const BERLIN_TZ = "Europe/Berlin";

export type CalendarDate = { year: number; month: number; day: number };

/** Strict calendar input, interpreted in Berlin regardless of server TZ. */
export function parseBerlinDate(value: string): Date | null {
  if (!/^\d{4}-\d{2}-\d{2}$/.test(value)) return null;
  const [year, month, day] = value.split("-").map(Number);
  if (year < 1000 || month < 1 || month > 12 || day < 1 || day > 31) return null;
  const date = berlinWallTimeToUTC({ year, month, day }, 0, 0);
  return berlinDateKey(date) === value ? date : null;
}

export function berlinDateKey(date: Date): string {
  const { year, month, day } = berlinParts(date);
  return `${year}-${String(month).padStart(2, "0")}-${String(day).padStart(2, "0")}`;
}

export function formatBerlinDate(date: Date, options: Intl.DateTimeFormatOptions = {}): string {
  return new Intl.DateTimeFormat("de-DE", { ...options, timeZone: BERLIN_TZ }).format(date);
}

export function formatBerlinTime(date: Date, options: Intl.DateTimeFormatOptions = {}): string {
  return new Intl.DateTimeFormat("de-DE", { ...options, timeZone: BERLIN_TZ }).format(date);
}

export function berlinParts(date: Date): CalendarDate & { hour: number; minute: number } {
  const parts = new Intl.DateTimeFormat("en-US", {
    timeZone: BERLIN_TZ,
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
    hour: "2-digit",
    minute: "2-digit",
    hourCycle: "h23",
  }).formatToParts(date);
  const get = (type: string) => Number(parts.find((part) => part.type === type)?.value ?? "0");
  return { year: get("year"), month: get("month"), day: get("day"), hour: get("hour"), minute: get("minute") };
}

export function berlinOffsetMinutes(date: Date): number {
  const tzName = new Intl.DateTimeFormat("en-US", { timeZone: BERLIN_TZ, timeZoneName: "shortOffset" })
    .formatToParts(date)
    .find((part) => part.type === "timeZoneName")?.value ?? "GMT+1";
  const match = tzName.match(/GMT([+-]\d+)(?::(\d+))?/);
  const hours = match ? Number(match[1]) : 1;
  const minutes = match?.[2] ? Number(match[2]) : 0;
  return hours * 60 + (hours < 0 ? -minutes : minutes);
}

/** Converts a Berlin wall-clock date/time into the UTC instant it represents. */
export function berlinWallTimeToUTC(date: CalendarDate, hour: number, minute: number): Date {
  const guess = Date.UTC(date.year, date.month - 1, date.day, hour, minute);
  return new Date(guess - berlinOffsetMinutes(new Date(guess)) * 60_000);
}

export function berlinDayBounds(date: Date): { key: string; start: Date; end: Date } {
  const current = berlinParts(date);
  const nextCalendarDay = new Date(Date.UTC(current.year, current.month - 1, current.day + 1));
  const next = berlinParts(nextCalendarDay);
  const start = berlinWallTimeToUTC(current, 0, 0);
  const nextStart = berlinWallTimeToUTC(next, 0, 0);
  return { key: berlinDateKey(date), start, end: new Date(nextStart.getTime() - 1) };
}

/**
 * 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",
];