<?php
namespace Bookzenix;

defined( 'ABSPATH' ) || exit;

/**
 * Slot engine.
 *
 * Working hours and days off are in the site's timezone; appointments are
 * stored in UTC. All interval math happens on Unix timestamps.
 *
 * $opts for slots()/available_dates(): ignore_limits (staff bookings skip the
 * minimum-notice and booking-window rules) plus the filters listed in slots().
 */
class Availability {

	/** Statuses that occupy a slot. */
	const BLOCKING = array( 'pending', 'approved' );

	public static function available_dates( $service_id, $employee_id, $year, $month ) {
		$tz    = wp_timezone();
		$first = new \DateTimeImmutable( sprintf( '%04d-%02d-01', $year, $month ), $tz );
		$days  = (int) $first->format( 't' );
		$out   = array();
		for ( $d = 0; $d < $days; $d++ ) {
			$date = $first->modify( "+{$d} days" )->format( 'Y-m-d' );
			if ( ! empty( self::slots( $service_id, $employee_id, $date ) ) ) {
				$out[] = $date;
			}
		}
		return $out;
	}

	/**
	 * Free start times for a local date.
	 * @return array [ ['time' => 'HH:MM', 'employees' => [ids]] ]
	 */
	public static function slots( $service_id, $employee_id, $date, array $opts = array() ) {
		$service = Repository::service( $service_id );
		if ( ! $service || 'active' !== $service['status'] ) {
			return array();
		}

		$settings = Settings::all();
		$tz       = wp_timezone();
		try {
			$day = new \DateTimeImmutable( $date . ' 00:00:00', $tz );
		} catch ( \Exception $e ) {
			return array();
		}
		if ( $day->format( 'Y-m-d' ) !== $date ) {
			return array();
		}

		$now        = time();
		$earliest   = $now + (int) $settings['min_notice'] * 60;
		$latest_day = ( new \DateTimeImmutable( 'today', $tz ) )->modify( '+' . (int) $settings['max_advance_days'] . ' days' );
		if ( empty( $opts['ignore_limits'] ) && ( $day > $latest_day || $day->getTimestamp() + DAY_IN_SECONDS < $now ) ) {
			return array();
		}

		$employees   = $employee_id ? array( (int) $employee_id ) : Repository::employee_ids_for_service( $service_id );

		$duration = (int) $service['duration'] * 60;
		$before   = (int) $service['buffer_before'] * 60;
		$after    = (int) $service['buffer_after'] * 60;
		$step     = max( 5, (int) $settings['time_slot_step'] ) * 60;
		$dow      = (int) $day->format( 'N' );

		$result = array();
		foreach ( $employees as $eid ) {
			if ( ! Repository::employee_offers( $eid, $service_id ) || Repository::is_day_off( $eid, $date ) ) {
				continue;
			}
			$windows = Repository::working_windows( $eid, $dow );
			if ( empty( $windows ) ) {
				continue;
			}
			$busy = self::busy_intervals( $eid, $day );
			foreach ( $windows as $w ) {
				$w_start = ( new \DateTimeImmutable( $date . ' ' . $w['start_time'], $tz ) )->getTimestamp();
				$w_end   = ( new \DateTimeImmutable( $date . ' ' . $w['end_time'], $tz ) )->getTimestamp();
				for ( $s = $w_start; $s + $duration <= $w_end; $s += $step ) {
					if ( empty( $opts['ignore_limits'] ) && $s < $earliest ) {
						continue;
					}
					$e = $s + $duration;
					if ( self::fits( $s - $before, $e + $after, $busy ) ) {
						$result[ wp_date( 'H:i', $s, $tz ) ][] = $eid;
					}
				}
			}
		}

		ksort( $result );
		$out = array();
		foreach ( $result as $time => $ids ) {
			$out[] = array( 'time' => $time, 'employees' => array_values( array_unique( $ids ) ) );
		}
		return apply_filters( 'bookzenix_available_slots', $out, $service_id, $employee_id, $date );
	}

	/** True when [from, to) overlaps none of the busy intervals. */
	private static function fits( $from, $to, array $busy ) {
		foreach ( $busy as $b ) {
			if ( $from < $b['to'] && $to > $b['from'] ) {
				return false;
			}
		}
		return true;
	}

	/** Busy intervals (incl. buffers) for an employee around a local day. */
	private static function busy_intervals( $employee_id, \DateTimeImmutable $day ) {
		global $wpdb;
		$from      = gmdate( 'Y-m-d H:i:s', $day->getTimestamp() - DAY_IN_SECONDS );
		$to        = gmdate( 'Y-m-d H:i:s', $day->getTimestamp() + 2 * DAY_IN_SECONDS );
		$a         = Plugin::table( 'appointments' );
		$s         = Plugin::table( 'services' );
		$status_in = "'" . implode( "','", self::BLOCKING ) . "'";
		// phpcs:ignore WordPress.DB.PreparedSQL.InterpolatedNotPrepared
		$rows = $wpdb->get_results( $wpdb->prepare( "SELECT a.start_utc, a.end_utc, s.buffer_before, s.buffer_after FROM {$a} a LEFT JOIN {$s} s ON s.id = a.service_id WHERE a.employee_id = %d AND a.status IN ({$status_in}) AND a.start_utc < %s AND a.end_utc > %s", $employee_id, $to, $from ), ARRAY_A );
		$busy = array();
		foreach ( (array) $rows as $r ) {
			$st     = strtotime( $r['start_utc'] . ' UTC' );
			$en     = strtotime( $r['end_utc'] . ' UTC' );
			$busy[] = array( 'from' => $st - (int) $r['buffer_before'] * 60, 'to' => $en + (int) $r['buffer_after'] * 60 );
		}
		return apply_filters( 'bookzenix_busy_intervals', $busy, $employee_id, $day );
	}

	/** Final server-side check right before writing. @return int|false employee id */
	public static function reserve_employee( $service_id, $employee_id, $date, $time, array $opts = array() ) {
		foreach ( self::slots( $service_id, $employee_id, $date, $opts ) as $slot ) {
			if ( $slot['time'] === $time ) {
				return (int) $slot['employees'][0];
			}
		}
		return false;
	}

}