<?php
/**
 * Reminder offset calculator (DST-safe).
 *
 * @package Bookline
 */

declare( strict_types=1 );

namespace Bookline\Domain\Notification;

use DateInterval;
use DateTimeImmutable;

/**
 * Computes reminder fire times from booking start + offset.
 *
 * @since 0.1.0
 */
final class ReminderScheduler {

	/**
	 * Fire time = start_utc + offset_minutes. Null when the instant is already past.
	 *
	 * @param DateTimeImmutable $start_utc      Booking start UTC.
	 * @param int               $offset_minutes Offset (negative = before).
	 * @param DateTimeImmutable $now_utc        Now UTC.
	 *
	 * @since 0.1.0
	 */
	public function fire_at( DateTimeImmutable $start_utc, int $offset_minutes, DateTimeImmutable $now_utc ): ?DateTimeImmutable {
		$absolute = abs( $offset_minutes );
		$interval = new DateInterval( 'PT' . $absolute . 'M' );
		$fire     = $offset_minutes < 0
			? $start_utc->sub( $interval )
			: $start_utc->add( $interval );

		if ( $fire <= $now_utc ) {
			return null;
		}

		return $fire;
	}
}