// Convert a datetime-local string ("YYYY-MM-DDTHH:MM") to a UTC Date,
// interpreting the wall-clock time in the given IANA timezone.
export function zonedLocalToUTC(localStr, tz) {
  if (!localStr || !tz) return null;
  const [datePart, timePart] = String(localStr).split("T");
  if (!datePart || !timePart) return null;
  const [y, mo, d] = datePart.split("-").map(Number);
  const [h, mi] = timePart.split(":").map(Number);
  if ([y, mo, d, h, mi].some((n) => Number.isNaN(n))) return null;

  // Treat the components as if they are already UTC. The result's wall-clock
  // in the target TZ will differ from what we want by exactly the TZ offset
  // for that moment, so we measure that offset and subtract it.
  const asUTC = new Date(Date.UTC(y, mo - 1, d, h, mi));
  const parts = new Intl.DateTimeFormat("en-US", {
    timeZone: tz,
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
    hour: "2-digit",
    minute: "2-digit",
    second: "2-digit",
    hour12: false,
  }).formatToParts(asUTC);
  const get = (type) => Number(parts.find((p) => p.type === type)?.value);
  const observed = Date.UTC(
    get("year"),
    get("month") - 1,
    get("day"),
    get("hour") % 24,
    get("minute"),
    get("second"),
  );
  const offsetMs = observed - asUTC.getTime();
  return new Date(asUTC.getTime() - offsetMs);
}

// Convert a UTC Date (or ISO string) to a datetime-local string
// ("YYYY-MM-DDTHH:MM") rendered in the given IANA timezone.
export function utcToZonedLocal(utc, tz) {
  if (!utc || !tz) return "";
  const d = utc instanceof Date ? utc : new Date(utc);
  if (Number.isNaN(d.getTime())) return "";
  const parts = new Intl.DateTimeFormat("en-US", {
    timeZone: tz,
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
    hour: "2-digit",
    minute: "2-digit",
    hour12: false,
  }).formatToParts(d);
  const get = (type) => parts.find((p) => p.type === type)?.value;
  const year = get("year");
  const month = get("month");
  const day = get("day");
  let hour = get("hour");
  const minute = get("minute");
  if (hour === "24") hour = "00";
  return `${year}-${month}-${day}T${hour}:${minute}`;
}

// Short timezone label (e.g., "EDT", "PDT") for a Date in a given IANA TZ.
export function shortTimezoneName(date, tz) {
  if (!date || !tz) return "";
  const d = date instanceof Date ? date : new Date(date);
  if (Number.isNaN(d.getTime())) return "";
  try {
    const parts = new Intl.DateTimeFormat("en-US", {
      timeZone: tz,
      timeZoneName: "short",
    }).formatToParts(d);
    return parts.find((p) => p.type === "timeZoneName")?.value || "";
  } catch {
    return "";
  }
}