<?php

abstract class WP_Background_Process extends WP_Async_Request {

	protected $action = 'background_process';

	protected $start_time = 0;

	protected $cron_hook_identifier;

	protected $cron_interval_identifier;

	public function __construct() {
		parent::__construct();

		$this->cron_hook_identifier     = $this->identifier . '_cron';
		$this->cron_interval_identifier = $this->identifier . '_cron_interval';

		add_action( $this->cron_hook_identifier, array( $this, 'handle_cron_healthcheck' ) );
		add_filter( 'cron_schedules', array( $this, 'schedule_cron_healthcheck' ) );
	}

	public function dispatch() {
		$this->schedule_event();

		return parent::dispatch();
	}

	public function push_to_queue( $data ) {
		$this->data[] = $data;

		return $this;
	}

	public function save() {
		$key = $this->generate_key();

		if ( ! empty( $this->data ) ) {
			update_site_option( $key, $this->data );
		}

		return $this;
	}

	public function update( $key, $data ) {
		if ( ! empty( $data ) ) {
			update_site_option( $key, $data );
		}

		return $this;
	}

	public function delete( $key ) {
		delete_site_option( $key );

		return $this;
	}

	protected function generate_key( $length = 64 ) {
		$unique  = md5( microtime() . rand() );
		$prepend = $this->identifier . '_batch_';

		return substr( $prepend . $unique, 0, $length );
	}

	public function maybe_handle() {
		session_write_close();

		if ( $this->is_process_running() ) {
			wp_die();
		}

		if ( $this->is_queue_empty() ) {
			wp_die();
		}

		check_ajax_referer( $this->identifier, 'nonce' );

		$this->handle();

		wp_die();
	}

	protected function is_queue_empty() {
		global $wpdb;

		$table  = $wpdb->options;
		$column = 'option_name';

		if ( is_multisite() ) {
			$table  = $wpdb->sitemeta;
			$column = 'meta_key';
		}

		$key = $wpdb->esc_like( $this->identifier . '_batch_' ) . '%';

		$count = $wpdb->get_var( $wpdb->prepare( "
			SELECT COUNT(*)
			FROM {$table}
			WHERE {$column} LIKE %s
		", $key ) );

		return ( $count > 0 ) ? false : true;
	}

	protected function is_process_running() {
		if ( get_site_transient( $this->identifier . '_process_lock' ) ) {
			return true;
		}

		return false;
	}

	protected function lock_process() {
		$this->start_time = time();

		$lock_duration = ( property_exists( $this, 'queue_lock_time' ) ) ? $this->queue_lock_time : 60;
		$lock_duration = apply_filters( $this->identifier . '_queue_lock_time', $lock_duration );

		set_site_transient( $this->identifier . '_process_lock', microtime(), $lock_duration );
	}

	protected function unlock_process() {
		delete_site_transient( $this->identifier . '_process_lock' );

		return $this;
	}

	protected function get_batch() {
		global $wpdb;

		$table        = $wpdb->options;
		$column       = 'option_name';
		$key_column   = 'option_id';
		$value_column = 'option_value';

		if ( is_multisite() ) {
			$table        = $wpdb->sitemeta;
			$column       = 'meta_key';
			$key_column   = 'meta_id';
			$value_column = 'meta_value';
		}

		$key = $wpdb->esc_like( $this->identifier . '_batch_' ) . '%';

		$query = $wpdb->get_row( $wpdb->prepare( "
			SELECT *
			FROM {$table}
			WHERE {$column} LIKE %s
			ORDER BY {$key_column} ASC
			LIMIT 1
		", $key ) );

		$batch       = new stdClass();
		$batch->key  = $query->$column;
		$batch->data = maybe_unserialize( $query->$value_column );

		return $batch;
	}

	protected function handle() {
		$this->lock_process();

		do {
			$batch = $this->get_batch();

			foreach ( $batch->data as $key => $value ) {
				$task = $this->task( $value );

				if ( false !== $task ) {
					$batch->data[ $key ] = $task;
				} else {
					unset( $batch->data[ $key ] );
				}

				if ( $this->time_exceeded() || $this->memory_exceeded() ) {
					break;
				}
			}

			if ( ! empty( $batch->data ) ) {
				$this->update( $batch->key, $batch->data );
			} else {
				$this->delete( $batch->key );
			}
		} while ( ! $this->time_exceeded() && ! $this->memory_exceeded() && ! $this->is_queue_empty() );

		$this->unlock_process();

		if ( ! $this->is_queue_empty() ) {
			$this->dispatch();
		} else {
			$this->complete();
		}

		wp_die();
	}

	protected function memory_exceeded() {
		$memory_limit   = $this->get_memory_limit() * 0.9;
		$current_memory = memory_get_usage( true );
		$return         = false;

		if ( $current_memory >= $memory_limit ) {
			$return = true;
		}

		return apply_filters( $this->identifier . '_memory_exceeded', $return );
	}

	protected function get_memory_limit() {
		if ( function_exists( 'ini_get' ) ) {
			$memory_limit = ini_get( 'memory_limit' );
		} else {
			$memory_limit = '128M';
		}

		if ( ! $memory_limit || - 1 === intval( $memory_limit ) ) {
			$memory_limit = '32000M';
		}

		return wp_convert_hr_to_bytes( $memory_limit );
	}

	protected function time_exceeded() {
		$finish = $this->start_time + apply_filters( $this->identifier . '_default_time_limit', 20 );
		$return = false;

		if ( time() >= $finish ) {
			$return = true;
		}

		return apply_filters( $this->identifier . '_time_exceeded', $return );
	}

	protected function complete() {
		$this->clear_scheduled_event();
	}

	public function schedule_cron_healthcheck( $schedules ) {
		$interval = apply_filters( $this->identifier . '_cron_interval', 5 );

		if ( property_exists( $this, 'cron_interval' ) ) {
			$interval = apply_filters( $this->identifier . '_cron_interval', $this->cron_interval );
		}

		$schedules[ $this->identifier . '_cron_interval' ] = array(
			'interval' => MINUTE_IN_SECONDS * $interval,
			'display'  => sprintf( __( 'Every %d Minutes' ), $interval ),
		);

		return $schedules;
	}

	public function handle_cron_healthcheck() {
		if ( $this->is_process_running() ) {
			exit;
		}

		if ( $this->is_queue_empty() ) {
			$this->clear_scheduled_event();
			exit;
		}

		$this->handle();

		exit;
	}

	protected function schedule_event() {
		if ( ! wp_next_scheduled( $this->cron_hook_identifier ) ) {
			wp_schedule_event( time(), $this->cron_interval_identifier, $this->cron_hook_identifier );
		}
	}

	protected function clear_scheduled_event() {
		$timestamp = wp_next_scheduled( $this->cron_hook_identifier );

		if ( $timestamp ) {
			wp_unschedule_event( $timestamp, $this->cron_hook_identifier );
		}
	}

	public function cancel_process() {
		if ( ! $this->is_queue_empty() ) {
			$batch = $this->get_batch();

			$this->delete( $batch->key );

			wp_clear_scheduled_hook( $this->cron_hook_identifier );
		}

	}

	abstract protected function task( $item );

}