/**
 * Format an ISO date string (YYYY-MM-DD) for display.
 * Uses T12:00:00 to avoid UTC timezone off-by-one when converting to local time.
 */
export function formatDate(isoDate: string): string {
	return new Intl.DateTimeFormat('de-DE', {
		day: 'numeric',
		month: 'long',
		year: 'numeric'
	}).format(new Date(isoDate + 'T12:00:00'));
}

/**
 * Converts an ISO date string (YYYY-MM-DD) to German display format (DD.MM.YYYY).
 * Returns an empty string for invalid or empty input.
 */
export function isoToGerman(iso: string): string {
	if (!iso || !/^\d{4}-\d{2}-\d{2}$/.test(iso)) return '';
	const [y, m, d] = iso.split('-');
	return `${d}.${m}.${y}`;
}

/**
 * Converts a German date string (DD.MM.YYYY) to ISO format (YYYY-MM-DD).
 * Returns an empty string for invalid or empty input.
 */
export function germanToIso(german: string): string {
	const match = german.match(/^(\d{2})\.(\d{2})\.(\d{4})$/);
	if (!match) return '';
	const [, d, m, y] = match;
	return `${y}-${m}-${d}`;
}

/**
 * Handles a date input event for German-format date fields (DD.MM.YYYY).
 * Strips non-digits, formats with dots, mutates the input's displayed value,
 * and returns the display string and its ISO equivalent.
 */
export function handleGermanDateInput(e: Event): { display: string; iso: string } {
	const input = e.target as HTMLInputElement;
	const digits = input.value.replace(/\D/g, '').slice(0, 8);
	let display: string;
	if (digits.length <= 2) {
		display = digits;
	} else if (digits.length <= 4) {
		display = `${digits.slice(0, 2)}.${digits.slice(2)}`;
	} else {
		display = `${digits.slice(0, 2)}.${digits.slice(2, 4)}.${digits.slice(4)}`;
	}
	input.value = display;
	return { display, iso: germanToIso(display) };
}