<?php
/*
 * *
 *  * calendar App
 *  *
 *  * @copyright 2023 Anna Larch <anna.larch@gmx.net>
 *  *
 *  * @author Anna Larch <anna.larch@gmx.net>
 *  *
 *  * This library is free software; you can redistribute it and/or
 *  * modify it under the terms of the GNU AFFERO GENERAL PUBLIC LICENSE
 *  * License as published by the Free Software Foundation; either
 *  * version 3 of the License, or any later version.
 *  *
 *  * This library is distributed in the hope that it will be useful,
 *  * but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  * GNU AFFERO GENERAL PUBLIC LICENSE for more details.
 *  *
 *  * You should have received a copy of the GNU Affero General Public
 *  * License along with this library.  If not, see <http://www.gnu.org/licenses/>.
 *  *
 *
 */

declare(strict_types=1);

/*
 * @copyright 2023 Anna Larch <anna.larch@gmx.net>
 *
 * @author 2023 Anna Larch <anna.larch@gmx.net>
 *
 * @license GNU AGPL version 3 or any later version
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

namespace OCA\Calendar\Service\Appointments;

use Sabre\VObject\Component;
use Sabre\VObject\Component\VCalendar;
use Sabre\VObject\Component\VTimeZone;
use Sabre\VObject\TimeZoneUtil;
use function max;
use function min;

class TimezoneGenerator {
	/**
	 * Returns a VTIMEZONE component for a Olson timezone identifier
	 * with daylight transitions covering the given date range.
	 *
	 * @link https://gist.github.com/thomascube/47ff7d530244c669825736b10877a200
	 *
	 * @param string $timezone Timezone
	 * @param integer $from Unix timestamp with first date/time in this timezone
	 * @param integer $to Unix timestap with last date/time in this timezone
	 * @psalm-suppress NoValue
	 *
	 * @return null|VTimeZone A Sabre\VObject\Component object representing a VTIMEZONE definition
	 *               or null if no timezone information is available
	 */
	public function generateVtimezone(string $timezone, int $from, int $to): ?VTimeZone {
		try {
			$tz = new \DateTimeZone($timezone);
		} catch (\Exception $e) {
			return null;
		}

		// get all transitions for one year back/ahead
		$year = 86400 * 360;
		$transitions = $tz->getTransitions($from - $year, $to + $year);

		$vcalendar = new VCalendar();
		$vtimezone = $vcalendar->createComponent('VTIMEZONE');
		$vtimezone->TZID = $timezone;

		$standard = $daylightStart = null;
		foreach ($transitions as $i => $trans) {
			$component = null;
			// daylight saving time definition
			if ($trans['isdst']) {
				$daylightDefinition = $trans['ts'];
				$daylightStart = $vcalendar->createComponent('DAYLIGHT');
				$component = $daylightStart;
			}
			// standard time definition
			else {
				$standardDefinition = $trans['ts'];
				$standard = $vcalendar->createComponent('STANDARD');
				$component = $standard;
			}

			if ($component) {
				if ($i === 0) {
					$date = new \DateTime('19700101T000000');
					$tzfrom = $trans['offset'] / 3600;
					$offset = $tzfrom;
				} else {
					$date = new \DateTime($trans['time']);
					$offset = $trans['offset'] / 3600;
				}
				
				$component->DTSTART = $date->format('Ymd\THis');
				$component->TZOFFSETFROM = sprintf('%s%02d%02d', $tzfrom >= 0 ? '+' : '-', abs(floor($tzfrom)), ($tzfrom - floor($tzfrom)) * 60);
				$component->TZOFFSETTO = sprintf('%s%02d%02d', $offset >= 0 ? '+' : '-', abs(floor($offset)), ($offset - floor($offset)) * 60);

				// add abbreviated timezone name if available
				if (!empty($trans['abbr'])) {
					$component->TZNAME = $trans['abbr'];
				}

				$tzfrom = $offset;
				$vtimezone->add($component);
			}

			// we covered the entire date range
			if ($standard && $daylightStart && min($standardDefinition, $daylightDefinition) < $from && max($standardDefinition, $daylightDefinition) > $to) {
				break;
			}
		}

		// add X-MICROSOFT-CDO-TZID if available
		$microsoftExchangeMap = array_flip(TimeZoneUtil::$microsoftExchangeMap);
		if (!empty($microsoftExchangeMap) && array_key_exists($tz->getName(), $microsoftExchangeMap)) {
			$vtimezone->add('X-MICROSOFT-CDO-TZID', $microsoftExchangeMap[$tz->getName()]);
		}

		return $vtimezone;
	}
}