/// ISO-8601 duration (`PT1H30M5S`) ↔ Dart `Duration`. Backend serialises
/// `java.time.Duration` in this format; Dart has no builtin parser.
class IsoDuration {
  IsoDuration._();

  static final RegExp _pattern = RegExp(
    r'^P(?:T(?:(\d+)H)?(?:(\d+)M)?(?:(\d+(?:\.\d+)?)S)?)?$',
  );

  static Duration? fromJson(String? iso) {
    if (iso == null || iso.isEmpty) return null;
    final match = _pattern.firstMatch(iso);
    if (match == null) return null;
    final hours = int.parse(match.group(1) ?? '0');
    final minutes = int.parse(match.group(2) ?? '0');
    final secondsRaw = match.group(3) ?? '0';
    final secondsValue = double.parse(secondsRaw);
    return Duration(
      hours: hours,
      minutes: minutes,
      milliseconds: (secondsValue * 1000).round(),
    );
  }

  static String? toJson(Duration? d) {
    if (d == null) return null;
    final hours = d.inHours;
    final minutes = d.inMinutes.remainder(60);
    final seconds = d.inSeconds.remainder(60);
    final buf = StringBuffer('PT');
    if (hours > 0) buf.write('${hours}H');
    if (minutes > 0) buf.write('${minutes}M');
    if (seconds > 0 || (hours == 0 && minutes == 0)) buf.write('${seconds}S');
    return buf.toString();
  }
}