import { DateTime } from 'luxon'

// Visitor timezone detection (browser); falls back to UTC on the server / when
// unavailable. All slot times are transmitted as UTC and rendered in this zone.
export function detectTimezone(): string {
  try {
    return Intl.DateTimeFormat().resolvedOptions().timeZone || 'UTC'
  } catch {
    return 'UTC'
  }
}

// UTC [from, to) covering one local calendar day in `tz`.
export function dayWindowUtc(isoDate: string, tz: string): { from: string; to: string } {
  const start = DateTime.fromISO(isoDate, { zone: tz }).startOf('day')
  return { from: start.toUTC().toISO()!, to: start.plus({ days: 1 }).toUTC().toISO()! }
}

export interface DayOption {
  iso: string // yyyy-MM-dd in tz
  weekday: string
  day: string
  month: string
}

// The next `count` calendar days starting today, in the visitor's tz.
export function nextDays(tz: string, count: number): DayOption[] {
  const today = DateTime.now().setZone(tz).startOf('day')
  return Array.from({ length: count }, (_, i) => {
    const d = today.plus({ days: i })
    return { iso: d.toFormat('yyyy-MM-dd'), weekday: d.toFormat('ccc'), day: d.toFormat('d'), month: d.toFormat('LLL') }
  })
}

export function fmtTime(utc: string, tz: string): string {
  return DateTime.fromISO(utc).setZone(tz).toFormat('HH:mm')
}

export function fmtDateLong(utc: string, tz: string): string {
  return DateTime.fromISO(utc).setZone(tz).toFormat('cccc d LLLL yyyy')
}

export function fmtRange(startUtc: string, endUtc: string, tz: string): string {
  const s = DateTime.fromISO(startUtc).setZone(tz)
  const e = DateTime.fromISO(endUtc).setZone(tz)
  return `${s.toFormat('cccc d LLLL')} · ${s.toFormat('HH:mm')}–${e.toFormat('HH:mm')} (${tz})`
}

// Google Calendar "add to calendar" template link (UTC basic format).
export function googleCalUrl(p: { title: string; startUtc: string; endUtc: string; details?: string; location?: string }): string {
  const fmt = (iso: string) => DateTime.fromISO(iso).toUTC().toFormat("yyyyLLdd'T'HHmmss'Z'")
  const params = new URLSearchParams({
    action: 'TEMPLATE',
    text: p.title,
    dates: `${fmt(p.startUtc)}/${fmt(p.endUtc)}`,
  })
  if (p.details) params.set('details', p.details)
  if (p.location) params.set('location', p.location)
  return `https://calendar.google.com/calendar/render?${params.toString()}`
}

// Common IANA zones for the manual switcher (visitor tz is added if missing).
export const COMMON_TIMEZONES = [
  'Europe/Copenhagen', 'Europe/London', 'Europe/Berlin', 'Europe/Paris', 'Europe/Madrid',
  'Europe/Stockholm', 'Europe/Helsinki', 'America/New_York', 'America/Chicago', 'America/Los_Angeles',
  'Asia/Dubai', 'Asia/Kolkata', 'Asia/Singapore', 'Asia/Tokyo', 'Australia/Sydney', 'UTC',
]