// Appointment times as the deck writes them, on the viewer's clock; the zone label only when it differs.

const instantOf = (iso?: string): Date | undefined => {
  if (!iso) return undefined;
  const at = new Date(iso);
  return Number.isNaN(at.getTime()) ? undefined : at;
};

const timeOf = (at: Date, zoneLabel?: string | null): string =>
  `${at.toLocaleTimeString("en-US", { hour: "numeric", minute: "2-digit" })}${zoneLabel ? ` ${zoneLabel}` : ""}`;

export const appointmentRowWhen = (iso?: string, zoneLabel?: string | null): string => {
  const at = instantOf(iso);
  if (!at) return "";
  const day = at.toLocaleDateString("en-US", { weekday: "short", month: "short", day: "numeric" });
  return `${day} · ${timeOf(at, zoneLabel)}`;
};

export const appointmentDetailDate = (iso?: string): string => {
  const at = instantOf(iso);
  if (!at) return "";
  const weekday = at.toLocaleDateString("en-US", { weekday: "short" });
  const date = at.toLocaleDateString("en-US", { month: "long", day: "numeric", year: "numeric" });
  return `${weekday} ${date}`;
};

export const appointmentDetailTime = (iso?: string, zoneLabel?: string | null): string => {
  const at = instantOf(iso);
  return at ? timeOf(at, zoneLabel) : "";
};

export const appointmentShortDate = (iso?: string): string => {
  const at = instantOf(iso);
  return at ? at.toLocaleDateString("en-US", { month: "short", day: "numeric", year: "numeric" }) : "";
};

const startOfDay = (at: Date): number => new Date(at.getFullYear(), at.getMonth(), at.getDate()).getTime();

// "in 6 days", "Tomorrow", "3 months ago": a distance a patient reads at a glance, in whole calendar
// days on their own clock, and in months once days stop meaning anything.
export const appointmentRelativeDay = (iso?: string, now: Date = new Date()): string => {
  const at = instantOf(iso);
  if (!at) return "";
  const days = Math.round((startOfDay(at) - startOfDay(now)) / 86_400_000);
  if (days === 0) return "Today";
  if (days === 1) return "Tomorrow";
  if (days === -1) return "Yesterday";
  const span = Math.abs(days);
  const amount = span >= 60 ? `${Math.round(span / 30.44)} months` : `${span} days`;
  return days > 0 ? `in ${amount}` : `${amount} ago`;
};