<?php
/**
 * Plugin Update Checker Library 3.1
 * http://w-shadow.com/
 *
 * Copyright 2016 Janis Elsts
 * Released under the MIT license. See license.txt for details.
 */

if ( !class_exists('PluginUpdateChecker_3_1', false) ):

    /**
     * A custom plugin update checker.
     *
     * @author Janis Elsts
     * @copyright 2016
     * @version 3.0
     * @access public
     */
    class PluginUpdateChecker_3_1 {
        public $metadataUrl = ''; //The URL of the plugin's metadata file.
        public $pluginAbsolutePath = ''; //Full path of the main plugin file.
        public $pluginFile = '';  //Plugin filename relative to the plugins directory. Many WP APIs use this to identify plugins.
        public $slug = '';        //Plugin slug.
        public $optionName = '';  //Where to store the update info.
        public $muPluginFile = ''; //For MU plugins, the plugin filename relative to the mu-plugins directory.

        public $debugMode = false; //Set to TRUE to enable error reporting. Errors are raised using trigger_error()
        //and should be logged to the standard PHP error log.
        public $scheduler;

        protected $upgraderStatus;

        private $cachedInstalledVersion = null;

        private $metadataHost = ''; //The host component of $metadataUrl.

        /**
         * Class constructor.
         *
         * @param string $metadataUrl The URL of the plugin's metadata file.
         * @param string $pluginFile Fully qualified path to the main plugin file.
         * @param string $slug The plugin's 'slug'. If not specified, the filename part of $pluginFile sans '.php' will be used as the slug.
         * @param integer $checkPeriod How often to check for updates (in hours). Defaults to checking every 12 hours. Set to 0 to disable automatic update checks.
         * @param string $optionName Where to store book-keeping info about update checks. Defaults to 'external_updates-$slug'.
         * @param string $muPluginFile Optional. The plugin filename relative to the mu-plugins directory.
         */
        public function __construct($metadataUrl, $pluginFile, $slug = '', $checkPeriod = 12, $optionName = '', $muPluginFile = ''){
            $this->metadataUrl = $metadataUrl;
            $this->pluginAbsolutePath = $pluginFile;
            $this->pluginFile = plugin_basename($this->pluginAbsolutePath);
            $this->muPluginFile = $muPluginFile;
            $this->slug = $slug;
            $this->optionName = $optionName;
            $this->debugMode = (bool)(constant('WP_DEBUG'));

            //If no slug is specified, use the name of the main plugin file as the slug.
            //For example, 'my-cool-plugin/cool-plugin.php' becomes 'cool-plugin'.
            if ( empty($this->slug) ){
                $this->slug = basename($this->pluginFile, '.php');
            }

            if ( empty($this->optionName) ){
                $this->optionName = 'external_updates-' . $this->slug;
            }

            //Backwards compatibility: If the plugin is a mu-plugin but no $muPluginFile is specified, assume
            //it's the same as $pluginFile given that it's not in a subdirectory (WP only looks in the base dir).
            if ( (strpbrk($this->pluginFile, '/\\') === false) && $this->isUnknownMuPlugin() ) {
                $this->muPluginFile = $this->pluginFile;
            }

            $this->scheduler = $this->createScheduler($checkPeriod);
            $this->upgraderStatus = new PucUpgraderStatus_3_1();

            $this->installHooks();
        }

        /**
         * Create an instance of the scheduler.
         *
         * This is implemented as a method to make it possible for plugins to subclass the update checker
         * and substitute their own scheduler.
         *
         * @param int $checkPeriod
         * @return PucScheduler_3_1
         */
        protected function createScheduler($checkPeriod) {
            return new PucScheduler_3_1($this, $checkPeriod);
        }

        /**
         * Install the hooks required to run periodic update checks and inject update info
         * into WP data structures.
         *
         * @return void
         */
        protected function installHooks(){
            //Override requests for plugin information
            add_filter('plugins_api', array($this, 'injectInfo'), 20, 3);

            //Insert our update info into the update array maintained by WP.
            add_filter('site_transient_update_plugins', array($this,'injectUpdate')); //WP 3.0+
            add_filter('transient_update_plugins', array($this,'injectUpdate')); //WP 2.8+
            add_filter('site_transient_update_plugins', array($this, 'injectTranslationUpdates'));

            add_filter('plugin_row_meta', array($this, 'addCheckForUpdatesLink'), 10, 2);
            add_action('admin_init', array($this, 'handleManualCheck'));
            add_action('all_admin_notices', array($this, 'displayManualCheckResult'));

            //Clear the version number cache when something - anything - is upgraded or WP clears the update cache.
            add_filter('upgrader_post_install', array($this, 'clearCachedVersion'));
            add_action('delete_site_transient_update_plugins', array($this, 'clearCachedVersion'));
            //Clear translation updates when WP clears the update cache.
            //This needs to be done directly because the library doesn't actually remove obsolete plugin updates,
            //it just hides them (see getUpdate()). We can't do that with translations - too much disk I/O.
            add_action('delete_site_transient_update_plugins', array($this, 'clearCachedTranslationUpdates'));

            //Rename the update directory to be the same as the existing directory.
            add_filter('upgrader_source_selection', array($this, 'fixDirectoryName'), 10, 3);

            //Enable language support (i18n).
            load_plugin_textdomain('plugin-update-checker', false, plugin_basename(dirname(__FILE__)) . '/languages');

            //Allow HTTP requests to the metadata URL even if it's on a local host.
            $this->metadataHost = @parse_url($this->metadataUrl, PHP_URL_HOST);
            add_filter('http_request_host_is_external', array($this, 'allowMetadataHost'), 10, 2);
        }

        /**
         * Explicitly allow HTTP requests to the metadata URL.
         *
         * WordPress has a security feature where the HTTP API will reject all requests that are sent to
         * another site hosted on the same server as the current site (IP match), a local host, or a local
         * IP, unless the host exactly matches the current site.
         *
         * This feature is opt-in (at least in WP 4.4). Apparently some people enable it.
         *
         * That can be a problem when you're developing your plugin and you decide to host the update information
         * on the same server as your test site. Update requests will mysteriously fail.
         *
         * We fix that by adding an exception for the metadata host.
         *
         * @param bool $allow
         * @param string $host
         * @return bool
         */
        public function allowMetadataHost($allow, $host) {
            if ( strtolower($host) === strtolower($this->metadataHost) ) {
                return true;
            }
            return $allow;
        }

        /**
         * Retrieve plugin info from the configured API endpoint.
         *
         * @uses wp_remote_get()
         *
         * @param array $queryArgs Additional query arguments to append to the request. Optional.
         * @return PluginInfo_3_1
         */
        public function requestInfo($queryArgs = array()){
            //Query args to append to the URL. Plugins can add their own by using a filter callback (see addQueryArgFilter()).
            $installedVersion = $this->getInstalledVersion();
            $queryArgs['installed_version'] = ($installedVersion !== null) ? $installedVersion : '';
            $queryArgs = apply_filters('puc_request_info_query_args-'.$this->slug, $queryArgs);

            //Various options for the wp_remote_get() call. Plugins can filter these, too.
            $options = array(
                'timeout' => 10, //seconds
                'headers' => array(
                    'Accept' => 'application/json'
                ),
            );
            $options = apply_filters('puc_request_info_options-'.$this->slug, $options);

            //The plugin info should be at 'http://your-api.com/url/here/$slug/info.json'
            $url = $this->metadataUrl;
            if ( !empty($queryArgs) ){
                $url = add_query_arg($queryArgs, $url);
            }

            $result = wp_remote_get(
                $url,
                $options
            );

            //Try to parse the response
            $status = $this->validateApiResponse($result);
            $pluginInfo = null;
            if ( !is_wp_error($status) ){
                $pluginInfo = PluginInfo_3_1::fromJson($result['body']);
                if ( $pluginInfo !== null ) {
                    $pluginInfo->filename = $this->pluginFile;
                    $pluginInfo->slug = $this->slug;
                }
            } else {
                $this->triggerError(
                    sprintf('The URL %s does not point to a valid plugin metadata file. ', $url)
                    . $status->get_error_message(),
                    E_USER_WARNING
                );
            }

            $pluginInfo = apply_filters('puc_request_info_result-'.$this->slug, $pluginInfo, $result);
            return $pluginInfo;
        }

        /**
         * Check if $result is a successful update API response.
         *
         * @param array|WP_Error $result
         * @return true|WP_Error
         */
        private function validateApiResponse($result) {
            if ( is_wp_error($result) ) { /** @var WP_Error $result */
                return new WP_Error($result->get_error_code(), 'WP HTTP Error: ' . $result->get_error_message());
            }

            if ( !isset($result['response']['code']) ) {
                return new WP_Error('puc_no_response_code', 'wp_remote_get() returned an unexpected result.');
            }

            if ( $result['response']['code'] !== 200 ) {
                return new WP_Error(
                    'puc_unexpected_response_code',
                    'HTTP response code is ' . $result['response']['code'] . ' (expected: 200)'
                );
            }

            if ( empty($result['body']) ) {
                return new WP_Error('puc_empty_response', 'The metadata file appears to be empty.');
            }

            return true;
        }

        /**
         * Retrieve the latest update (if any) from the configured API endpoint.
         *
         * @uses PluginUpdateChecker::requestInfo()
         *
         * @return PluginUpdate_3_1 An instance of PluginUpdate, or NULL when no updates are available.
         */
        public function requestUpdate(){
            //For the sake of simplicity, this function just calls requestInfo()
            //and transforms the result accordingly.
            $pluginInfo = $this->requestInfo(array('checking_for_updates' => '1'));
            if ( $pluginInfo == null ){
                return null;
            }
            $update = PluginUpdate_3_1::fromPluginInfo($pluginInfo);

            //Keep only those translation updates that apply to this site.
            $update->translations = $this->filterApplicableTranslations($update->translations);

            return $update;
        }

        /**
         * Filter a list of translation updates and return a new list that contains only updates
         * that apply to the current site.
         *
         * @param array $translations
         * @return array
         */
        private function filterApplicableTranslations($translations) {
            $languages = array_flip(array_values(get_available_languages()));
            $installedTranslations = wp_get_installed_translations('plugins');
            if ( isset($installedTranslations[$this->slug]) ) {
                $installedTranslations = $installedTranslations[$this->slug];
            } else {
                $installedTranslations = array();
            }

            $applicableTranslations = array();
            foreach($translations as $translation) {
                //Does it match one of the available core languages?
                $isApplicable = array_key_exists($translation->language, $languages);
                //Is it more recent than an already-installed translation?
                if ( isset($installedTranslations[$translation->language]) ) {
                    $updateTimestamp = strtotime($translation->updated);
                    $installedTimestamp = strtotime($installedTranslations[$translation->language]['PO-Revision-Date']);
                    $isApplicable = $updateTimestamp > $installedTimestamp;
                }

                if ( $isApplicable ) {
                    $applicableTranslations[] = $translation;
                }
            }

            return $applicableTranslations;
        }

        /**
         * Get the currently installed version of the plugin.
         *
         * @return string Version number.
         */
        public function getInstalledVersion(){
            if ( isset($this->cachedInstalledVersion) ) {
                return $this->cachedInstalledVersion;
            }

            $pluginHeader = $this->getPluginHeader();
            if ( isset($pluginHeader['Version']) ) {
                $this->cachedInstalledVersion = $pluginHeader['Version'];
                return $pluginHeader['Version'];
            } else {
                //This can happen if the filename points to something that is not a plugin.
                $this->triggerError(
                    sprintf(
                        "Can't to read the Version header for '%s'. The filename is incorrect or is not a plugin.",
                        $this->pluginFile
                    ),
                    E_USER_WARNING
                );
                return null;
            }
        }

        /**
         * Get plugin's metadata from its file header.
         *
         * @return array
         */
        protected function getPluginHeader() {
            if ( !is_file($this->pluginAbsolutePath) ) {
                //This can happen if the plugin filename is wrong.
                $this->triggerError(
                    sprintf(
                        "Can't to read the plugin header for '%s'. The file does not exist.",
                        $this->pluginFile
                    ),
                    E_USER_WARNING
                );
                return array();
            }

            if ( !function_exists('get_plugin_data') ){
                /** @noinspection PhpIncludeInspection */
                require_once( ABSPATH . '/wp-admin/includes/plugin.php' );
            }
            return get_plugin_data($this->pluginAbsolutePath, false, false);
        }

        /**
         * Check for plugin updates.
         * The results are stored in the DB option specified in $optionName.
         *
         * @return PluginUpdate_3_1|null
         */
        public function checkForUpdates(){
            $installedVersion = $this->getInstalledVersion();
            //Fail silently if we can't find the plugin or read its header.
            if ( $installedVersion === null ) {
                $this->triggerError(
                    sprintf('Skipping update check for %s - installed version unknown.', $this->pluginFile),
                    E_USER_WARNING
                );
                return null;
            }

            $state = $this->getUpdateState();
            if ( empty($state) ){
                $state = new stdClass;
                $state->lastCheck = 0;
                $state->checkedVersion = '';
                $state->update = null;
            }

            $state->lastCheck = time();
            $state->checkedVersion = $installedVersion;
            $this->setUpdateState($state); //Save before checking in case something goes wrong

            $state->update = $this->requestUpdate();
            $this->setUpdateState($state);

            return $this->getUpdate();
        }

        /**
         * Load the update checker state from the DB.
         *
         * @return stdClass|null
         */
        public function getUpdateState() {
            $state = get_site_option($this->optionName, null);
            if ( empty($state) || !is_object($state)) {
                $state = null;
            }

            if ( isset($state, $state->update) && is_object($state->update) ) {
                $state->update = PluginUpdate_3_1::fromObject($state->update);
            }
            return $state;
        }


        /**
         * Persist the update checker state to the DB.
         *
         * @param StdClass $state
         * @return void
         */
        private function setUpdateState($state) {
            if ( isset($state->update) && is_object($state->update) && method_exists($state->update, 'toStdClass') ) {
                $update = $state->update; /** @var PluginUpdate_3_1 $update */
                $state->update = $update->toStdClass();
            }
            update_site_option($this->optionName, $state);
        }

        /**
         * Reset update checker state - i.e. last check time, cached update data and so on.
         *
         * Call this when your plugin is being uninstalled, or if you want to
         * clear the update cache.
         */
        public function resetUpdateState() {
            delete_site_option($this->optionName);
        }

        /**
         * Intercept plugins_api() calls that request information about our plugin and
         * use the configured API endpoint to satisfy them.
         *
         * @see plugins_api()
         *
         * @param mixed $result
         * @param string $action
         * @param array|object $args
         * @return mixed
         */
        public function injectInfo($result, $action = null, $args = null){
            $relevant = ($action == 'plugin_information') && isset($args->slug) && (
                    ($args->slug == $this->slug) || ($args->slug == dirname($this->pluginFile))
                );
            if ( !$relevant ) {
                return $result;
            }

            $pluginInfo = $this->requestInfo();
            $pluginInfo = apply_filters('puc_pre_inject_info-' . $this->slug, $pluginInfo);
            if ( $pluginInfo ) {
                return $pluginInfo->toWpFormat();
            }

            return $result;
        }

        /**
         * Insert the latest update (if any) into the update list maintained by WP.
         *
         * @param StdClass $updates Update list.
         * @return StdClass Modified update list.
         */
        public function injectUpdate($updates){
            //Is there an update to insert?
            $update = $this->getUpdate();

            //No update notifications for mu-plugins unless explicitly enabled. The MU plugin file
            //is usually different from the main plugin file so the update wouldn't show up properly anyway.
            if ( $this->isUnknownMuPlugin() ) {
                $update = null;
            }

            if ( !empty($update) ) {
                //Let plugins filter the update info before it's passed on to WordPress.
                $update = apply_filters('puc_pre_inject_update-' . $this->slug, $update);
                $updates = $this->addUpdateToList($updates, $update);
            } else {
                //Clean up any stale update info.
                $updates = $this->removeUpdateFromList($updates);
            }

            return $updates;
        }

        /**
         * @param StdClass|null $updates
         * @param PluginUpdate_3_1 $updateToAdd
         * @return StdClass
         */
        private function addUpdateToList($updates, $updateToAdd) {
            if ( !is_object($updates) ) {
                $updates = new stdClass();
                $updates->response = array();
            }

            $wpUpdate = $updateToAdd->toWpFormat();
            $pluginFile = $this->pluginFile;

            if ( $this->isMuPlugin() ) {
                //WP does not support automatic update installation for mu-plugins, but we can still display a notice.
                $wpUpdate->package = null;
                $pluginFile = $this->muPluginFile;
            }
            $updates->response[$pluginFile] = $wpUpdate;
            return $updates;
        }

        /**
         * @param stdClass|null $updates
         * @return stdClass|null
         */
        private function removeUpdateFromList($updates) {
            if ( isset($updates, $updates->response) ) {
                unset($updates->response[$this->pluginFile]);
                if ( !empty($this->muPluginFile) ) {
                    unset($updates->response[$this->muPluginFile]);
                }
            }
            return $updates;
        }

        /**
         * Insert translation updates into the list maintained by WordPress.
         *
         * @param stdClass $updates
         * @return stdClass
         */
        public function injectTranslationUpdates($updates) {
            $translationUpdates = $this->getTranslationUpdates();
            if ( empty($translationUpdates) ) {
                return $updates;
            }

            //Being defensive.
            if ( !is_object($updates) ) {
                $updates = new stdClass();
            }
            if ( !isset($updates->translations) ) {
                $updates->translations = array();
            }

            //In case there's a name collision with a plugin hosted on wordpress.org,
            //remove any preexisting updates that match our plugin.
            $translationType = 'plugin';
            $filteredTranslations = array();
            foreach($updates->translations as $translation) {
                if ( ($translation['type'] === $translationType) && ($translation['slug'] === $this->slug) ) {
                    continue;
                }
                $filteredTranslations[] = $translation;
            }
            $updates->translations = $filteredTranslations;

            //Add our updates to the list.
            foreach($translationUpdates as $update) {
                $convertedUpdate = array_merge(
                    array(
                        'type' => $translationType,
                        'slug' => $this->slug,
                        'autoupdate' => 0,
                        //AFAICT, WordPress doesn't actually use the "version" field for anything.
                        //But lets make sure it's there, just in case.
                        'version' => isset($update->version) ? $update->version : ('1.' . strtotime($update->updated)),
                    ),
                    (array)$update
                );

                $updates->translations[] = $convertedUpdate;
            }

            return $updates;
        }

        /**
         * Rename the update directory to match the existing plugin directory.
         *
         * When WordPress installs a plugin or theme update, it assumes that the ZIP file will contain
         * exactly one directory, and that the directory name will be the same as the directory where
         * the plugin/theme is currently installed.
         *
         * GitHub and other repositories provide ZIP downloads, but they often use directory names like
         * "project-branch" or "project-tag-hash". We need to change the name to the actual plugin folder.
         *
         * This is a hook callback. Don't call it from a plugin.
         *
         * @param string $source The directory to copy to /wp-content/plugins. Usually a subdirectory of $remoteSource.
         * @param string $remoteSource WordPress has extracted the update to this directory.
         * @param WP_Upgrader $upgrader
         * @return string|WP_Error
         */
        public function fixDirectoryName($source, $remoteSource, $upgrader) {
            global $wp_filesystem; /** @var WP_Filesystem_Base $wp_filesystem */

            //Basic sanity checks.
            if ( !isset($source, $remoteSource, $upgrader, $upgrader->skin, $wp_filesystem) ) {
                return $source;
            }

            //If WordPress is upgrading anything other than our plugin, leave the directory name unchanged.
            if ( !$this->isPluginBeingUpgraded($upgrader) ) {
                return $source;
            }

            //Rename the source to match the existing plugin directory.
            $pluginDirectoryName = dirname($this->pluginFile);
            if ( $pluginDirectoryName === '.' ) {
                return $source;
            }
            $correctedSource = trailingslashit($remoteSource) . $pluginDirectoryName . '/';
            if ( $source !== $correctedSource ) {
                //The update archive should contain a single directory that contains the rest of plugin files. Otherwise,
                //WordPress will try to copy the entire working directory ($source == $remoteSource). We can't rename
                //$remoteSource because that would break WordPress code that cleans up temporary files after update.
                if ( $this->isBadDirectoryStructure($remoteSource) ) {
                    return new WP_Error(
                        'puc-incorrect-directory-structure',
                        sprintf(
                            'The directory structure of the update is incorrect. All plugin files should be inside ' .
                            'a directory named <span class="code">%s</span>, not at the root of the ZIP file.',
                            htmlentities($this->slug)
                        )
                    );
                }

                /** @var WP_Upgrader_Skin $upgrader->skin */
                $upgrader->skin->feedback(sprintf(
                    'Renaming %s to %s&#8230;',
                    '<span class="code">' . basename($source) . '</span>',
                    '<span class="code">' . $pluginDirectoryName . '</span>'
                ));

                if ( $wp_filesystem->move($source, $correctedSource, true) ) {
                    $upgrader->skin->feedback('Plugin directory successfully renamed.');
                    return $correctedSource;
                } else {
                    return new WP_Error(
                        'puc-rename-failed',
                        'Unable to rename the update to match the existing plugin directory.'
                    );
                }
            }

            return $source;
        }

        /**
         * Check for incorrect update directory structure. An update must contain a single directory,
         * all other files should be inside that directory.
         *
         * @param string $remoteSource Directory path.
         * @return bool
         */
        private function isBadDirectoryStructure($remoteSource) {
            global $wp_filesystem; /** @var WP_Filesystem_Base $wp_filesystem */

            $sourceFiles = $wp_filesystem->dirlist($remoteSource);
            if ( is_array($sourceFiles) ) {
                $sourceFiles = array_keys($sourceFiles);
                $firstFilePath = trailingslashit($remoteSource) . $sourceFiles[0];
                return (count($sourceFiles) > 1) || (!$wp_filesystem->is_dir($firstFilePath));
            }

            //Assume it's fine.
            return false;
        }

        /**
         * Is there and update being installed RIGHT NOW, for this specific plugin?
         *
         * @param WP_Upgrader|null $upgrader The upgrader that's performing the current update.
         * @return bool
         */
        public function isPluginBeingUpgraded($upgrader = null) {
            return $this->upgraderStatus->isPluginBeingUpgraded($this->pluginFile, $upgrader);
        }

        /**
         * Get the details of the currently available update, if any.
         *
         * If no updates are available, or if the last known update version is below or equal
         * to the currently installed version, this method will return NULL.
         *
         * Uses cached update data. To retrieve update information straight from
         * the metadata URL, call requestUpdate() instead.
         *
         * @return PluginUpdate_3_1|null
         */
        public function getUpdate() {
            $state = $this->getUpdateState(); /** @var StdClass $state */

            //Is there an update available?
            if ( isset($state, $state->update) ) {
                $update = $state->update;
                //Check if the update is actually newer than the currently installed version.
                $installedVersion = $this->getInstalledVersion();
                if ( ($installedVersion !== null) && version_compare($update->version, $installedVersion, '>') ){
                    $update->filename = $this->pluginFile;
                    return $update;
                }
            }
            return null;
        }

        /**
         * Get a list of available translation updates.
         *
         * This method will return an empty array if there are no updates.
         * Uses cached update data.
         *
         * @return array
         */
        public function getTranslationUpdates() {
            $state = $this->getUpdateState();
            if ( isset($state, $state->update, $state->update->translations) ) {
                return $state->update->translations;
            }
            return array();
        }

        /**
         * Remove all cached translation updates.
         *
         * @see wp_clean_update_cache
         */
        public function clearCachedTranslationUpdates() {
            $state = $this->getUpdateState();
            if ( isset($state, $state->update, $state->update->translations) ) {
                $state->update->translations = array();
                $this->setUpdateState($state);
            }
        }

        /**
         * Add a "Check for updates" link to the plugin row in the "Plugins" page. By default,
         * the new link will appear after the "Visit plugin site" link.
         *
         * You can change the link text by using the "puc_manual_check_link-$slug" filter.
         * Returning an empty string from the filter will disable the link.
         *
         * @param array $pluginMeta Array of meta links.
         * @param string $pluginFile
         * @return array
         */
        public function addCheckForUpdatesLink($pluginMeta, $pluginFile) {
            $isRelevant = ($pluginFile == $this->pluginFile)
                || (!empty($this->muPluginFile) && $pluginFile == $this->muPluginFile);

            if ( $isRelevant && current_user_can('update_plugins') ) {
                $linkUrl = wp_nonce_url(
                    add_query_arg(
                        array(
                            'puc_check_for_updates' => 1,
                            'puc_slug' => $this->slug,
                        ),
                        self_admin_url('plugins.php')
                    ),
                    'puc_check_for_updates'
                );

                $linkText = apply_filters('puc_manual_check_link-' . $this->slug, __('Check for updates', 'plugin-update-checker'));
                if ( !empty($linkText) ) {
                    $pluginMeta[] = sprintf('<a href="%s">%s</a>', esc_attr($linkUrl), $linkText);
                }
            }
            return $pluginMeta;
        }

        /**
         * Check for updates when the user clicks the "Check for updates" link.
         * @see self::addCheckForUpdatesLink()
         *
         * @return void
         */
        public function handleManualCheck() {
            $shouldCheck =
                isset($_GET['puc_check_for_updates'], $_GET['puc_slug'])
                && $_GET['puc_slug'] == $this->slug
                && current_user_can('update_plugins')
                && check_admin_referer('puc_check_for_updates');

            if ( $shouldCheck ) {
                $update = $this->checkForUpdates();
                $status = ($update === null) ? 'no_update' : 'update_available';
                wp_redirect(add_query_arg(
                    array(
                        'puc_update_check_result' => $status,
                        'puc_slug' => $this->slug,
                    ),
                    self_admin_url('plugins.php')
                ));
            }
        }

        /**
         * Display the results of a manual update check.
         * @see self::handleManualCheck()
         *
         * You can change the result message by using the "puc_manual_check_message-$slug" filter.
         */
        public function displayManualCheckResult() {
            if ( isset($_GET['puc_update_check_result'], $_GET['puc_slug']) && ($_GET['puc_slug'] == $this->slug) ) {
                $status = strval($_GET['puc_update_check_result']);
                if ( $status == 'no_update' ) {
                    $message = __('This plugin is up to date.', 'plugin-update-checker');
                } else if ( $status == 'update_available' ) {
                    $message = __('A new version of this plugin is available.', 'plugin-update-checker');
                } else {
                    $message = sprintf(__('Unknown update checker status "%s"', 'plugin-update-checker'), htmlentities($status));
                }
                printf(
                    '<div class="updated notice is-dismissible"><p>%s</p></div>',
                    apply_filters('puc_manual_check_message-' . $this->slug, $message, $status)
                );
            }
        }

        /**
         * Check if the plugin file is inside the mu-plugins directory.
         *
         * @return bool
         */
        protected function isMuPlugin() {
            static $cachedResult = null;

            if ( $cachedResult === null ) {
                //Convert both paths to the canonical form before comparison.
                $muPluginDir = realpath(WPMU_PLUGIN_DIR);
                $pluginPath  = realpath($this->pluginAbsolutePath);

                $cachedResult = (strpos($pluginPath, $muPluginDir) === 0);
            }

            return $cachedResult;
        }

        /**
         * MU plugins are partially supported, but only when we know which file in mu-plugins
         * corresponds to this plugin.
         *
         * @return bool
         */
        protected function isUnknownMuPlugin() {
            return empty($this->muPluginFile) && $this->isMuPlugin();
        }

        /**
         * Clear the cached plugin version. This method can be set up as a filter (hook) and will
         * return the filter argument unmodified.
         *
         * @param mixed $filterArgument
         * @return mixed
         */
        public function clearCachedVersion($filterArgument = null) {
            $this->cachedInstalledVersion = null;
            return $filterArgument;
        }

        /**
         * Register a callback for filtering query arguments.
         *
         * The callback function should take one argument - an associative array of query arguments.
         * It should return a modified array of query arguments.
         *
         * @uses add_filter() This method is a convenience wrapper for add_filter().
         *
         * @param callable $callback
         * @return void
         */
        public function addQueryArgFilter($callback){
            add_filter('puc_request_info_query_args-'.$this->slug, $callback);
        }

        /**
         * Register a callback for filtering arguments passed to wp_remote_get().
         *
         * The callback function should take one argument - an associative array of arguments -
         * and return a modified array or arguments. See the WP documentation on wp_remote_get()
         * for details on what arguments are available and how they work.
         *
         * @uses add_filter() This method is a convenience wrapper for add_filter().
         *
         * @param callable $callback
         * @return void
         */
        public function addHttpRequestArgFilter($callback){
            add_filter('puc_request_info_options-'.$this->slug, $callback);
        }

        /**
         * Register a callback for filtering the plugin info retrieved from the external API.
         *
         * The callback function should take two arguments. If the plugin info was retrieved
         * successfully, the first argument passed will be an instance of  PluginInfo. Otherwise,
         * it will be NULL. The second argument will be the corresponding return value of
         * wp_remote_get (see WP docs for details).
         *
         * The callback function should return a new or modified instance of PluginInfo or NULL.
         *
         * @uses add_filter() This method is a convenience wrapper for add_filter().
         *
         * @param callable $callback
         * @return void
         */
        public function addResultFilter($callback){
            add_filter('puc_request_info_result-'.$this->slug, $callback, 10, 2);
        }

        /**
         * Register a callback for one of the update checker filters.
         *
         * Identical to add_filter(), except it automatically adds the "puc_" prefix
         * and the "-$plugin_slug" suffix to the filter name. For example, "request_info_result"
         * becomes "puc_request_info_result-your_plugin_slug".
         *
         * @param string $tag
         * @param callable $callback
         * @param int $priority
         * @param int $acceptedArgs
         */
        public function addFilter($tag, $callback, $priority = 10, $acceptedArgs = 1) {
            add_filter('puc_' . $tag . '-' . $this->slug, $callback, $priority, $acceptedArgs);
        }


        /**
         * Trigger a PHP error, but only when $debugMode is enabled.
         *
         * @param string $message
         * @param int $errorType
         */
        protected function triggerError($message, $errorType) {
            if ( $this->debugMode ) {
                trigger_error($message, $errorType);
            }
        }
    }

endif;

if ( !class_exists('PluginInfo_3_1', false) ):

    /**
     * A container class for holding and transforming various plugin metadata.
     *
     * @author Janis Elsts
     * @copyright 2016
     * @version 3.0
     * @access public
     */
    class PluginInfo_3_1 {
        //Most fields map directly to the contents of the plugin's info.json file.
        //See the relevant docs for a description of their meaning.
        public $name;
        public $slug;
        public $version;
        public $homepage;
        public $sections = array();
        public $banners;
        public $translations = array();
        public $download_url;

        public $author;
        public $author_homepage;

        public $requires;
        public $tested;
        public $upgrade_notice;

        public $rating;
        public $num_ratings;
        public $downloaded;
        public $active_installs;
        public $last_updated;

        public $id = 0; //The native WP.org API returns numeric plugin IDs, but they're not used for anything.

        public $filename; //Plugin filename relative to the plugins directory.

        /**
         * Create a new instance of PluginInfo from JSON-encoded plugin info
         * returned by an external update API.
         *
         * @param string $json Valid JSON string representing plugin info.
         * @return PluginInfo_3_1|null New instance of PluginInfo, or NULL on error.
         */
        public static function fromJson($json){
            /** @var StdClass $apiResponse */
            $apiResponse = json_decode($json);
            if ( empty($apiResponse) || !is_object($apiResponse) ){
                trigger_error(
                    "Failed to parse plugin metadata. Try validating your .json file with http://jsonlint.com/",
                    E_USER_NOTICE
                );
                return null;
            }

            $valid = self::validateMetadata($apiResponse);
            if ( is_wp_error($valid) ){
                trigger_error($valid->get_error_message(), E_USER_NOTICE);
                return null;
            }

            $info = new self();
            foreach(get_object_vars($apiResponse) as $key => $value){
                $info->$key = $value;
            }

            //json_decode decodes assoc. arrays as objects. We want it as an array.
            $info->sections = (array)$info->sections;

            return $info;
        }

        /**
         * Very, very basic validation.
         *
         * @param StdClass $apiResponse
         * @return bool|WP_Error
         */
        protected static function validateMetadata($apiResponse) {
            if (
                !isset($apiResponse->name, $apiResponse->version)
                || empty($apiResponse->name)
                || empty($apiResponse->version)
            ) {
                return new WP_Error(
                    'puc-invalid-metadata',
                    "The plugin metadata file does not contain the required 'name' and/or 'version' keys."
                );
            }
            return true;
        }


        /**
         * Transform plugin info into the format used by the native WordPress.org API
         *
         * @return object
         */
        public function toWpFormat(){
            $info = new stdClass;

            //The custom update API is built so that many fields have the same name and format
            //as those returned by the native WordPress.org API. These can be assigned directly.
            $sameFormat = array(
                'name', 'slug', 'version', 'requires', 'tested', 'rating', 'upgrade_notice',
                'num_ratings', 'downloaded', 'active_installs', 'homepage', 'last_updated',
            );
            foreach($sameFormat as $field){
                if ( isset($this->$field) ) {
                    $info->$field = $this->$field;
                } else {
                    $info->$field = null;
                }
            }

            //Other fields need to be renamed and/or transformed.
            $info->download_link = $this->download_url;
            $info->author = $this->getFormattedAuthor();
            $info->sections = array_merge(array('description' => ''), $this->sections);

            if ( !empty($this->banners) ) {
                //WP expects an array with two keys: "high" and "low". Both are optional.
                //Docs: https://wordpress.org/plugins/about/faq/#banners
                $info->banners = is_object($this->banners) ? get_object_vars($this->banners) : $this->banners;
                $info->banners = array_intersect_key($info->banners, array('high' => true, 'low' => true));
            }

            return $info;
        }

        protected function getFormattedAuthor() {
            if ( !empty($this->author_homepage) ){
                return sprintf('<a href="%s">%s</a>', $this->author_homepage, $this->author);
            }
            return $this->author;
        }
    }

endif;

if ( !class_exists('PluginUpdate_3_1', false) ):

    /**
     * A simple container class for holding information about an available update.
     *
     * @author Janis Elsts
     * @copyright 2016
     * @version 3.0
     * @access public
     */
    class PluginUpdate_3_1 {
        public $id = 0;
        public $slug;
        public $version;
        public $homepage;
        public $download_url;
        public $upgrade_notice;
        public $tested;
        public $translations = array();
        public $filename; //Plugin filename relative to the plugins directory.

        private static $fields = array(
            'id', 'slug', 'version', 'homepage', 'tested',
            'download_url', 'upgrade_notice', 'filename',
            'translations'
        );

        /**
         * Create a new instance of PluginUpdate from its JSON-encoded representation.
         *
         * @param string $json
         * @return PluginUpdate_3_1|null
         */
        public static function fromJson($json){
            //Since update-related information is simply a subset of the full plugin info,
            //we can parse the update JSON as if it was a plugin info string, then copy over
            //the parts that we care about.
            $pluginInfo = PluginInfo_3_1::fromJson($json);
            if ( $pluginInfo != null ) {
                return self::fromPluginInfo($pluginInfo);
            } else {
                return null;
            }
        }

        /**
         * Create a new instance of PluginUpdate based on an instance of PluginInfo.
         * Basically, this just copies a subset of fields from one object to another.
         *
         * @param PluginInfo_3_1 $info
         * @return PluginUpdate_3_1
         */
        public static function fromPluginInfo($info){
            return self::fromObject($info);
        }

        /**
         * Create a new instance of PluginUpdate by copying the necessary fields from
         * another object.
         *
         * @param StdClass|PluginInfo_3_1|PluginUpdate_3_1 $object The source object.
         * @return PluginUpdate_3_1 The new copy.
         */
        public static function fromObject($object) {
            $update = new self();
            $fields = self::$fields;
            if ( !empty($object->slug) ) {
                $fields = apply_filters('puc_retain_fields-' . $object->slug, $fields);
            }
            foreach($fields as $field){
                if (property_exists($object, $field)) {
                    $update->$field = $object->$field;
                }
            }
            return $update;
        }

        /**
         * Create an instance of StdClass that can later be converted back to
         * a PluginUpdate. Useful for serialization and caching, as it avoids
         * the "incomplete object" problem if the cached value is loaded before
         * this class.
         *
         * @return StdClass
         */
        public function toStdClass() {
            $object = new stdClass();
            $fields = self::$fields;
            if ( !empty($this->slug) ) {
                $fields = apply_filters('puc_retain_fields-' . $this->slug, $fields);
            }
            foreach($fields as $field){
                if (property_exists($this, $field)) {
                    $object->$field = $this->$field;
                }
            }
            return $object;
        }


        /**
         * Transform the update into the format used by WordPress native plugin API.
         *
         * @return object
         */
        public function toWpFormat(){
            $update = new stdClass;

            $update->id = $this->id;
            $update->slug = $this->slug;
            $update->new_version = $this->version;
            $update->url = $this->homepage;
            $update->package = $this->download_url;
            $update->tested = $this->tested;
            $update->plugin = $this->filename;

            if ( !empty($this->upgrade_notice) ){
                $update->upgrade_notice = $this->upgrade_notice;
            }

            return $update;
        }
    }

endif;

if ( !class_exists('PucScheduler_3_1', false) ):

    /**
     * The scheduler decides when and how often to check for updates.
     * It calls @see PluginUpdateChecker::checkForUpdates() to perform the actual checks.
     *
     * @version 3.0
     */
    class PucScheduler_3_1 {
        public $checkPeriod = 12; //How often to check for updates (in hours).
        public $throttleRedundantChecks = false; //Check less often if we already know that an update is available.
        public $throttledCheckPeriod = 72;

        /**
         * @var PluginUpdateChecker_3_1
         */
        protected $updateChecker;

        private $cronHook = null;

        /**
         * Scheduler constructor.
         *
         * @param PluginUpdateChecker_3_1 $updateChecker
         * @param int $checkPeriod How often to check for updates (in hours).
         */
        public function __construct($updateChecker, $checkPeriod) {
            $this->updateChecker = $updateChecker;
            $this->checkPeriod = $checkPeriod;

            //Set up the periodic update checks
            $this->cronHook = 'check_plugin_updates-' . $this->updateChecker->slug;
            if ( $this->checkPeriod > 0 ){

                //Trigger the check via Cron.
                //Try to use one of the default schedules if possible as it's less likely to conflict
                //with other plugins and their custom schedules.
                $defaultSchedules = array(
                    1  => 'hourly',
                    12 => 'twicedaily',
                    24 => 'daily',
                );
                if ( array_key_exists($this->checkPeriod, $defaultSchedules) ) {
                    $scheduleName = $defaultSchedules[$this->checkPeriod];
                } else {
                    //Use a custom cron schedule.
                    $scheduleName = 'every' . $this->checkPeriod . 'hours';
                    add_filter('cron_schedules', array($this, '_addCustomSchedule'));
                }

                if ( !wp_next_scheduled($this->cronHook) && !defined('WP_INSTALLING') ) {
                    wp_schedule_event(time(), $scheduleName, $this->cronHook);
                }
                add_action($this->cronHook, array($this, 'maybeCheckForUpdates'));

                register_deactivation_hook($this->updateChecker->pluginFile, array($this, '_removeUpdaterCron'));

                //In case Cron is disabled or unreliable, we also manually trigger
                //the periodic checks while the user is browsing the Dashboard.
                add_action( 'admin_init', array($this, 'maybeCheckForUpdates') );

                //Like WordPress itself, we check more often on certain pages.
                /** @see wp_update_plugins */
                add_action('load-update-core.php', array($this, 'maybeCheckForUpdates'));
                add_action('load-plugins.php', array($this, 'maybeCheckForUpdates'));
                add_action('load-update.php', array($this, 'maybeCheckForUpdates'));
                //This hook fires after a bulk update is complete.
                add_action('upgrader_process_complete', array($this, 'maybeCheckForUpdates'), 11);

            } else {
                //Periodic checks are disabled.
                wp_clear_scheduled_hook($this->cronHook);
            }
        }

        /**
         * Check for updates if the configured check interval has already elapsed.
         * Will use a shorter check interval on certain admin pages like "Dashboard -> Updates" or when doing cron.
         *
         * You can override the default behaviour by using the "puc_check_now-$slug" filter.
         * The filter callback will be passed three parameters:
         *     - Current decision. TRUE = check updates now, FALSE = don't check now.
         *     - Last check time as a Unix timestamp.
         *     - Configured check period in hours.
         * Return TRUE to check for updates immediately, or FALSE to cancel.
         *
         * This method is declared public because it's a hook callback. Calling it directly is not recommended.
         */
        public function maybeCheckForUpdates(){
            if ( empty($this->checkPeriod) ){
                return;
            }

            $state = $this->updateChecker->getUpdateState();
            $shouldCheck =
                empty($state) ||
                !isset($state->lastCheck) ||
                ( (time() - $state->lastCheck) >= $this->getEffectiveCheckPeriod() );

            //Let plugin authors substitute their own algorithm.
            $shouldCheck = apply_filters(
                'puc_check_now-' . $this->updateChecker->slug,
                $shouldCheck,
                (!empty($state) && isset($state->lastCheck)) ? $state->lastCheck : 0,
                $this->checkPeriod
            );

            if ( $shouldCheck ) {
                $this->updateChecker->checkForUpdates();
            }
        }

        /**
         * Calculate the actual check period based on the current status and environment.
         *
         * @return int Check period in seconds.
         */
        protected function getEffectiveCheckPeriod() {
            $currentFilter = current_filter();
            if ( in_array($currentFilter, array('load-update-core.php', 'upgrader_process_complete')) ) {
                //Check more often when the user visits "Dashboard -> Updates" or does a bulk update.
                $period = 60;
            } else if ( in_array($currentFilter, array('load-plugins.php', 'load-update.php')) ) {
                //Also check more often on the "Plugins" page and /wp-admin/update.php.
                $period = 3600;
            } else if ( $this->throttleRedundantChecks && ($this->updateChecker->getUpdate() !== null) ) {
                //Check less frequently if it's already known that an update is available.
                $period = $this->throttledCheckPeriod * 3600;
            } else if ( defined('DOING_CRON') && constant('DOING_CRON') ) {
                //WordPress cron schedules are not exact, so lets do an update check even
                //if slightly less than $checkPeriod hours have elapsed since the last check.
                $cronFuzziness = 20 * 60;
                $period = $this->checkPeriod * 3600 - $cronFuzziness;
            } else {
                $period = $this->checkPeriod * 3600;
            }

            return $period;
        }

        /**
         * Add our custom schedule to the array of Cron schedules used by WP.
         *
         * @param array $schedules
         * @return array
         */
        public function _addCustomSchedule($schedules){
            if ( $this->checkPeriod && ($this->checkPeriod > 0) ){
                $scheduleName = 'every' . $this->checkPeriod . 'hours';
                $schedules[$scheduleName] = array(
                    'interval' => $this->checkPeriod * 3600,
                    'display' => sprintf('Every %d hours', $this->checkPeriod),
                );
            }
            return $schedules;
        }

        /**
         * Remove the scheduled cron event that the library uses to check for updates.
         *
         * @return void
         */
        public function _removeUpdaterCron(){
            wp_clear_scheduled_hook($this->cronHook);
        }

        /**
         * Get the name of the update checker's WP-cron hook. Mostly useful for debugging.
         *
         * @return string
         */
        public function getCronHookName() {
            return $this->cronHook;
        }
    }

endif;


if ( !class_exists('PucUpgraderStatus_3_1', false) ):

    /**
     * A utility class that helps figure out which plugin WordPress is upgrading.
     *
     * It may seem strange to have an separate class just for that, but the task is surprisingly complicated.
     * Core classes like Plugin_Upgrader don't expose the plugin file name during an in-progress update (AFAICT).
     * This class uses a few workarounds and heuristics to get the file name.
     */
    class PucUpgraderStatus_3_1 {
        private $upgradedPluginFile = null; //The plugin that is currently being upgraded by WordPress.

        public function __construct() {
            //Keep track of which plugin WordPress is currently upgrading.
            add_filter('upgrader_pre_install', array($this, 'setUpgradedPlugin'), 10, 2);
            add_filter('upgrader_package_options', array($this, 'setUpgradedPluginFromOptions'), 10, 1);
            add_filter('upgrader_post_install', array($this, 'clearUpgradedPlugin'), 10, 1);
            add_action('upgrader_process_complete', array($this, 'clearUpgradedPlugin'), 10, 1);
        }

        /**
         * Is there and update being installed RIGHT NOW, for a specific plugin?
         *
         * Caution: This method is unreliable. WordPress doesn't make it easy to figure out what it is upgrading,
         * and upgrader implementations are liable to change without notice.
         *
         * @param string $pluginFile The plugin to check.
         * @param WP_Upgrader|null $upgrader The upgrader that's performing the current update.
         * @return bool True if the plugin identified by $pluginFile is being upgraded.
         */
        public function isPluginBeingUpgraded($pluginFile, $upgrader = null) {
            if ( isset($upgrader) ) {
                $upgradedPluginFile = $this->getPluginBeingUpgradedBy($upgrader);
                if ( !empty($upgradedPluginFile) ) {
                    $this->upgradedPluginFile = $upgradedPluginFile;
                }
            }
            return ( !empty($this->upgradedPluginFile) && ($this->upgradedPluginFile === $pluginFile) );
        }

        /**
         * Get the file name of the plugin that's currently being upgraded.
         *
         * @param Plugin_Upgrader|WP_Upgrader $upgrader
         * @return string|null
         */
        private function getPluginBeingUpgradedBy($upgrader) {
            if ( !isset($upgrader, $upgrader->skin) ) {
                return null;
            }

            //Figure out which plugin is being upgraded.
            $pluginFile = null;
            $skin = $upgrader->skin;
            if ( $skin instanceof Plugin_Upgrader_Skin ) {
                if ( isset($skin->plugin) && is_string($skin->plugin) && ($skin->plugin !== '') ) {
                    $pluginFile = $skin->plugin;
                }
            } elseif ( isset($skin->plugin_info) && is_array($skin->plugin_info) ) {
                //This case is tricky because Bulk_Plugin_Upgrader_Skin (etc) doesn't actually store the plugin
                //filename anywhere. Instead, it has the plugin headers in $plugin_info. So the best we can
                //do is compare those headers to the headers of installed plugins.
                $pluginFile = $this->identifyPluginByHeaders($skin->plugin_info);
            }

            return $pluginFile;
        }

        /**
         * Identify an installed plugin based on its headers.
         *
         * @param array $searchHeaders The plugin file header to look for.
         * @return string|null Plugin basename ("foo/bar.php"), or NULL if we can't identify the plugin.
         */
        private function identifyPluginByHeaders($searchHeaders) {
            if ( !function_exists('get_plugins') ){
                /** @noinspection PhpIncludeInspection */
                require_once( ABSPATH . '/wp-admin/includes/plugin.php' );
            }

            $installedPlugins = get_plugins();
            $matches = array();
            foreach($installedPlugins as $pluginBasename => $headers) {
                $diff1 = array_diff_assoc($headers, $searchHeaders);
                $diff2 = array_diff_assoc($searchHeaders, $headers);
                if ( empty($diff1) && empty($diff2) ) {
                    $matches[] = $pluginBasename;
                }
            }

            //It's possible (though very unlikely) that there could be two plugins with identical
            //headers. In that case, we can't unambiguously identify the plugin that's being upgraded.
            if ( count($matches) !== 1 ) {
                return null;
            }

            return reset($matches);
        }

        /**
         * @access private
         *
         * @param mixed $input
         * @param array $hookExtra
         * @return mixed Returns $input unaltered.
         */
        public function setUpgradedPlugin($input, $hookExtra) {
            if (!empty($hookExtra['plugin']) && is_string($hookExtra['plugin'])) {
                $this->upgradedPluginFile = $hookExtra['plugin'];
            } else {
                $this->upgradedPluginFile = null;
            }
            return $input;
        }

        /**
         * @access private
         *
         * @param array $options
         * @return array
         */
        public function setUpgradedPluginFromOptions($options) {
            if (isset($options['hook_extra']['plugin']) && is_string($options['hook_extra']['plugin'])) {
                $this->upgradedPluginFile = $options['hook_extra']['plugin'];
            } else {
                $this->upgradedPluginFile = null;
            }
            return $options;
        }

        /**
         * @access private
         *
         * @param mixed $input
         * @return mixed Returns $input unaltered.
         */
        public function clearUpgradedPlugin($input = null) {
            $this->upgradedPluginFile = null;
            return $input;
        }
    }

endif;


if ( !class_exists('PucFactory', false) ):

    /**
     * A factory that builds instances of other classes from this library.
     *
     * When multiple versions of the same class have been loaded (e.g. PluginUpdateChecker 1.2
     * and 1.3), this factory will always use the latest available version. Register class
     * versions by calling {@link PucFactory::addVersion()}.
     *
     * At the moment it can only build instances of the PluginUpdateChecker class. Other classes
     * are intended mainly for internal use and refer directly to specific implementations. If you
     * want to instantiate one of them anyway, you can use {@link PucFactory::getLatestClassVersion()}
     * to get the class name and then create it with <code>new $class(...)</code>.
     */
    class PucFactory {
        protected static $classVersions = array();
        protected static $sorted = false;

        /**
         * Create a new instance of PluginUpdateChecker.
         *
         * @see PluginUpdateChecker::__construct()
         *
         * @param $metadataUrl
         * @param $pluginFile
         * @param string $slug
         * @param int $checkPeriod
         * @param string $optionName
         * @param string $muPluginFile
         * @return PluginUpdateChecker_3_1
         */
        public static function buildUpdateChecker($metadataUrl, $pluginFile, $slug = '', $checkPeriod = 12, $optionName = '', $muPluginFile = '') {
            $class = self::getLatestClassVersion('PluginUpdateChecker');
            return new $class($metadataUrl, $pluginFile, $slug, $checkPeriod, $optionName, $muPluginFile);
        }

        /**
         * Get the specific class name for the latest available version of a class.
         *
         * @param string $class
         * @return string|null
         */
        public static function getLatestClassVersion($class) {
            if ( !self::$sorted ) {
                self::sortVersions();
            }

            if ( isset(self::$classVersions[$class]) ) {
                return reset(self::$classVersions[$class]);
            } else {
                return null;
            }
        }

        /**
         * Sort available class versions in descending order (i.e. newest first).
         */
        protected static function sortVersions() {
            foreach ( self::$classVersions as $class => $versions ) {
                uksort($versions, array(__CLASS__, 'compareVersions'));
                self::$classVersions[$class] = $versions;
            }
            self::$sorted = true;
        }

        protected static function compareVersions($a, $b) {
            return -version_compare($a, $b);
        }

        /**
         * Register a version of a class.
         *
         * @access private This method is only for internal use by the library.
         *
         * @param string $generalClass Class name without version numbers, e.g. 'PluginUpdateChecker'.
         * @param string $versionedClass Actual class name, e.g. 'PluginUpdateChecker_1_2'.
         * @param string $version Version number, e.g. '1.2'.
         */
        public static function addVersion($generalClass, $versionedClass, $version) {
            if ( !isset(self::$classVersions[$generalClass]) ) {
                self::$classVersions[$generalClass] = array();
            }
            self::$classVersions[$generalClass][$version] = $versionedClass;
            self::$sorted = false;
        }
    }

endif;

//Register classes defined in this file with the factory.
PucFactory::addVersion('PluginUpdateChecker', 'PluginUpdateChecker_3_1', '3.1');
PucFactory::addVersion('PluginUpdate', 'PluginUpdate_3_1', '3.1');
PucFactory::addVersion('PluginInfo', 'PluginInfo_3_1', '3.1');