/**
 * Date utility for HTML input elements.
 */
export default class HTMLInputElementDateUtility {
	/**
	 * Returns iso week number from given date
	 *
	 * @see https://stackoverflow.com/a/6117889
	 * @param date Date or number.
	 * @returns Iso-week string.
	 */
	public static dateIsoWeek(date: Date | number): string {
		date = typeof date === 'number' ? new Date(date) : date;
		// Copy date so don't modify original
		date = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()));
		// Set to nearest Thursday: current date + 4 - current day number
		// Make Sunday's day number 7
		date.setUTCDate(date.getUTCDate() + 4 - (date.getUTCDay() || 7));
		// Get first day of year
		const yearStart = new Date(Date.UTC(date.getUTCFullYear(), 0, 1));
		// Calculate full weeks to nearest Thursday
		const weekNo = Math.ceil(
			((<number>(<unknown>date) - <number>(<unknown>yearStart)) / 86400000 + 1) / 7
		);
		return `${date.getUTCFullYear()}-W${weekNo < 10 ? '0' : ''}${weekNo}`;
	}

	/**
	 * Returns a date object for monday of given iso week string (\d\d\d\d-W\d\d)
	 *
	 * @param isoWeek Iso-week string.
	 * @returns Date.
	 */
	public static isoWeekDate(isoWeek: string): Date {
		// Algorythm adapted from https://en.wikipedia.org/wiki/ISO_week_date
		const [, Y, W] = isoWeek.match(/^(\d{4})-W(\d{2})$/) || [];
		if (!Y || !W || Number(W) > 53 || Number(W) < 1) {
			return new Date('x'); // Return an invalid date
		}
		const date = new Date(`${Y}-01-01T00:00Z`);
		const jan4th = new Date(`${Y}-01-04T00:00Z`);
		const jan4thDay = (jan4th.getUTCDay() + 6) % 7;
		const ordinalDate = 1 + (Number(W) - 1) * 7 - jan4thDay + 3;
		date.setUTCDate(ordinalDate);
		if (date.getUTCFullYear() > Number(Y)) {
			return new Date('x'); // Return an invalid date
		}
		return date;
	}
}