const BERLIN_TZ = "Europe/Berlin";

/**
 * Converts a JS Date (a UTC instant) into its Europe/Berlin wall-clock
 * representation for iCalendar's local `TZID` date-time form
 * (`DTSTART;TZID=Europe/Berlin:YYYYMMDDTHHMMSS`). Uses the ICU timezone
 * database via Intl rather than manual UTC+1/+2 arithmetic, so daylight
 * saving transitions are always handled correctly — no naive UTC offset
 * math that would drift by an hour half the year.
 */
export function toBerlinICSDateTime(date: Date): string {
  const parts = new Intl.DateTimeFormat("en-US", {
    timeZone: BERLIN_TZ,
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
    hour: "2-digit",
    minute: "2-digit",
    second: "2-digit",
    hourCycle: "h23",
  }).formatToParts(date);

  const get = (type: string) => parts.find((p) => p.type === type)?.value ?? "00";
  return `${get("year")}${get("month")}${get("day")}T${get("hour")}${get("minute")}${get("second")}`;
}

/** UTC timestamp form (`YYYYMMDDTHHMMSSZ`) — used only for DTSTAMP, which marks generation time, not a local event time. */
export function toUTCICSStamp(date: Date): string {
  return date.toISOString().replace(/[-:]/g, "").split(".")[0] + "Z";
}

/**
 * Standard IANA Europe/Berlin VTIMEZONE block (the same one commonly
 * embedded by calendar exporters): CEST from the last Sunday in March,
 * CET from the last Sunday in October — the actual EU DST rule, not an
 * approximation.
 */
export const BERLIN_VTIMEZONE_LINES = [
  "BEGIN:VTIMEZONE",
  "TZID:Europe/Berlin",
  "X-LIC-LOCATION:Europe/Berlin",
  "BEGIN:DAYLIGHT",
  "TZOFFSETFROM:+0100",
  "TZOFFSETTO:+0200",
  "TZNAME:CEST",
  "DTSTART:19700329T020000",
  "RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU",
  "END:DAYLIGHT",
  "BEGIN:STANDARD",
  "TZOFFSETFROM:+0200",
  "TZOFFSETTO:+0100",
  "TZNAME:CET",
  "DTSTART:19701025T030000",
  "RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU",
  "END:STANDARD",
  "END:VTIMEZONE",
];