export const BERLIN_TIME_ZONE = 'Europe/Berlin';

export type CalendarDay = {
	date: string;
	dayNumber: number;
	inSelectedMonth: boolean;
	isToday: boolean;
};

const berlinDateFormatter = new Intl.DateTimeFormat('en-CA', {
	timeZone: BERLIN_TIME_ZONE,
	year: 'numeric',
	month: '2-digit',
	day: '2-digit'
});

function padded(value: number) {
	return String(value).padStart(2, '0');
}

function dateParts(value: string | Date) {
	const parts = berlinDateFormatter.formatToParts(
		typeof value === 'string' ? new Date(value) : value
	);
	const read = (type: Intl.DateTimeFormatPartTypes) =>
		Number(parts.find((part) => part.type === type)?.value);
	return { year: read('year'), month: read('month'), day: read('day') };
}

export function berlinDateKey(value: string | Date) {
	const parts = dateParts(value);
	return `${parts.year}-${padded(parts.month)}-${padded(parts.day)}`;
}

export function berlinMonthKey(value: string | Date) {
	return berlinDateKey(value).slice(0, 7);
}

export function normalizeMonthKey(value: string | null | undefined, fallback: string) {
	if (!value || !/^\d{4}-(0[1-9]|1[0-2])$/.test(value)) return fallback;
	return value;
}

export function shiftMonth(monthKey: string, delta: number) {
	const [year, month] = monthKey.split('-').map(Number);
	const shifted = new Date(Date.UTC(year, month - 1 + delta, 1));
	return `${shifted.getUTCFullYear()}-${padded(shifted.getUTCMonth() + 1)}`;
}

export function formatMonthLabel(monthKey: string) {
	const [year, month] = monthKey.split('-').map(Number);
	return new Intl.DateTimeFormat('de-DE', {
		timeZone: 'UTC',
		month: 'long',
		year: 'numeric'
	}).format(new Date(Date.UTC(year, month - 1, 1)));
}

export function buildCalendarDays(monthKey: string, todayKey: string): CalendarDay[] {
	const [year, month] = monthKey.split('-').map(Number);
	const first = new Date(Date.UTC(year, month - 1, 1));
	const mondayOffset = (first.getUTCDay() + 6) % 7;
	const start = new Date(Date.UTC(year, month - 1, 1 - mondayOffset));
	return Array.from({ length: 42 }, (_, index) => {
		const date = new Date(start);
		date.setUTCDate(start.getUTCDate() + index);
		const dateKey = `${date.getUTCFullYear()}-${padded(date.getUTCMonth() + 1)}-${padded(date.getUTCDate())}`;
		return {
			date: dateKey,
			dayNumber: date.getUTCDate(),
			inSelectedMonth: dateKey.startsWith(monthKey),
			isToday: dateKey === todayKey
		};
	});
}