// Offer dates are stored in UTC and edited in the shop's timezone (brief §3).

function offsetMinutes(timeZone: string, at: Date) {
  try {
    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 p = Object.fromEntries(parts.map((x) => [x.type, Number(x.value)]));
    const asUtc = Date.UTC(p.year, p.month - 1, p.day, p.hour, p.minute, p.second);
    return Math.round((asUtc - at.getTime()) / 60000);
  } catch {
    return 0;
  }
}

/** "2026-10-20T00:00" in the shop timezone -> UTC ISO string (null for empty). */
export function shopLocalToUtc(local: string | null | undefined, timeZone: string): string | null {
  if (!local) return null;
  const m = /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2})/.exec(local);
  if (!m) return null;
  const guess = Date.UTC(+m[1], +m[2] - 1, +m[3], +m[4], +m[5]);
  let t = guess - offsetMinutes(timeZone, new Date(guess)) * 60000;
  t = guess - offsetMinutes(timeZone, new Date(t)) * 60000; // second pass across DST changes
  return new Date(t).toISOString();
}

/** UTC -> "YYYY-MM-DDTHH:mm" in the shop timezone, for <input type="datetime-local">. */
export function utcToShopLocal(utc: Date | string | null | undefined, timeZone: string): string {
  if (!utc) return "";
  const d = new Date(utc);
  const local = new Date(d.getTime() + offsetMinutes(timeZone, d) * 60000);
  return local.toISOString().slice(0, 16);
}

export function formatShopDate(utc: Date | string | null | undefined, timeZone: string, withTime = true) {
  if (!utc) return "";
  try {
    return new Intl.DateTimeFormat("en", {
      timeZone, dateStyle: "medium", ...(withTime ? { timeStyle: "short" } : {}),
    }).format(new Date(utc));
  } catch {
    return new Date(utc).toISOString();
  }
}