interface IRowWithTimezone {
	TimezoneGmtOffset?: number;
	TimezoneDstOffset?: number;
}

export default class TimezoneHelper {
	public static isDST(targetDate: Temporal.PlainDateTime): boolean {
		const currMonth = targetDate.month;
		if (currMonth < 3) {
			return false;
		} else if (currMonth > 3 && currMonth < 10) {
			return true;
		} else if (currMonth == 3) {
			if (targetDate.day >= this.getLastSunday(targetDate.month, targetDate.year).day) {
				return true;
			} else {
				return false;
			}
		} else if (currMonth == 10) {
			if (targetDate.day >= this.getLastSunday(targetDate.month, targetDate.year).day) {
				return false;
			} else {
				return true;
			}
		} else {
			return false;
		}
	}

	public static getLocalDateFromUTC(dateUTC: Temporal.PlainDateTime, dataRow: IRowWithTimezone): Temporal.PlainDateTime;
	public static getLocalDateFromUTC(dateUTC: Temporal.PlainDateTime, dstOffset: number, gmtOffset: number): Temporal.PlainDateTime;
	public static getLocalDateFromUTC(
		dateUTC: Temporal.PlainDateTime,
		dstOffsetOrRow: number | IRowWithTimezone,
		gmtOffset?: number,
	): Temporal.PlainDateTime {
		if (dateUTC == null) {
			return null as any;
		}

		const tzRow = this.getRowWithTimezone(dstOffsetOrRow, gmtOffset);
		return dateUTC.add({
			milliseconds: ((this.isDST(dateUTC) ? tzRow.TimezoneDstOffset : tzRow.TimezoneGmtOffset) as any) * -1 * 60 * 1000,
		});
	}

	public static getUTCFromLocalDate(dateUTC: Temporal.PlainDateTime, dataRow: IRowWithTimezone): Temporal.PlainDateTime;
	public static getUTCFromLocalDate(dateUTC: Temporal.PlainDateTime, dstOffset: number, gmtOffset: number): Temporal.PlainDateTime;
	public static getUTCFromLocalDate(
		dateUTC: Temporal.PlainDateTime,
		dstOffsetOrRow: number | IRowWithTimezone,
		gmtOffset?: number,
	): Temporal.PlainDateTime {
		if (dateUTC == null) {
			return null as any;
		}

		const tzRow = this.getRowWithTimezone(dstOffsetOrRow, gmtOffset);
		return dateUTC.add({
			milliseconds: ((this.isDST(dateUTC) ? tzRow.TimezoneDstOffset : tzRow.TimezoneGmtOffset) as any) * 60 * 1000,
		});
	}

	private static getRowWithTimezone(dstOffsetOrRow: number | IRowWithTimezone, gmtOffset?: number): IRowWithTimezone {
		const tzRow = dstOffsetOrRow as IRowWithTimezone;
		if (tzRow.TimezoneDstOffset != null) {
			return tzRow;
		}

		return {
			TimezoneDstOffset: dstOffsetOrRow as number,
			TimezoneGmtOffset: gmtOffset,
		};
	}

	private static getLastSunday(month: number, year: number): Temporal.PlainDateTime {
		// first day of next month
		const firstOfNextMonth = Temporal.PlainYearMonth.from({ year, month })
			.add({ months: 1 })
			.toPlainDate({ day: 1 });

		// last day of target month
		const lastOfMonth = firstOfNextMonth.subtract({ days: 1 });

		// Temporal dayOfWeek: 1=Mon ... 7=Sun
		const daysToSubtract = lastOfMonth.dayOfWeek % 7; // 0 if already Sunday
		const lastSunday = lastOfMonth.subtract({ days: daysToSubtract });

		// return as PlainDateTime at 00:00
		return lastSunday.toPlainDateTime({ hour: 0, minute: 0, second: 0, millisecond: 0 });
	}
}