import { describe, expect, it } from 'vitest';
import {
	berlinDateKey,
	buildCalendarDays,
	formatMonthLabel,
	normalizeMonthKey,
	shiftMonth
} from './calendar.js';

describe('show calendar', () => {
	it('groups UTC instants by their Berlin civil date', () => {
		expect(berlinDateKey('2026-09-30T22:30:00.000Z')).toBe('2026-10-01');
		expect(berlinDateKey('2026-12-31T23:30:00.000Z')).toBe('2027-01-01');
	});

	it('builds a Monday-first six-week month without dropping boundary days', () => {
		const days = buildCalendarDays('2026-10', '2026-10-24');
		expect(days).toHaveLength(42);
		expect(days[0]).toMatchObject({ date: '2026-09-28', inSelectedMonth: false });
		expect(days.find((day) => day.date === '2026-10-24')?.isToday).toBe(true);
		expect(days.at(-1)?.date).toBe('2026-11-08');
	});

	it('moves across year boundaries and validates month keys', () => {
		expect(shiftMonth('2026-12', 1)).toBe('2027-01');
		expect(shiftMonth('2026-01', -1)).toBe('2025-12');
		expect(normalizeMonthKey('2026-13', '2026-10')).toBe('2026-10');
		expect(normalizeMonthKey('2026-02', '2026-10')).toBe('2026-02');
		expect(formatMonthLabel('2026-10')).toBe('Oktober 2026');
	});
});