<?php

if (!defined('ABSPATH')) die('No direct access allowed');

if (!class_exists('WP_Optimize_Load_Url_Task')) {
	require_once(WPO_PLUGIN_MAIN_PATH . 'cache/class-wpo-load-url-task.php');
}

class WP_Optimize_Page_Cache_Preloader extends WP_Optimize_Preloader {
	
	/**
	 * @var string
	 */
	protected $preload_type = 'page_cache';
	
	/**
	 * @var string
	 */
	protected $task_type = 'load-url-task';
	
	/**
	 * @var null | WP_Optimize_Page_Cache_Preloader
	 */
	static protected $_instance = null;

	/**
	 * List of URLs for which a task will be created to preload them into cache
	 *
	 * @var array
	 */
	private $url_preload_list = array();

	/**
	 * WP_Optimize_Page_Cache_Preloader constructor.
	 */
	public function __construct() {
		parent::__construct();

		add_action('wpo_page_cache_schedule_preload', array($this, 'run_scheduled_cache_preload'));
		add_action('wpo_page_cache_run_preload', array($this, 'run_preload_cron_job'));
		add_filter('wpo_preload_headers', array($this, 'preload_headers'));
	}

	/**
	 * Check if cache is active.
	 *
	 * @return bool
	 */
	public function is_option_active() {
		return WP_Optimize()->get_page_cache()->is_enabled();
	}

	/**
	 * Schedule or delete automatic preload action on cache settings update.
	 *
	 * @param array $new_settings      The new settings
	 * @param array $previous_settings Settings before saving
	 */
	public function cache_settings_updated($new_settings, $previous_settings) {
		if (!$new_settings['enable_page_caching']) {
			wp_clear_scheduled_hook('wpo_page_cache_schedule_preload');
			wp_clear_scheduled_hook('wpo_page_cache_run_preload');
			$this->delete_preload_continue_action();
			return;
		}

		if (!empty($new_settings['enable_schedule_preload'])) {

			$last_schedule_type = $previous_settings['preload_schedule_type'];

			$last_use_nighttime = (int) $this->options->get_option('page_cache_schedule_preload_nighttime');
			// By default, we schedule preload events to the nighttime.
			$use_nighttime = apply_filters('wpo_page_cache_schedule_preload_use_nighttime', 1);

			$this->options->update_option('page_cache_schedule_preload_nighttime', $use_nighttime);

			if (wp_next_scheduled('wpo_page_cache_schedule_preload')) {
				// if already scheduled this schedule type
				if ($new_settings['preload_schedule_type'] === $last_schedule_type && ($last_use_nighttime === $use_nighttime)) {
					// If the schedule type is cache lifespan, check if the cache lifespan changed.
					if ('wpo_use_cache_lifespan' === $new_settings['preload_schedule_type']) {
						// Else, if the settings cache lifespan settings haven't changed, returns
						if ($new_settings['page_cache_length_value'] === $previous_settings['page_cache_length_value'] && $new_settings['page_cache_length_unit'] === $previous_settings['page_cache_length_unit']) {
							return;
						}
					} else {
						return;
					}
				}
				// clear currently scheduled preload action.
				wp_clear_scheduled_hook('wpo_page_cache_schedule_preload');
			}

			// schedule preload action.
			$timestamp = time() + $this->get_schedule_interval($new_settings['preload_schedule_type']);

			if ($use_nighttime) {
				$timestamp = $this->make_nighttime($timestamp);
			}

			wp_schedule_event($timestamp, $new_settings['preload_schedule_type'], 'wpo_page_cache_schedule_preload');
		} else {
			wp_clear_scheduled_hook('wpo_page_cache_schedule_preload');
		}

	}

	/**
	 * Clear active preload tasks, reschedule preload action.
	 */
	public function reschedule_preload() {
		// clear scheduled action.
		if (wp_next_scheduled('wpo_page_cache_schedule_preload')) {
			wp_clear_scheduled_hook('wpo_page_cache_schedule_preload');
		}

		// schedule preload action if need.
		if ($this->is_scheduled_preload_enabled()) {
			$preload_schedule_type = $this->get_cache_config('preload_schedule_type');
			// By default, we schedule preload events to the nighttime.
			$use_nighttime = apply_filters('wpo_page_cache_schedule_preload_use_nighttime', 1);

			$timestamp = time() + $this->get_schedule_interval($preload_schedule_type);
			if ($use_nighttime) {
				$timestamp = $this->make_nighttime($timestamp);
			}

			wp_schedule_event($timestamp, $preload_schedule_type, 'wpo_page_cache_schedule_preload');
		}
	}

	/**
	 * Check if scheduled preload enabled.
	 *
	 * @return bool
	 */
	public function is_scheduled_preload_enabled() {
		$enable_schedule_preload = $this->get_cache_config('enable_schedule_preload');
		return !empty($enable_schedule_preload);
	}

	/**
	 * Check if we need run cache preload and run it.
	 */
	public function run_scheduled_cache_preload() {

		$schedule_type = WPO_Cache_Config::instance()->get_option('preload_schedule_type');
		if (!$schedule_type) return;

		// Don't run preload if cache lifespan option enabled and cache not expired yet.
		if ('wpo_use_cache_lifespan' === $schedule_type) {

			/**
			 * Filters the allowed time difference between the cache exiry and the current time, in seconds.
			 * If the cache expires in less than $allowed_time_difference, preload. Otherwise leave it.
			 *
			 * @param integer $allowed_time_difference The time difference, in seconds (default is same as changed time limit)
			 */
			$time_limit = (defined('WP_OPTIMIZE_SET_TIME_LIMIT') && WP_OPTIMIZE_SET_TIME_LIMIT > 15) ? WP_OPTIMIZE_SET_TIME_LIMIT : 1800;

			$allowed_time_difference = apply_filters('wpo_preload_allowed_time_difference', $time_limit);
			$page_cache_lifespan = WPO_Cache_Config::instance()->get_option('page_cache_length', 0);
			$last_preload_time = $this->options->get_option('wpo_last_page_cache_preload', 0);
			$time_since_last_preload = time() - $last_preload_time;
			$minimum_time_to_next_schedule_preload = $page_cache_lifespan - $allowed_time_difference;
			// Skip this if the last preload is not as old as the cache lifespan minus $allowed_time_difference
			if ($page_cache_lifespan > 0 && $time_since_last_preload < $minimum_time_to_next_schedule_preload) return;
		}

		$this->run();
	}

	/**
	 * Runs manual cache preload and reschedules if scheduled preload is enabled
	 *
	 * @return void
	 */
	public function run_preload_cron_job() {
		$wpo_page_cache = WP_Optimize()->get_page_cache();

		if ($this->is_scheduled_preload_enabled()) {
			$this->cancel_preload();
			$this->reschedule_preload();
			$this->run('scheduled', null, true);
		} elseif ($wpo_page_cache->should_auto_preload_purged_contents()) {
			$this->run('manual', null, true);
		}
	}

	/**
	 * Get cache config option value.
	 *
	 * @param string $option
	 *
	 * @return mixed
	 */
	public function get_cache_config($option) {
		static $config = null;

		if (null === $config) $config = WPO_Page_Cache::instance()->config->get();

		if (is_array($config) && array_key_exists($option, $config)) {
			return $config[$option];
		}

		return false;
	}

	/**
	 * Create tasks (WP_Optimize_Load_Url_Task) for preload all urls from site.
	 *
	 * @param string $type The preload type (currently: scheduled, manual)
	 * @return void
	 */
	public function create_tasks_for_preload_site_urls($type) {
		$urls = $this->get_site_urls();

		if (!empty($urls)) {

			$this->log(__('Creating tasks for preload site urls.', 'wp-optimize'));

			foreach ($urls as $url) {
				if (wpo_url_in_exceptions($url)) continue;

				if ($this->url_is_already_cached($url, $type)) {
					continue;
				}

				// this description is being used for internal purposes.
				$description = 'Preload - '.$url;
				$options = array('url' => $url, 'preload_type' => $type, 'anonymous_user_allowed' => (defined('DOING_CRON') && DOING_CRON) || (defined('WP_CLI') && WP_CLI));

				WP_Optimize_Load_Url_Task::create_task($this->task_type, $description, $options, 'WP_Optimize_Load_Url_Task');
			}

			$this->log(__('Tasks for preload site urls created.', 'wp-optimize'));
		}
	}

	/**
	 * Preload mobile version from $url.
	 *
	 * @param string $url
	 *
	 * @return void
	 */
	public function preload_mobile($url) {
		static $is_mobile_caching_enabled;
		if (!isset($is_mobile_caching_enabled)) {
			$is_mobile_caching_enabled = $this->get_cache_config('enable_mobile_caching');
		}

		// Only run if option is active
		if (!$is_mobile_caching_enabled) return;

		$mobile_args = array(
			'httpversion' => '1.1',
			'user-agent'  => WP_Optimize_Utils::get_user_agent('mobile'),
			'timeout'     => 10,
			'headers'     => apply_filters('wpo_preload_headers', array()),
		);

		$mobile_args = apply_filters('wpo_page_cache_preloader_mobile_args', $mobile_args, $url);

		$this->log('preload_mobile - ' . $url);

		wp_remote_get($url, $mobile_args);
	}

	/**
	 * Preload amp version from $url.
	 *
	 * @param string $url
	 *
	 * @return void
	 */
	public function preload_amp($url) {
		if (!apply_filters('wpo_should_preload_amp', false, $url)) return;

		$amp_args = array(
			'httpversion' => '1.1',
			'user-agent'  => WP_Optimize_Utils::get_user_agent(),
			'timeout'     => 10,
			'headers'     => array(
				'X-WP-Optimize-Cache-Preload' => 'Yes',
			),
		);

		$url = untrailingslashit($url) . '/amp/';

		$amp_args = apply_filters('wpo_page_cache_preloader_amp_args', $amp_args, $url);

		$this->log('preload_amp - ' . $url);

		wp_remote_get($url, $amp_args);
	}

	/**
	 * Check if sitemap exists then returns list of urls from sitemap file otherwise returns all posts urls.
	 *
	 * @return array
	 */
	public function get_site_urls() {

		$urls = $this->get_sitemap_urls();

		if (!empty($urls)) {
			$this->options->update_option('wpo_last_page_cache_preload_type', 'sitemap');
		} else {
			$urls = $this->get_post_urls();
			$this->options->update_option('wpo_last_page_cache_preload_type', 'posts');
		}

		// translators: %d: Number of URLs found
		$this->log(sprintf(_n('%d url found.', '%d urls found.', count($urls), 'wp-optimize'), count($urls)));

		/**
		 * Filter the URLs which will be preloaded
		 *
		 * @param array $urls
		 * @return array
		 */
		return apply_filters('wpo_preload_get_site_urls', $urls);
	}

	/**
	 * Loads sitemap file and returns list of urls.
	 *
	 * @param string $sitemap_url
	 *
	 * @return array|bool
	 */
	public function get_sitemap_urls($sitemap_url = '') {

		$urls = array();

		// if sitemap url is empty then use main sitemap file name.
		$sitemap_url = ('' === $sitemap_url) ? site_url('/'.$this->get_sitemap_filename()) : $sitemap_url;

		// if simplexml_load_string not available then we don't load sitemap.
		if (!function_exists('simplexml_load_string')) {
			return $urls;
		}

		// load sitemap file.
		$response = wp_remote_get($sitemap_url, array('timeout' => 30));

		// if we get error then
		// sometimes returns error due to timeout
		if (is_wp_error($response)) {
			$response = @file_get_contents($sitemap_url); // phpcs:ignore Generic.PHP.NoSilencedErrors.Discouraged -- suppress warnings when there is network error

			// if response is empty then try load from file.
			if (empty($response)) {
				$sitemap_file = $this->get_local_sitemap_file();

				if (is_file($sitemap_file)) {
					$response = file_get_contents($sitemap_file);
				}
			}

			if (empty($response)) return $urls;

			$xml = @simplexml_load_string($response); // phpcs:ignore Generic.PHP.NoSilencedErrors.Discouraged -- suppress warnings when error found in XML data
		} else {
			// parse xml answer.
			$xml = @simplexml_load_string(wp_remote_retrieve_body($response)); // phpcs:ignore Generic.PHP.NoSilencedErrors.Discouraged -- suppress warnings when error found in XML data
		}

		// xml file has not valid xml content then return false.
		if (false === $xml) return false;

		// if exists urls then return them.
		if (isset($xml->url)) {
			foreach ($xml->url as $element) {
				if (!isset($element->loc)) continue;
				$urls[] = (string) $element->loc;
			}
		} elseif (isset($xml->sitemap)) {
			// if has links to other sitemap files then get urls from them.
			foreach ($xml->sitemap as $element) {
				if (!isset($element->loc)) continue;

				$sitemap_urls = $this->get_sitemap_urls($element->loc);

				if (is_array($sitemap_urls)) {
					$urls = array_merge($urls, $sitemap_urls);
				}
			}
		}

		return $urls;
	}

	/**
	 * Get all posts of any post type and returns urls for them.
	 *
	 * @return array
	 */
	public function get_post_urls() {
		global $post;

		$offset = 0;
		$posts_per_page = 1000;
		$urls = array();

		$urls[] = site_url('/');

		do {
			$query = new WP_Query(array(
				'post_type'         => 'any',
				'post_status'       => 'publish',
				'posts_per_page'    => $posts_per_page,
				'offset'            => $offset,
				'orderby'           => 'ID',
				'order'             => 'ASC',
				'cache_results'     => false, // disable cache to avoid memory error.
			));

			$posts_loaded = $query->post_count;

			while ($query->have_posts()) {
				$query->the_post();
				$permalink = get_permalink();
				$urls[] = $permalink;

				// check page separators in the post content
				preg_match_all('/\<\!--nextpage--\>/', $post->post_content, $matches);
				// if there any separators add urls for each page
				if (count($matches[0])) {
					$prefix = strpos($permalink, '?') ? '&page=' : '';
					for ($page = 0; $page < count($matches[0]); $page++) {
						if ('' !== $prefix) {
							$urls[] = $permalink . $prefix . ($page+2);
						} else {
							$urls[] = trailingslashit($permalink) . ($page+2);
						}
					}
				}
			}

			$offset += $posts_loaded;
		} while ($posts_loaded > 0);

		/**
		 * If domain mapping enabled then replace domains in urls.
		 */
		if ($this->is_domain_mapping_enabled()) {
			$blog_id = get_current_blog_id();

			$mapped_domain = $this->get_mapped_domain($blog_id);
			$blog_details = get_blog_details($blog_id);

			if ($mapped_domain) {
				foreach ($urls as $i => $url) {
					$urls[$i] = preg_replace('/'.$blog_details->domain.'/i', $mapped_domain, $url, 1);
				}
			}
		}

		wp_reset_postdata();

		return $urls;
	}

	/**
	 * Check if domain mapping enabled.
	 *
	 * @return bool
	 */
	public function is_domain_mapping_enabled() {
		// SUNRISE constant is defined with installation WordPress MU Domain Mapping plugin.
		$enabled = is_multisite() && defined('SUNRISE') && 'on' === strtolower(SUNRISE);

		/**
		 * Filters if Multisite Domain mapping is enabled.
		 * Currently, we can only detect if the WordPress MU Domain Mapping plugin is in use.
		 * Using the WP Core functionality should not require this, unless if the domain name is set somewhere else but in the site url option.
		 */
		return apply_filters('wpo_is_domain_mapping_enabled', $enabled);
	}

	/**
	 * Return mapped domain by $blog_id.
	 *
	 * @param int $blog_id
	 *
	 * @return string
	 */
	public function get_mapped_domain($blog_id) {
		global $wpdb;

		$domain = '';
		$multisite_plugin_table_name = $wpdb->base_prefix.'domain_mapping';
		// Check if table exists
		if ($wpdb->get_var("SHOW TABLES LIKE '" . esc_sql($multisite_plugin_table_name) . "'") !== $multisite_plugin_table_name) {
			// This table created in WordPress MU Domain Mapping plugin.
			$row = $wpdb->get_row($wpdb->prepare("SELECT `domain` FROM " .esc_sql($multisite_plugin_table_name) . " WHERE `blog_id` = %d AND `active` = %d", $blog_id, 1), ARRAY_A);
			if (!empty($row)) {
				$domain = $row['domain'];
			}
		} else {
			// When using the WP Core method, the site url option contains the mapped domain.
			$domain = get_site_url($blog_id);
		}

		/**
		 * Filters the mapped domain name
		 *
		 * @param string  $domain  The domain name
		 * @param integer $blog_id The blog ID
		 */
		return apply_filters('wpo_get_mapped_domain', $domain, $blog_id);
	}

	/**
	 * Actually add the URLs to be preloaded into cache during `shutdown` hook
	 *
	 * @return void
	 */
	public function create_tasks_for_auto_preload_urls() {
		foreach ($this->url_preload_list as $url) {
			if (wpo_url_in_exceptions($url)) continue;

			$description = 'Preload - '.$url;
			$options = array('url' => $url, 'preload_type' => 'manual', 'anonymous_user_allowed' => (defined('DOING_CRON') && DOING_CRON) || (defined('WP_CLI') && WP_CLI));

			WP_Optimize_Load_Url_Task::create_task($this->task_type, $description, $options, 'WP_Optimize_Load_Url_Task');
		}

		$this->run('manual', null, true);
	}

	/**
	 * Prepare the URL list that will need to be tasked to be preloaded
	 *
	 * @param string $url The URL to be preloaded
	 * @return void
	 */
	public function add_url_to_preload_list($url) {
		$this->url_preload_list[] = $url;
	}

	/**
	 * Captures and logs any interesting messages
	 *
	 * @param string $message    - the error message
	 * @param string $error_type - the error type
	 */
	public function log($message, $error_type = 'info') {

		if (isset($this->loggers)) {
			foreach ($this->loggers as $logger) {
				$logger->log($message, $error_type);
			}
		}
	}

	/**
	 * Instance of WP_Optimize_Page_Cache_Preloader.
	 *
	 * @return WP_Optimize_Page_Cache_Preloader
	 */
	public static function instance() {
		if (empty(self::$_instance)) {
			self::$_instance = new WP_Optimize_Page_Cache_Preloader();
		}

		return self::$_instance;
	}

	/**
	 * Check if the URL is already cached, or needs to be preloaded
	 *
	 * @param string $url          The preloaded url
	 * @param string $preload_type The preload type (manual | scheduled)
	 * @return boolean
	 */
	private function url_is_already_cached($url, $preload_type) {
		static $files = array();
		$regenerate_count = 0;
		$folder = trailingslashit(WPO_CACHE_FILES_DIR) . wpo_get_url_path($url);
		// If the folder does not exist, consider the URL as cleared
		if (!is_dir($folder)) return false;

		if (empty($files)) {
			// Check only the base files
			$files[] = 'index.html';

			if (WPO_Cache_Config::instance()->get_option('enable_mobile_caching')) {
				$files[] = 'mobile.index.html';
			}
			$files = apply_filters('wpo_maybe_clear_files_list', $files);
		}

		foreach ($files as $file) {
			$file_path = trailingslashit($folder).$file;
			if (!file_exists($file_path)) {
				// The file does not exist, count it as "deleted"
				$regenerate_count++;
				continue;
			}

			if ($this->should_regenerate_file($file_path, $preload_type)) {
				// delete the expired cache file
				wp_delete_file($file_path);
				$regenerate_count++;
			}
		}

		// if 0 == $regenerate_count, nothing all the expected files exist, and none were deleted.
		return 0 === $regenerate_count;
	}

	/**
	 * Determine if a file should be regenerated
	 *
	 * @param string $path         The file to check
	 * @param string $preload_type The preload type (manual | scheduled)
	 *
	 * @return boolean
	 */
	private function should_regenerate_file($path, $preload_type) {
		// Store the variables, as they'll be used for each file and each file
		static $is_preloader_scheduled = null;
		static $lifespan = null;
		static $schedule_type = null;
		static $schedule_interval = null;
		static $lifespan_expiry_threshold = null;
		static $always_regenerate_file_if_preload_is_manual = null;
		static $always_regenerate_file_if_preload_is_scheduled = null;
		static $regenerate_file_when_no_expiry_date = null;

		// Sets the variables once per request:
		if (null === $is_preloader_scheduled) {
			$is_preloader_scheduled = WPO_Cache_Config::instance()->get_option('enable_schedule_preload');
			$schedule_type = WPO_Cache_Config::instance()->get_option('preload_schedule_type');
			$lifespan = (int) WPO_Cache_Config::instance()->get_option('page_cache_length');
			$schedule_interval = $this->get_schedule_interval($schedule_type);

			/**
			 * Expiry threshold: the current file will be considered stale if within the threshold. Default: 600s (10min)
			 */
			$lifespan_expiry_threshold = apply_filters('wpo_lifespan_expiry_threshold', 600);

			/**
			 * Filters if a cache should systematically be regenerated when running a manual preload. Default: false
			 */
			$always_regenerate_file_if_preload_is_manual = apply_filters('wpo_always_regenerate_file_if_preload_is_manual', false);

			/**
			 * Filters if a cache should systematically be regenerated when running a scheduled preload. Default: false
			 */
			$always_regenerate_file_if_preload_is_scheduled = apply_filters('wpo_always_regenerate_file_if_preload_is_scheduled', false);

			/**
			 * Filters if a cache should systematically be regenerated when running a preload and no schedule is set, and cache does not expire. Default: true
			 */
			$regenerate_file_when_no_expiry_date = apply_filters('wpo_regenerate_file_when_no_expiry_date', true);
		}

		if (($always_regenerate_file_if_preload_is_manual && 'manual' === $preload_type) || ($always_regenerate_file_if_preload_is_scheduled && 'scheduled' === $preload_type)) {
			$result = true;
		} else {

			$modified_time = (int) filemtime($path);

			// cache lifespan is set.
			if (0 !== $lifespan) {
				$expiry_time = $modified_time + $lifespan - $lifespan_expiry_threshold;
				$result = time() > $expiry_time;
			} elseif ($is_preloader_scheduled) {
				$expiry_time = $modified_time + $schedule_interval - $lifespan_expiry_threshold;
				$result = time() > $expiry_time;
			} else {
				$result = $regenerate_file_when_no_expiry_date;
			}
			
		}
		
		return apply_filters('wpo_preloader_should_regenerate_file', $result, $path, $preload_type);
	}

	/**
	 * Add preloader headers
	 *
	 * @param array $headers Preload headers
	 */
	public function preload_headers($headers) {
		$headers['X-WP-Optimize-Cache-Preload'] = 'Yes';
		return $headers;
	}

	/**
	 * Option disabled error message
	 *
	 * @return array
	 */
	protected function get_option_disabled_error() {
		return array(
			'success' => false,
			'error' => __('Page cache is disabled.', 'wp-optimize')
		);
	}

	/**
	 * Get preload already running error message
	 *
	 * @return array
	 */
	protected function get_preload_already_running_error() {
		return array(
			'success' => false,
			'error' => __('Probably page cache preload is running already.', 'wp-optimize')
		);
	}
	
	/**
	 * Returns preload data
	 *
	 * @return array
	 */
	protected function get_preload_data() {
		return WP_Optimize()->get_page_cache()->get_cache_size();
	}
	
	/**
	 * Returns preloading message
	 *
	 * @param array $cache_size
	 *
	 * @return array
	 */
	protected function get_preloading_message($cache_size) {
		return array(
			'done' => false,
			'message' => __('Loading URLs...', 'wp-optimize'),
			'size' => WP_Optimize()->format_size($cache_size['size']),
			'file_count' => $cache_size['file_count']
		);
	}
	
	/**
	 * Returns last preload message
	 *
	 * @param array  $cache_size
	 * @param string $last_preload_time_str
	 *
	 * @return array
	 */
	protected function get_last_preload_message($cache_size, $last_preload_time_str) {
		return array(
			'done' => true,
			// translators: %s is the last preload time
			'message' => sprintf(__('Last preload finished at %s', 'wp-optimize'), $last_preload_time_str),
			'size' => WP_Optimize()->format_size($cache_size['size']),
			'file_count' => $cache_size['file_count']
		);
	}
	
	/**
	 * Returns preload success message
	 *
	 * @param array $cache_size
	 *
	 * @return array
	 */
	protected function get_preload_success_message($cache_size) {
		return array(
			'done' => true,
			'size' => WP_Optimize()->format_size($cache_size['size']),
			'file_count' => $cache_size['file_count']
		);
	}
	
	/**
	 * Returns preload progress message
	 *
	 * @param array  $cache_size
	 * @param string $preloaded_message
	 * @param int    $preload_resuming_in
	 *
	 * @return array
	 */
	protected function get_preload_progress_message($cache_size, $preloaded_message, $preload_resuming_in) {
		return array(
			'done' => false,
			'message' => $preloaded_message,
			'size' => WP_Optimize()->format_size($cache_size['size']),
			'file_count' => $cache_size['file_count'],
			'resume_in' => $preload_resuming_in
		);
	}

	/**
	 * Shifts the given timestamp to nighttime.
	 *
	 * @param int $time
	 *
	 * @return int
	 */
	private function make_nighttime($time) {
		
		$gmt_offset = WP_Optimize_Utils::get_gmt_offset();

		// Set time to random time from 1 a.m. to 4:59 a.m.
		$random_time = sprintf('%02d:%02d', wp_rand(1, 4), wp_rand(0, 59));
		$nighttime = strtotime(gmdate("Y-m-d {$random_time}:00", $time)) - $gmt_offset;
		
		if ($nighttime < time()) $nighttime += 24 * 3600;

		return $nighttime;
	}
}
