✨ first version

src/lib/opening-hours.ts

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+import { formatDateOnly } from '#/lib/datetime'
+
+export const RESTAURANT_TIME_ZONE = 'Europe/Berlin'
+export const RESERVATION_INTERVAL_MINUTES = 15
+
+export type OpeningRange = { start: string; end: string }
+
+// Keyed by the JS `Date#getDay()` convention: 0 = Sunday … 6 = Saturday.
+const OPENING_HOURS_BY_WEEKDAY: Record<number, OpeningRange[]> = {
+  0: [
+    { start: '11:30', end: '13:30' },
+    { start: '17:00', end: '22:00' },
+  ],
+  1: [],
+  2: [{ start: '17:00', end: '21:30' }],
+  3: [{ start: '17:00', end: '21:30' }],
+  4: [{ start: '17:00', end: '21:30' }],
+  5: [{ start: '17:00', end: '22:00' }],
+  6: [{ start: '17:00', end: '22:00' }],
+}
+
+const WEEKDAY_LABELS_MONDAY_FIRST: Array<{ day: string; weekday: number }> = [
+  { day: 'Montag', weekday: 1 },
+  { day: 'Dienstag', weekday: 2 },
+  { day: 'Mittwoch', weekday: 3 },
+  { day: 'Donnerstag', weekday: 4 },
+  { day: 'Freitag', weekday: 5 },
+  { day: 'Samstag', weekday: 6 },
+  { day: 'Sonntag', weekday: 0 },
+]
+
+const WEEKDAY_SHORT_NAMES = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat']
+
+const berlinPartsFormatter = new Intl.DateTimeFormat('en-US', {
+  timeZone: RESTAURANT_TIME_ZONE,
+  weekday: 'short',
+  hour: '2-digit',
+  minute: '2-digit',
+  hourCycle: 'h23',
+})
+
+function toMinutes(time: string) {
+  const [hours, minutes] = time.split(':').map(Number)
+  return hours * 60 + minutes
+}
+
+function getBerlinTimeParts(date: Date) {
+  const parts = berlinPartsFormatter.formatToParts(date)
+  const byType = Object.fromEntries(parts.map((part) => [part.type, part.value]))
+  return {
+    weekday: WEEKDAY_SHORT_NAMES.indexOf(byType.weekday),
+    minutesSinceMidnight: Number(byType.hour) * 60 + Number(byType.minute),
+  }
+}
+
+export function isOnReservationInterval(date: Date, intervalMinutes: number = RESERVATION_INTERVAL_MINUTES) {
+  return date.getUTCMinutes() % intervalMinutes === 0 && date.getUTCSeconds() === 0
+}
+
+// `durationMinutes` requires the whole reservation — not just its start — to fit
+// before closing, so a slot too close to close time is rejected outright.
+export function isWithinOpeningHours(date: Date, durationMinutes = 0) {
+  const { weekday, minutesSinceMidnight } = getBerlinTimeParts(date)
+  const ranges = OPENING_HOURS_BY_WEEKDAY[weekday] ?? []
+
+  return ranges.some(
+    (range) =>
+      minutesSinceMidnight >= toMinutes(range.start) &&
+      minutesSinceMidnight + durationMinutes <= toMinutes(range.end),
+  )
+}
+
+export function getOpeningHoursForDisplay() {
+  return WEEKDAY_LABELS_MONDAY_FIRST.map(({ day, weekday }) => {
+    const ranges = OPENING_HOURS_BY_WEEKDAY[weekday]
+    return {
+      day,
+      hours: ranges.length ? ranges.map((range) => `${range.start} – ${range.end}`) : null,
+    }
+  })
+}
+
+export function getTodayLabelBerlin() {
+  return new Intl.DateTimeFormat('de-DE', { timeZone: RESTAURANT_TIME_ZONE, weekday: 'long' }).format(new Date())
+}
+
+// The functions below drive the customer-facing date/time picker. They work with the
+// browser's local time (not RESTAURANT_TIME_ZONE) since visitors are assumed to be
+// browsing from the restaurant's own timezone — same assumption the rest of the
+// booking form already makes when it builds a Date from an unqualified local string.
+
+export function getOpeningRangesForWeekday(weekday: number): OpeningRange[] {
+  return OPENING_HOURS_BY_WEEKDAY[weekday] ?? []
+}
+
+// `durationMinutes` (how long a reservation booked at a slot would occupy the
+// table) defaults to `intervalMinutes` so a bare call still behaves sensibly —
+// but every real caller should pass the restaurant's actual reservation
+// duration, so a slot too close to closing time to fit the whole reservation
+// is excluded rather than offered and then silently running past close.
+export function generateTimeOptions(
+  ranges: OpeningRange[],
+  intervalMinutes: number = RESERVATION_INTERVAL_MINUTES,
+  durationMinutes: number = intervalMinutes,
+): string[] {
+  const options: string[] = []
+
+  for (const range of ranges) {
+    const startMinutes = toMinutes(range.start)
+    const endMinutes = toMinutes(range.end)
+
+    for (let minutes = startMinutes; minutes + durationMinutes <= endMinutes; minutes += intervalMinutes) {
+      const hours = Math.floor(minutes / 60)
+      const remainder = minutes % 60
+      options.push(`${String(hours).padStart(2, '0')}:${String(remainder).padStart(2, '0')}`)
+    }
+  }
+
+  return options
+}
+
+export function getNextAvailableSlot(
+  from: Date,
+  intervalMinutes: number = RESERVATION_INTERVAL_MINUTES,
+  durationMinutes: number = intervalMinutes,
+): { date: string; time: string } | null {
+  for (let dayOffset = 0; dayOffset < 14; dayOffset++) {
+    const day = new Date(from)
+    day.setDate(from.getDate() + dayOffset)
+
+    const dateStr = formatDateOnly(day)
+    const options = generateTimeOptions(getOpeningRangesForWeekday(day.getDay()), intervalMinutes, durationMinutes)
+
+    for (const time of options) {
+      const slot = new Date(`${dateStr}T${time}`)
+      if (slot.getTime() >= from.getTime()) {
+        return { date: dateStr, time }
+      }
+    }
+  }
+
+  return null
+}