const BRUSSELS_TIME_ZONE = "Europe/Brussels";

const partsFormatter = new Intl.DateTimeFormat("en-CA", {
  timeZone: BRUSSELS_TIME_ZONE,
  year: "numeric",
  month: "2-digit",
  day: "2-digit",
  hour: "2-digit",
  minute: "2-digit",
  second: "2-digit",
  hourCycle: "h23",
});

function partsAt(value: Date): Record<string, string> {
  return Object.fromEntries(
    partsFormatter.formatToParts(value)
      .filter((part) => part.type !== "literal")
      .map((part) => [part.type, part.value]),
  );
}

export function toBrusselsDateTimeLocal(value: Date): string {
  const parts = partsAt(value);
  return `${toBrusselsDate(value)}T${parts.hour}:${parts.minute}`;
}

/** Render an instant as the Fleet Ops Europe/Brussels calendar date (YYYY-MM-DD). */
export function toBrusselsDate(value: Date): string {
  const parts = partsAt(value);
  return `${parts.year}-${parts.month}-${parts.day}`;
}

export function brusselsDateTimeFromNow(hours: number): string {
  const value = new Date(Date.now() + hours * 60 * 60 * 1000);
  value.setUTCMinutes(0, 0, 0);
  return toBrusselsDateTimeLocal(value);
}

/** Convert a Fleet Ops Europe/Brussels wall-clock value to an unambiguous UTC instant. */
export function brusselsLocalToIso(value: string): string {
  const match = /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2})$/.exec(value);
  if (!match) throw new Error("Invalid Brussels date-time");
  const [, year, month, day, hour, minute] = match;
  const wallClockUtc = Date.UTC(+year, +month - 1, +day, +hour, +minute);
  let candidate = wallClockUtc;
  for (let iteration = 0; iteration < 3; iteration += 1) {
    const parts = partsAt(new Date(candidate));
    const represented = Date.UTC(+parts.year, +parts.month - 1, +parts.day, +parts.hour, +parts.minute);
    candidate += wallClockUtc - represented;
  }
  const result = new Date(candidate);
  // Date.UTC normalises impossible calendar values and Brussels' spring transition
  // contains a wall-clock hour that does not exist. Never silently shift either one.
  if (toBrusselsDateTimeLocal(result) !== value) {
    throw new Error("Invalid or non-existent Brussels date-time");
  }
  return result.toISOString();
}

export function tryBrusselsLocalToIso(value: string): string | null {
  try {
    return brusselsLocalToIso(value);
  } catch {
    return null;
  }
}

/** Start of the given Brussels calendar day (YYYY-MM-DD) as a UTC ISO instant. */
export function brusselsDayStartIso(day: string): string {
  return brusselsLocalToIso(`${day}T00:00`);
}

/** Start of the day *after* the given Brussels calendar day, i.e. an exclusive upper bound. */
export function brusselsNextDayStartIso(day: string): string {
  const [year, month, date] = day.split("-").map(Number);
  const next = new Date(Date.UTC(year, month - 1, date + 1));
  return brusselsLocalToIso(`${next.toISOString().slice(0, 10)}T00:00`);
}