// Approximate local wall-clock from longitude (v0.11.0) — display-only airport
// popup field. The zone is the fixed-offset band N = round(lon/15), no DST and no
// political boundaries — good enough for a game map, labelled approximate in the UI.

// Etc/GMT±N name for the band (POSIX sign flip: UTC+1 is "Etc/GMT-1"). Exported as
// the documented pure mapping; approxLocalTime below does NOT pass it to Intl,
// because some runtimes ship a reduced ICU that lacks the positive-offset zones.
export function approxZoneForLon(lonDeg: number): string {
  const n = Math.max(-12, Math.min(12, Math.round(lonDeg / 15)));
  return `Etc/GMT${n === 0 ? '' : -n}`;
}

// "HH:MM" in the approximate zone at epoch ms. Implemented as a UTC-format of a
// shifted timestamp (UTC + N hours) so it works on every ICU build, including
// small-icu runtimes missing Etc/GMT+N zones.
export function approxLocalTime(lonDeg: number, atMs: number): string {
  const n = Math.max(-12, Math.min(12, Math.round(lonDeg / 15)));
  const shifted = new Date(atMs + n * 3_600_000);
  return new Intl.DateTimeFormat('en-GB', {
    timeZone: 'UTC',
    hour: '2-digit',
    minute: '2-digit',
    hourCycle: 'h23',
  }).format(shifted);
}