<?php
declare(strict_types=1);

namespace WebCalendar\WP\Security;

defined( 'ABSPATH' ) || exit;

use DateTimeImmutable;

/**
 * Bounds on a caller-chosen window that recurring events are expanded over.
 *
 * Core expands each series from its DTSTART up to the window's end and only
 * then drops what falls before the start, so the work grows with how far the
 * end lies from DTSTART, not with how wide the window is. An anonymous
 * request for 0001-01-01..9999-12-31, or for one day in 9999, therefore costs
 * seconds of CPU and the whole memory limit. Every public path that lets the
 * caller pick the window checks it here first.
 *
 * Two bounds, both filterable:
 *   - span: no window wider than `webcal_expansion_max_span_days` (400), which
 *     covers every calendar view and a year of upcoming events;
 *   - horizon: no window ending more than `webcal_expansion_horizon_years`
 *     (10) from today, which is what keeps a narrow far-future window cheap.
 */
final class ExpansionWindow {

	/**
	 * Whether a window is within both bounds.
	 *
	 * @param DateTimeImmutable      $start Window start.
	 * @param DateTimeImmutable      $end   Window end.
	 * @param DateTimeImmutable|null $now   Today, for tests.
	 */
	public static function allows( DateTimeImmutable $start, DateTimeImmutable $end, ?DateTimeImmutable $now = null ): bool {
		$max_days = max( 1, (int) apply_filters( 'webcal_expansion_max_span_days', 400 ) );
		$horizon  = max( 1, (int) apply_filters( 'webcal_expansion_horizon_years', 10 ) );
		$now      = $now ?? new DateTimeImmutable();

		if ( $end < $start ) {
			return false;
		}

		if ( (int) $start->diff( $end )->days > $max_days ) {
			return false;
		}

		return $end <= $now->modify( '+' . $horizon . ' years' );
	}
}