key The cache key to use for retrieval later
     * @param mixed $var The data to add to the cache store
     * @param string $group The group to add the cache to
     * @param int $ttl When the cache data should be expired
     *
     * @return bool False if cache key and group already exist, true on success
     */
    public function add($key, $var, $group = 'default', $ttl = 0)
    {
        if (function_exists('wp_suspend_cache_addition') && wp_suspend_cache_addition()) {
            return false;
        }

        $key = $this->_key($key, $group);

        if (!$this->_apcuAvailable || $this->_is_non_persistent_group($group)) {
            return $this->_add_np($key, $var);
        }

        return $this->_add($key, $var, $ttl);
    }

    /**
     * Adds multiple values to the cache in one call.
     *
     * @param array  $data   Array of keys and values to be added.
     * @param string $group  Optional. Where the cache contents are grouped. Default default.
     * @param int    $ttl    Optional. When to expire the cache contents, in seconds.
     *                       Default 0 (no expiration).
     * @return bool[] Array of return values, grouped by key. Each value is either
     *                true on success, or false if cache key and group already exist.
     */
    public function add_multiple(array $data, $group = 'default', $ttl = 0)
    {
        $values = [];

        foreach ($data as $key => $value) {
            $values[$key] = $this->add($key, $value, $group, $ttl);
        }

        return $values;
    }

    /**
     * Adds data to APCu cache, if the cache key does not already exist.
     *
     * @param string $key The cache key to use for retrieval later
     * @param mixed $var The data to add to the cache store
     * @param int $ttl When the cache data should be expired
     *
     * @return bool False if cache key and group already exist, true on success
     */
    private function _add($key, $var, $ttl)
    {
        if (apcu_add($key, $var, max((int)$ttl, 0))) {
            if (WP_APCU_LOCAL_CACHE) {
                $this->_localCache[$key] = is_object($var) ? clone $var : $var;
            }
            return true;
        }
        return false;
    }

    /**
     * Adds data to non persistent cache, if the cache key does not already exist.
     *
     * @param string $key The cache key to use for retrieval later
     * @param mixed $var The data to add to the cache store
     *
     * @return bool False if cache key and group already exist, true on success
     */
    private function _add_np($key, $var)
    {
        if ($this->_exists_np($key)) {
            return false;
        }

        return $this->_set_np($key, $var);
    }

    /**
     * Sets the list of global groups.
     *
     * @param string|array $groups List of groups that are global.
     */
    public function add_global_groups($groups)
    {
        foreach ((array)$groups as $group) {
            $this->_globalGroups[$group] = true;
        }
    }

    /**
     * Sets the list of non persistent groups.
     *
     * @param string|array $groups List of groups that are non persistent.
     */
    public function add_non_persistent_groups($groups)
    {
        foreach ((array)$groups as $group) {
            $this->_nonPersistentGroups[$group] = true;
        }
    }

    /**
     * Decrement numeric cache item's value
     *
     * @param int|string $key The cache key to increment
     * @param int $offset The amount by which to decrement the item's value. Default is 1.
     * @param string $group The group the key is in.
     *
     * @return false|int False on failure, the item's new value on success.
     */
    public function decr($key, $offset = 1, $group = 'default')
    {
        $key = $this->_key($key, $group);

        if (!$this->_apcuAvailable || $this->_is_non_persistent_group($group)) {
            return $this->_decr_np($key, $offset);
        }

        return $this->_decr($key, $offset);
    }

    /**
     * Decrement numeric APCu cache item's value
     *
     * @param string $key The cache key to increment
     * @param int $offset The amount by which to decrement the item's value. Default is 1.
     *
     * @return false|int False on failure, the item's new value on success.
     */
    private function _decr($key, $offset)
    {
        $this->_get($key, $success);
        if (!$success) {
            return false;
        }

        $value = apcu_dec($key, max((int)$offset, 0));
        if ($value !== false && WP_APCU_LOCAL_CACHE) {
            $this->_localCache[$key] = $value;
        }
        return $value;
    }

    /**
     * Decrement numeric non persistent cache item's value
     *
     * @param string $key The cache key to increment
     * @param int $offset The amount by which to decrement the item's value. Default is 1.
     *
     * @return false|int False on failure, the item's new value on success.
     */
    private function _decr_np($key, $offset)
    {
        if (!$this->_exists_np($key)) {
            return false;
        }

        $offset = max((int)$offset, 0);
        $var = $this->_get_np($key);
        $var = is_numeric($var) ? $var : 0;
        $var -= $offset;

        return $this->_set_np($key, $var);
    }

    /**
     * Remove the contents of the cache key in the group
     *
     * If the cache key does not exist in the group, then nothing will happen.
     *
     * @param int|string $key What the contents in the cache are called
     * @param string $group Where the cache contents are grouped
     * @param bool $deprecated Deprecated.
     *
     * @return bool False if the contents weren't deleted and true on success
     */
    public function delete($key, $group = 'default', $deprecated = false)
    {
        $key = $this->_key($key, $group);

        if (!$this->_apcuAvailable || $this->_is_non_persistent_group($group)) {
            return $this->_delete_np($key);
        }

        return $this->_delete($key);
    }

    /**
     * Deletes multiple values from the cache in one call.
     *
     * @param array  $keys  Array of keys to be deleted.
     * @param string $group Optional. Where the cache contents are grouped. Default default.
     * @return bool[] Array of return values, grouped by key. Each value is either
     *                true on success, or false if the contents were not deleted.
     */
    public function delete_multiple(array $keys, $group = 'default')
    {
        $values = [];

        foreach ($keys as $key) {
            $values[$key] = $this->delete($key, $group);
        }

        return $values;
    }

    /**
     * Remove the contents of the APCu cache key in the group
     *
     * If the cache key does not exist in the group, then nothing will happen.
     *
     * @param string $key What the contents in the cache are called
     *
     * @return bool False if the contents weren't deleted and true on success
     */
    private function _delete($key)
    {
        unset($this->_localCache[$key]);
        return apcu_delete($key);
    }

    /**
     * Remove the contents of the non persistent cache key in the group
     *
     * If the cache key does not exist in the group, then nothing will happen.
     *
     * @param string $key What the contents in the cache are called
     *
     * @return bool False if the contents weren't deleted and true on success
     */
    private function _delete_np($key)
    {
        if (array_key_exists($key, $this->_nonPersistentCache)) {
            unset($this->_nonPersistentCache[$key]);

            return true;
        }

        return false;
    }

    /**
     * Checks if the cached non persistent key exists
     *
     * @param string $key What the contents in the cache are called
     *
     * @return bool True if cache key exists else false
     */
    private function _exists_np($key)
    {
        return array_key_exists($key, $this->_nonPersistentCache);
    }

    /**
     * Clears the object cache of all data
     *
     * @return bool Always returns true
     */
    public function flush()
    {
        $this->_nonPersistentCache = [];

        if (WP_APCU_LOCAL_CACHE) {
            $this->_localCache = [];
        }

        if ($this->_apcuAvailable) {
            apcu_clear_cache();
        }

        return true;
    }

    /**
     * if using local cache clear that else flush all
     *
     * @return bool Always returns true
     */
    public function flush_runtime()
    {
        $this->_nonPersistentCache = [];

        if (WP_APCU_LOCAL_CACHE) {
            $this->_localCache = [];
        } elseif ($this->_apcuAvailable) {
            apcu_clear_cache();
        }

        return true;
    }

    /**
     * Invalidate a groups object cache
     *
     * @param mixed $groups A group or an array of groups to invalidate
     *
     * @return bool
     */
    public function flush_groups($groups)
    {
        $groups = (array)$groups;

        if (empty($groups)) {
            return false;
        }

        foreach ($groups as $group) {
            $version = $this->_get_group_cache_version($group);
            $this->_set_group_cache_version($group, $version + 1);
        }

        return true;
    }

    /**
     * Invalidate a site's object cache
     *
     * @param mixed $sites Sites ID's that want flushing.
     *                     Don't pass a site to flush current site
     *
     * @return bool
     */
    public function flush_sites($sites)
    {
        $sites = (array)$sites;

        if (empty($sites)) {
            $sites = [$this->_blogPrefix];
        }

        // Add global groups (site 0) to be flushed.
        if (!in_array(0, $sites, false)) {
            $sites[] = 0;
        }

        foreach ($sites as $site) {
            $version = $this->_get_site_cache_version($site);
            $this->_set_site_cache_version($site, $version + 1);
        }

        return true;
    }

    /**
     * Retrieves the cache contents, if it exists
     *
     * The contents will be first attempted to be retrieved by searching by the
     * key in the cache key. If the cache is hit (success) then the contents
     * are returned.
     *
     * On failure, the number of cache misses will be incremented.
     *
     * @param int|string $key What the contents in the cache are called
     * @param string $group Where the cache contents are grouped
     * @param bool $force Not used.
     * @param bool       &$success
     *
     * @return bool|mixed False on failure to retrieve contents or the cache contents on success
     */
    public function get($key, $group = 'default', $force = false, &$success = null)
    {
        $key = $this->_key($key, $group);

        if (!$this->_apcuAvailable || $this->_is_non_persistent_group($group)) {
            $var = $this->_get_np($key, $success);
        } else {
            $var = $this->_get($key, $success);
        }

        if ($success) {
            $this->cache_hits++;
        } else {
            $this->cache_misses++;
        }

        return $var;
    }

    /**
     * Retrieves the APCu cache contents, if it exists
     *
     * @param string $key What the contents in the cache are called
     * @param bool   &$success
     *
     * @return bool|mixed False on failure to retrieve contents or the cache contents on success
     */
    private function _get($key, &$success = null)
    {
        if (WP_APCU_LOCAL_CACHE && array_key_exists($key, $this->_localCache)
        ) {
            $success = true;
            $var = $this->_localCache[$key];
        } else {
            $var = apcu_fetch($key, $success);
            if ($success && WP_APCU_LOCAL_CACHE) {
                $this->_localCache[$key] = $var;
            }
        }

        if (is_object($var)) {
            $var = clone $var;
        }

        return $var;
    }

    /**
     * Retrieves the non persistent cache contents, if it exists
     *
     * @param string $key What the contents in the cache are called
     * @param bool   &$success
     *
     * @return bool|mixed False on failure to retrieve contents or the cache contents on success
     */
    private function _get_np($key, &$success = null)
    {
        if (array_key_exists($key, $this->_nonPersistentCache)) {
            $success = true;
            return $this->_nonPersistentCache[$key];
        }

        $success = false;
        return false;
    }

    /**
     * Get the cache version of a given key
     *
     * @param string $key
     *
     * @return int cache version
     */
    private function _get_cache_version($key)
    {
        if ($this->_apcuAvailable) {
            $version = (int)apcu_fetch($key);
        } elseif (array_key_exists($key, $this->_nonPersistentCache)) {
            $version = (int)$this->_nonPersistentCache[$key];
        } else {
            $version = 0;
        }

        return $version;
    }

    /**
     * Build cache version key
     *
     * @param string $type Type of key, for site or group
     * @param mixed $value the group or site id
     *
     * @return string The key
     */
    private function _get_cache_version_key($type, $value)
    {
        return WP_APCU_KEY_SALT . ':' . $this->_absPath . ':' . $type . ':' . $value;
    }

    /**
     * Get the groups cache version
     *
     * @param string $group The group to get version for
     *
     * @return int The group cache version
     */
    private function _get_group_cache_version($group)
    {
        if (!isset($this->_groupVersions[$group])) {
            $this->_groupVersions[$group] = $this->_get_cache_version(
                $this->_get_cache_version_key(
                    'GroupVersion',
                    $group
                )
            );
        }

        return $this->_groupVersions[$group];
    }


    /**
     * Retrieves multiple values from the cache in one call.
     *
     * @param array  $keys  Array of keys under which the cache contents are stored.
     * @param string $group Optional. Where the cache contents are grouped. Default 'default'.
     * @param bool   $force Optional. Whether to force an update of the local cache
     *                      from the persistent cache. Default false.
     * @return array Array of return values, grouped by key. Each value is either
     *               the cache contents on success, or false on failure.
     */
    public function get_multiple($keys, $group = 'default', $force = false)
    {
        $values = [];

        foreach ($keys as $key) {
            $values[$key] = $this->get($key, $group, $force);
        }

        return $values;
    }


    /**
     * Get the sites cache version
     *
     * @param int $site The site to get version for
     *
     * @return int The site cache version
     */
    private function _get_site_cache_version($site)
    {
        if (!isset($this->_siteVersions[$site])) {
            $this->_siteVersions[$site] = $this->_get_cache_version(
                $this->_get_cache_version_key(
                    'SiteVersion',
                    $site
                )
            );
        }

        return $this->_siteVersions[$site];
    }

    /**
     * Increment numeric cache item's value
     *
     * @param int|string $key The cache key to increment
     * @param int $offset The amount by which to increment the item's value. Default is 1.
     * @param string $group The group the key is in.
     *
     * @return false|int False on failure, the item's new value on success.
     */
    public function incr($key, $offset = 1, $group = 'default')
    {
        $key = $this->_key($key, $group);

        if (!$this->_apcuAvailable || $this->_is_non_persistent_group($group)) {
            return $this->_incr_np($key, $offset);
        }

        return $this->_incr($key, $offset);
    }

    /**
     * Increment numeric APCu cache item's value
     *
     * @param string $key The cache key to increment
     * @param int $offset The amount by which to increment the item's value. Default is 1.
     *
     * @return false|int False on failure, the item's new value on success.
     */
    private function _incr($key, $offset)
    {
        $this->_get($key, $success);
        if (!$success) {
            return false;
        }

        $value = apcu_inc($key, max((int)$offset, 0));
        if ($value !== false && WP_APCU_LOCAL_CACHE) {
            $this->_localCache[$key] = $value;
        }
        return $value;
    }

    /**
     * Increment numeric non persistent cache item's value
     *
     * @param string $key The cache key to increment
     * @param int $offset The amount by which to increment the item's value. Default is 1.
     *
     * @return false|int False on failure, the item's new value on success.
     */
    private function _incr_np($key, $offset)
    {
        if (!$this->_exists_np($key)) {
            return false;
        }

        $offset = max((int)$offset, 0);
        $var = $this->_get_np($key);
        $var = is_numeric($var) ? $var : 0;
        $var += $offset;

        return $this->_set_np($key, $var);
    }

    /**
     * Checks if the given group is a non persistent group
     *
     * @param string $group The group to be checked
     *
     * @return bool True if the group is a non persistent group else false
     */
    private function _is_non_persistent_group($group)
    {
        return isset($this->_nonPersistentGroups[$group]);
    }

    /**
     * Works out a cache key based on a given key and group
     *
     * @param int|string $key The key
     * @param string $group The group
     *
     * @return string Returns the calculated cache key
     */
    private function _key($key, $group)
    {
        if (empty($group)) {
            $group = 'default';
        }

        $prefix = 0;

        if (!isset($this->_globalGroups[$group])) {
            $prefix = $this->_blogPrefix;
        }

        $group_version = $this->_get_group_cache_version($group);
        $site_version = $this->_get_site_cache_version($prefix);

        return WP_APCU_KEY_SALT . ':' . $this->_absPath . ':' . $prefix . ':' . $group . ':' . $key . ':v' . $site_version . '.' . $group_version;
    }

    /**
     * Replace the contents in the cache, if contents already exist
     *
     * @param int|string $key What to call the contents in the cache
     * @param mixed $var The contents to store in the cache
     * @param string $group Where to group the cache contents
     * @param int $ttl When to expire the cache contents
     *
     * @return bool False if not exists, true if contents were replaced
     */
    public function replace($key, $var, $group = 'default', $ttl = 0)
    {
        $key = $this->_key($key, $group);

        if (!$this->_apcuAvailable || $this->_is_non_persistent_group($group)) {
            return $this->_replace_np($key, $var);
        }

        return $this->_replace($key, $var, $ttl);
    }

    /**
     * Replace the contents in the APCu cache, if contents already exist
     *
     * @param string $key What to call the contents in the cache
     * @param mixed $var The contents to store in the cache
     * @param int $ttl When to expire the cache contents
     *
     * @return bool False if not exists, true if contents were replaced
     */
    private function _replace($key, $var, $ttl)
    {
        $this->_get($key, $success);
        if ($success) {
            return false;
        }

        return $this->_set($key, $var, $ttl);
    }

    /**
     * Replace the contents in the non persistent cache, if contents already exist
     *
     * @param string $key What to call the contents in the cache
     * @param mixed $var The contents to store in the cache
     *
     * @return bool False if not exists, true if contents were replaced
     */
    private function _replace_np($key, $var)
    {
        if (!$this->_exists_np($key)) {
            return false;
        }

        return $this->_set_np($key, $var);
    }

    /**
     * Sets the data contents into the cache
     *
     * @param int|string $key What to call the contents in the cache
     * @param mixed $var The contents to store in the cache
     * @param string $group Where to group the cache contents
     * @param int $ttl When the cache data should be expired
     *
     * @return bool True if cache set successfully else false
     */
    public function set($key, $var, $group = 'default', $ttl = 0)
    {
        $key = $this->_key($key, $group);

        if (!$this->_apcuAvailable || $this->_is_non_persistent_group($group)) {
            return $this->_set_np($key, $var);
        }

        return $this->_set($key, $var, $ttl);
    }

    /**
     * Sets multiple values to the cache in one call.
     *
     * @param array  $data   Array of key and value to be set.
     * @param string $group  Optional. Where the cache contents are grouped. Default default.
     * @param int    $ttl    Optional. When to expire the cache contents, in seconds.
     *                       Default 0 (no expiration).
     * @return bool[] Array of return values, grouped by key. Each value is always true.
     */
    public function set_multiple(array $data, $group = 'default', $ttl = 0)
    {
        $values = [];

        foreach ( $data as $key => $var ) {
            $values[$key] = $this->set($key, $var, $group, $ttl);
        }

        return $values;
    }

    /**
     * Sets the data contents into the APCu cache
     *
     * @param string $key What to call the contents in the cache
     * @param mixed $var The contents to store in the cache
     * @param int $ttl When the cache data should be expired
     *
     * @return bool True if cache set successfully else false
     */
    private function _set($key, $var, $ttl)
    {
        if (is_object($var)) {
            $var = clone $var;
        }

        if (apcu_store($key, $var, max((int)$ttl, 0))) {
            if (WP_APCU_LOCAL_CACHE) {
                $this->_localCache[$key] = $var;
            }
            return true;
        }

        return false;
    }

    /**
     * Sets the data contents into the non persistent cache
     *
     * @param string $key What to call the contents in the cache
     * @param mixed $var The contents to store in the cache
     *
     * @return bool True if cache set successfully else false
     */
    private function _set_np($key, $var)
    {
        if (is_object($var)) {
            $var = clone $var;
        }

        return $this->_nonPersistentCache[$key] = $var;
    }

    /**
     * Set the cache version for a given key
     *
     * @param string $key
     * @param int $version
     *
     * @return mixed
     */
    private function _set_cache_version($key, $version)
    {
        if ($this->_apcuAvailable) {
            return apcu_store($key, $version);
        }

        return $this->_nonPersistentCache[$key] = $version;
    }

    /**
     * Set the version for a groups cache
     *
     * @param string $group
     * @param int $version
     */
    private function _set_group_cache_version($group, $version)
    {
        $this->_set_cache_version($this->_get_cache_version_key('GroupVersion', $group), $version);
    }

    /**
     * Set the version for a sites cache
     *
     * @param int $site
     * @param int $version
     */
    private function _set_site_cache_version($site, $version)
    {
        $this->_set_cache_version($this->_get_cache_version_key('SiteVersion', $site), $version);
    }

    /**
     * Switch the internal blog id.
     *
     * This changes the blog id used to create keys in blog specific groups.
     *
     * @param int $blog_id Blog ID
     */
    public function switch_to_blog($blog_id)
    {
        $this->_blogPrefix = $this->_multiSite ? $blog_id : 1;
    }

    /**
     * @return string
     */
    public function getAbsPath()
    {
        return $this->_absPath;
    }

    /**
     * @return bool
     */
    public function getApcuAvailable()
    {
        return $this->_apcuAvailable;
    }

    /**
     * @return int
     */
    public function getBlogPrefix()
    {
        return $this->_blogPrefix;
    }

    /**
     * @return int
     */
    public function getCacheHits()
    {
        return $this->cache_hits;
    }

    /**
     * @return int
     */
    public function getCacheMisses()
    {
        return $this->cache_misses;
    }

    /**
     * @return array
     */
    public function getGlobalGroups()
    {
        return $this->_globalGroups;
    }

    /**
     * @return array
     */
    public function getGroupVersions()
    {
        return $this->_groupVersions;
    }

    /**
     * @return bool
     */
    public function getMultiSite()
    {
        return $this->_multiSite;
    }

    /**
     * @return array
     */
    public function getNonPersistentCache()
    {
        return $this->_nonPersistentCache;
    }

    /**
     * @return array
     */
    public function getNonPersistentGroups()
    {
        return $this->_nonPersistentGroups;
    }

    /**
     * @return array
     */
    public function getSiteVersions()
    {
        return $this->_siteVersions;
    }
}