<?php
namespace Akram;

defined( 'ABSPATH' ) || exit;

/**
 * Real-time co-editing ("Edit together").
 *
 * Every editor of a page pushes the elements it changed since its last sync and pulls everyone
 * else's changes, about once a second, through one REST call. The server keeps a short ordered log
 * per page; clients merge remote changes field by field (lists of children are merged, so
 * simultaneous additions all survive) and the latest change to the same field wins. Selections
 * travel with each sync so collaborators see each other on the canvas.
 *
 * Storage is a non-autoloaded option per page guarded by a database lock, so it works on any host
 * without WebSockets, extra tables or a persistent object cache.
 */
final class Sync {

	const KEEP    = 400; // ops kept per page
	const IDLE    = 30;  // seconds before a client counts as gone
	const COLORS  = array( '#2563eb', '#db2777', '#7c3aed', '#0891b2', '#ca8a04', '#dc2626', '#059669', '#4f46e5' );

	public static function rest( \WP_REST_Request $r ) {
		$id     = (int) $r['id'];
		$p      = (array) $r->get_json_params();
		$client = preg_replace( '/[^a-z0-9]/i', '', (string) ( $p['client'] ?? '' ) );
		if ( ! $client || strlen( $client ) > 32 ) {
			return new \WP_Error( 'akram_sync', __( 'Missing client id.', 'akram' ), array( 'status' => 400 ) );
		}
		$since = max( 0, (int) ( $p['since'] ?? 0 ) );
		$ops   = array();
		foreach ( array_slice( (array) ( $p['ops'] ?? array() ), 0, 500 ) as $op ) {
			$op = self::clean_op( $op );
			if ( $op ) {
				$ops[] = $op;
			}
		}
		$sel = array_values( array_filter( array_slice( (array) ( $p['sel'] ?? array() ), 0, 20 ), static function ( $x ) {
			return is_string( $x ) && preg_match( '/^[a-z0-9_-]{1,24}$/i', $x );
		} ) );

		if ( ! self::lock( $id ) ) {
			return new \WP_Error( 'akram_sync_busy', __( 'Busy, retrying.', 'akram' ), array( 'status' => 503 ) );
		}
		$key = 'akram_sync_' . $id;
		wp_cache_delete( $key, 'options' );
		$log = get_option( $key );
		$log = is_array( $log ) ? $log : array(
			'seq'     => 0,
			'ops'     => array(),
			'clients' => array(),
		);
		$me  = get_current_user_id();
		$u   = wp_get_current_user();

		// Clients that went quiet leave the session.
		foreach ( $log['clients'] as $cid => $c ) {
			if ( time() - (int) $c['seen'] > self::IDLE ) {
				unset( $log['clients'][ $cid ] );
			}
		}
		if ( ! empty( $p['leave'] ) ) {
			unset( $log['clients'][ $client ] );
		} else {
			$known = $log['clients'][ $client ] ?? null;
			$log['clients'][ $client ] = array(
				'user'  => $me,
				'name'  => $u->display_name,
				'color' => $known['color'] ?? self::color( $log['clients'] ),
				'sel'   => $sel,
				'seen'  => time(),
			);
		}
		// A client that fell behind the kept window must reload the shared document.
		$oldest = $log['ops'] ? (int) $log['ops'][0]['seq'] : $log['seq'] + 1;
		$reset  = $since > 0 && $since < $oldest - 1;

		$remote = array();
		if ( ! $reset ) {
			foreach ( $log['ops'] as $op ) {
				if ( $op['seq'] > $since && $op['client'] !== $client ) {
					$remote[] = $op;
				}
			}
		}
		foreach ( $ops as $op ) {
			$op['seq']    = ++$log['seq'];
			$op['client'] = $client;
			$op['user']   = $me;
			$log['ops'][] = $op;
		}
		if ( count( $log['ops'] ) > self::KEEP ) {
			$log['ops'] = array_slice( $log['ops'], -self::KEEP );
		}
		if ( ! $log['clients'] ) {
			delete_option( $key ); // nobody left: the log is no longer needed
		} else {
			update_option( $key, $log, false );
		}
		self::unlock( $id );

		$peers = array();
		foreach ( $log['clients'] as $cid => $c ) {
			if ( $cid !== $client ) {
				$peers[] = array(
					'client' => $cid,
					'user'   => (int) $c['user'],
					'name'   => $c['name'],
					'color'  => $c['color'],
					'sel'    => $c['sel'],
				);
			}
		}
		$out = array(
			'seq'   => $log['seq'],
			'ops'   => $remote,
			'peers' => $peers,
			'hash'  => Document::hash( $id ),
		);
		if ( $reset ) {
			$out['reset'] = true;
		}
		return $out;
	}

	/** Shared-session snapshot for a client that joins (or fell behind): latest saved/draft doc + log position. */
	public static function join( $id ) {
		$log = get_option( 'akram_sync_' . $id );
		return is_array( $log ) ? array(
			'seq' => (int) $log['seq'],
			'ops' => $log['ops'],
		) : array(
			'seq' => 0,
			'ops' => array(),
		);
	}

	/** Number of other people in the shared session (used to decide whether to keep syncing). */
	public static function active( $id ) {
		$log = get_option( 'akram_sync_' . $id );
		if ( ! is_array( $log ) ) {
			return 0;
		}
		$n = 0;
		foreach ( $log['clients'] as $c ) {
			if ( time() - (int) $c['seen'] <= self::IDLE ) {
				$n++;
			}
		}
		return $n;
	}

	private static function clean_op( $op ) {
		if ( ! is_array( $op ) ) {
			return null;
		}
		if ( array_key_exists( 'root', $op ) ) {
			return is_array( $op['root'] ) ? array( 'root' => array_values( array_filter( $op['root'], array( __CLASS__, 'valid_id' ) ) ) ) : null;
		}
		if ( array_key_exists( 'page', $op ) ) {
			return is_array( $op['page'] ) ? array( 'page' => $op['page'] ) : null;
		}
		$id = (string) ( $op['id'] ?? '' );
		if ( ! self::valid_id( $id ) ) {
			return null;
		}
		if ( null === ( $op['el'] ?? null ) ) {
			return array(
				'id' => $id,
				'el' => null,
			);
		}
		$el = Sanitize::element( $id, $op['el'] );
		return $el ? array(
			'id' => $id,
			'el' => $el,
		) : null;
	}

	public static function valid_id( $id ) {
		return is_string( $id ) && (bool) preg_match( '/^[a-z0-9_-]{1,24}$/i', $id );
	}

	private static function color( array $clients ) {
		$used = wp_list_pluck( $clients, 'color' );
		foreach ( self::COLORS as $c ) {
			if ( ! in_array( $c, $used, true ) ) {
				return $c;
			}
		}
		return self::COLORS[ count( $clients ) % count( self::COLORS ) ];
	}

	/** Row-level mutex: add_option fails while another request holds it (stale after 5 s). */
	private static function lock( $id ) {
		global $wpdb;
		$name = 'akram_sync_lock_' . $id;
		for ( $i = 0; $i < 60; $i++ ) {
			// phpcs:ignore WordPress.DB.DirectDatabaseQuery
			if ( $wpdb->query( $wpdb->prepare( "INSERT IGNORE INTO {$wpdb->options} (option_name, option_value, autoload) VALUES (%s, %s, 'no')", $name, (string) time() ) ) ) {
				return true;
			}
			// phpcs:ignore WordPress.DB.DirectDatabaseQuery
			$t = (int) $wpdb->get_var( $wpdb->prepare( "SELECT option_value FROM {$wpdb->options} WHERE option_name = %s", $name ) );
			if ( $t && time() - $t > 5 ) {
				self::unlock( $id );
				continue;
			}
			usleep( 50000 );
		}
		return false;
	}

	private static function unlock( $id ) {
		global $wpdb;
		// phpcs:ignore WordPress.DB.DirectDatabaseQuery
		$wpdb->query( $wpdb->prepare( "DELETE FROM {$wpdb->options} WHERE option_name = %s", 'akram_sync_lock_' . $id ) );
	}
}