/**
 * Timezone helpers (client-safe, Intl only). Planned dates are entered as wall-clock time in the
 * tenant timezone (TenantSettings.timezone) and stored as UTC.
 */

export const DEFAULT_TIMEZONE = "Europe/Berlin";

export function safeTimeZone(tz: string | null | undefined): string {
  if (!tz) return DEFAULT_TIMEZONE;
  try {
    new Intl.DateTimeFormat("en-US", { timeZone: tz });
    return tz;
  } catch {
    return DEFAULT_TIMEZONE;
  }
}

/** Offset (ms) of `timeZone` relative to UTC at instant `at`. */
export function tzOffsetMs(at: Date, timeZone: string): number {
  const parts = new Intl.DateTimeFormat("en-US", {
    timeZone,
    hourCycle: "h23",
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
    hour: "2-digit",
    minute: "2-digit",
    second: "2-digit",
  }).formatToParts(at);
  const n = (t: string) => Number(parts.find((p) => p.type === t)?.value);
  const asUtc = Date.UTC(n("year"), n("month") - 1, n("day"), n("hour"), n("minute"), n("second"));
  return asUtc - Math.floor(at.getTime() / 1000) * 1000;
}

/** "2026-09-14T08:30" (wall time in tz) → UTC Date. Returns undefined for invalid input. */
export function wallTimeToUtc(value: string, timeZone: string): Date | undefined {
  const m = /^(\d{4})-(\d{2})-(\d{2})(?:T(\d{2}):(\d{2}))?$/.exec(value.trim());
  if (!m) return undefined;
  const tz = safeTimeZone(timeZone);
  const guess = Date.UTC(+m[1], +m[2] - 1, +m[3], m[4] ? +m[4] : 0, m[5] ? +m[5] : 0);
  const first = guess - tzOffsetMs(new Date(guess), tz);
  // second pass corrects DST boundaries
  const result = guess - tzOffsetMs(new Date(first), tz);
  const d = new Date(result);
  return Number.isNaN(d.getTime()) ? undefined : d;
}

/** UTC Date → "YYYY-MM-DDTHH:mm" wall time in tz (for <input type="datetime-local">). */
export function toWallTimeInput(date: Date | null | undefined, timeZone: string): string {
  if (!date) return "";
  const parts = new Intl.DateTimeFormat("en-CA", {
    timeZone: safeTimeZone(timeZone),
    hourCycle: "h23",
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
    hour: "2-digit",
    minute: "2-digit",
  }).formatToParts(date);
  const p = (t: string) => parts.find((x) => x.type === t)?.value ?? "00";
  return `${p("year")}-${p("month")}-${p("day")}T${p("hour")}:${p("minute")}`;
}

/** Start/end of the calendar day containing `now` in the given timezone. */
export function zonedDayBounds(now: Date, timeZone: string): { start: Date; end: Date } {
  const tz = safeTimeZone(timeZone);
  const [y, m, d] = new Intl.DateTimeFormat("en-CA", { timeZone: tz, year: "numeric", month: "2-digit", day: "2-digit" })
    .format(now)
    .split("-")
    .map(Number);
  const guessStart = Date.UTC(y, m - 1, d, 0, 0, 0);
  const start = new Date(guessStart - tzOffsetMs(new Date(guessStart), tz));
  const guessEnd = Date.UTC(y, m - 1, d, 23, 59, 59, 999);
  const end = new Date(guessEnd - tzOffsetMs(new Date(guessEnd), tz));
  return { start, end };
}

export function formatDateTime(date: Date | null | undefined, locale: string, timeZone: string): string {
  if (!date) return "";
  return new Intl.DateTimeFormat(locale === "en" ? "en-GB" : "de-DE", { timeZone: safeTimeZone(timeZone), dateStyle: "medium", timeStyle: "short" }).format(date);
}

export function formatDate(date: Date | null | undefined, locale: string, timeZone: string): string {
  if (!date) return "";
  return new Intl.DateTimeFormat(locale === "en" ? "en-GB" : "de-DE", { timeZone: safeTimeZone(timeZone), dateStyle: "medium" }).format(date);
}