/**
 * Returns the ISO Monday (YYYY-MM-DD) for the week containing `date`.
 */
export function getWeekStart(date: Date): string {
	const d = new Date(date);
	const day = d.getUTCDay(); // 0=Sun, 1=Mon, …
	const diff = day === 0 ? -6 : 1 - day; // shift to Monday
	d.setUTCDate(d.getUTCDate() + diff);
	return d.toISOString().slice(0, 10);
}

/**
 * Returns the Monday of the previous week relative to `weekStart`.
 */
export function prevWeek(weekStart: string): string {
	const d = new Date(weekStart + 'T00:00:00Z');
	d.setUTCDate(d.getUTCDate() - 7);
	return d.toISOString().slice(0, 10);
}

/**
 * Returns the Monday of the next week relative to `weekStart`.
 */
export function nextWeek(weekStart: string): string {
	const d = new Date(weekStart + 'T00:00:00Z');
	d.setUTCDate(d.getUTCDate() + 7);
	return d.toISOString().slice(0, 10);
}

/**
 * Formats a date string (YYYY-MM-DD) as a localized day abbreviation.
 */
export function formatDayAbbr(dateStr: string, length: 'narrow' | 'short' = 'narrow'): string {
	const d = new Date(dateStr + 'T00:00:00Z');
	return d.toLocaleDateString('de-DE', { weekday: length, timeZone: 'UTC' });
}

/**
 * Returns an array of 7 date strings for the week starting on `weekStart`.
 */
export function weekDays(weekStart: string): string[] {
	const days: string[] = [];
	for (let i = 0; i < 7; i++) {
		const d = new Date(weekStart + 'T00:00:00Z');
		d.setUTCDate(d.getUTCDate() + i);
		days.push(d.toISOString().slice(0, 10));
	}
	return days;
}

/**
 * Formats a date string as "Mo, 30.03." style label.
 */
export function formatDayLabel(dateStr: string): string {
	const d = new Date(dateStr + 'T00:00:00Z');
	const day = d.toLocaleDateString('de-DE', { weekday: 'short', timeZone: 'UTC' });
	const date = d.toLocaleDateString('de-DE', { day: '2-digit', month: '2-digit', timeZone: 'UTC' });
	return `${day}, ${date}`;
}

/**
 * Formats a date string as "30. März" style label.
 */
export function formatDayFull(dateStr: string): string {
	const d = new Date(dateStr + 'T00:00:00Z');
	return d.toLocaleDateString('de-DE', { day: 'numeric', month: 'long', timeZone: 'UTC' });
}

/**
 * Returns true if dateStr is today (UTC date).
 * Uses UTC consistently with all other date functions in this module.
 */
export function isToday(dateStr: string): boolean {
	const todayStr = new Date().toISOString().slice(0, 10);
	return dateStr === todayStr;
}

const EFFORT_ORDER: Record<string, number> = { easy: 0, medium: 1, hard: 2 };

/**
 * Returns a new array of recipes sorted easiest first (effort ASC, cookTimeMin ASC).
 * Used for the J4 mid-week swap context — different from variety-first sorting in J2.
 */
export function sortEasiestFirst<T extends { effort?: string | null; cookTimeMin?: number | null }>(
	recipes: T[]
): T[] {
	return [...recipes].sort((a, b) => {
		const ea = a.effort != null ? (EFFORT_ORDER[a.effort] ?? 99) : 99;
		const eb = b.effort != null ? (EFFORT_ORDER[b.effort] ?? 99) : 99;
		if (ea !== eb) return ea - eb;
		const ta = a.cookTimeMin ?? Infinity;
		const tb = b.cookTimeMin ?? Infinity;
		return ta - tb;
	});
}

/**
 * Formats a week range: "30. Mär – 5. Apr 2026".
 */
export function formatWeekRange(weekStart: string): string {
	const start = new Date(weekStart + 'T00:00:00Z');
	const end = new Date(weekStart + 'T00:00:00Z');
	end.setUTCDate(end.getUTCDate() + 6);
	const startStr = start.toLocaleDateString('de-DE', { day: 'numeric', month: 'short', timeZone: 'UTC' });
	const endStr = end.toLocaleDateString('de-DE', { day: 'numeric', month: 'short', year: 'numeric', timeZone: 'UTC' });
	return `${startStr} – ${endStr}`;
}