/** Iran standard time (Asia/Tehran, UTC+3:30; no DST since 2022). */
export const IRAN_TIMEZONE = "Asia/Tehran";

const IRAN_UTC_OFFSET = "+03:30";

const DATE_ONLY_RE = /^\d{4}-\d{2}-\d{2}$/;
const TIME_HM_RE = /^(\d{1,2}):(\d{2})(?::(\d{2}))?$/;
const NAIVE_ISO_DATETIME_RE =
  /^(\d{4}-\d{2}-\d{2})[T\s](\d{1,2}:\d{2}(?::\d{2})?)$/;

/**
 * Gregorian calendar date (`YYYY-MM-DD`) for an instant in Iran.
 */
export function gregorianDateInIran(d: Date): string {
  return new Intl.DateTimeFormat("en-CA", {
    timeZone: IRAN_TIMEZONE,
    year: "numeric",
    month: "2-digit",
    day: "2-digit",
  }).format(d);
}

function normalizeTimeToHms(time: string): string | null {
  const m = String(time).trim().match(TIME_HM_RE);
  if (!m) return null;
  const hh = m[1].padStart(2, "0");
  const mm = m[2];
  const ss = m[3] ?? "00";
  return `${hh}:${mm}:${ss}`;
}

function parseIranLocalIso(isoDate: string, timeHms: string): Date | null {
  const d = new Date(`${isoDate}T${timeHms}${IRAN_UTC_OFFSET}`);
  return Number.isNaN(d.getTime()) ? null : d;
}

/**
 * Combines a calendar date and optional `HH:MM` / `HH:MM:SS` as Iran local
 * time and returns the corresponding UTC instant.
 */
export function parseIranLocalDateTime(
  date: Date | string,
  time?: string,
): Date | null {
  if (date instanceof Date) {
    if (Number.isNaN(date.getTime())) return null;
    const cleanTime = (time ?? "").trim();
    if (!cleanTime) return date;
    const timeHms = normalizeTimeToHms(cleanTime);
    if (!timeHms) return null;
    return parseIranLocalIso(gregorianDateInIran(date), timeHms);
  }

  const raw = String(date).trim();
  if (!raw) return null;

  if (/[zZ]|[+-]\d{2}:?\d{2}$/.test(raw)) {
    const d = new Date(raw);
    return Number.isNaN(d.getTime()) ? null : d;
  }

  const naive = raw.match(NAIVE_ISO_DATETIME_RE);
  if (naive) {
    const timeHms = normalizeTimeToHms(naive[2]);
    if (!timeHms) return null;
    return parseIranLocalIso(naive[1], timeHms);
  }

  if (DATE_ONLY_RE.test(raw)) {
    const cleanTime = (time ?? "").trim();
    if (!cleanTime) {
      const d = new Date(`${raw}T00:00:00${IRAN_UTC_OFFSET}`);
      return Number.isNaN(d.getTime()) ? null : d;
    }
    const timeHms = normalizeTimeToHms(cleanTime);
    if (!timeHms) return null;
    return parseIranLocalIso(raw, timeHms);
  }

  const d = new Date(raw);
  return Number.isNaN(d.getTime()) ? null : d;
}