const FALLBACK_TIME_ZONE = "UTC";

export function isValidTimeZone(value: string): boolean {
  if (!value.trim()) return false;
  try {
    new Intl.DateTimeFormat("en-US", { timeZone: value }).format(0);
    return true;
  } catch {
    return false;
  }
}

export function getLocalTimeZone(): string {
  const resolved = Intl.DateTimeFormat().resolvedOptions().timeZone;
  return resolved && isValidTimeZone(resolved) ? resolved : FALLBACK_TIME_ZONE;
}

export function formatZonedDateTime(
  value: Date | string | number,
  timeZone = getLocalTimeZone(),
  options: Intl.DateTimeFormatOptions = {},
): string {
  const date = value instanceof Date ? value : new Date(value);
  if (Number.isNaN(date.getTime())) return "Invalid date";

  return new Intl.DateTimeFormat("en-US", {
    dateStyle: "medium",
    timeStyle: "short",
    ...options,
    timeZone: isValidTimeZone(timeZone) ? timeZone : FALLBACK_TIME_ZONE,
  }).format(date);
}

export function getDefaultScheduledSendAt(now = new Date()): Date {
  const result = new Date(now);
  result.setMinutes(0, 0, 0);
  result.setHours(result.getHours() + 1);
  return result;
}

export function toLocalDateTimeInputValue(value: Date): string {
  const pad = (part: number) => String(part).padStart(2, "0");
  return `${value.getFullYear()}-${pad(value.getMonth() + 1)}-${pad(value.getDate())}T${pad(value.getHours())}:${pad(value.getMinutes())}`;
}