/// Metric or imperial readouts (GO 4's General → Units, plan P6).
///
/// The aircraft always reports metres and m/s, and every setting is still
/// written in metres; only what the pilot reads changes.
enum UnitSystem {
  metric('Metric (m, m/s)'),
  imperial('Imperial (ft, mph)');

  const UnitSystem(this.label);
  final String label;
}

abstract final class Units {
  static const double feetPerMetre = 3.28084;
  static const double mphPerMs = 2.23694;
  static const double kmhPerMs = 3.6;

  /// A height or short distance: `12 m` / `39 ft`.
  static String length(double? m, UnitSystem u, {int decimals = 0}) {
    if (m == null) return '—';
    return u == UnitSystem.metric ? '${m.toStringAsFixed(decimals)} m' : '${(m * feetPerMetre).toStringAsFixed(decimals)} ft';
  }

  /// A distance that can run long: metres then km, or feet then miles.
  static String distance(double? m, UnitSystem u) {
    if (m == null) return '—';
    if (u == UnitSystem.metric) return m < 1000 ? '${m.round()} m' : '${(m / 1000).toStringAsFixed(m < 10000 ? 2 : 1)} km';
    final double ft = m * feetPerMetre;
    return ft < 5280 ? '${ft.round()} ft' : '${(ft / 5280).toStringAsFixed(ft < 52800 ? 2 : 1)} mi';
  }

  /// A speed: `8.2 m/s` / `18.3 mph`.
  static String speed(double? ms, UnitSystem u, {int decimals = 1}) {
    if (ms == null) return '—';
    return u == UnitSystem.metric ? '${ms.toStringAsFixed(decimals)} m/s' : '${(ms * mphPerMs).toStringAsFixed(decimals)} mph';
  }

  /// The bare number for a HUD readout whose unit is drawn apart.
  static double lengthValue(double m, UnitSystem u) => u == UnitSystem.metric ? m : m * feetPerMetre;
  static double speedValue(double ms, UnitSystem u) => u == UnitSystem.metric ? ms : ms * mphPerMs;

  static String lengthUnit(UnitSystem u) => u == UnitSystem.metric ? 'm' : 'ft';
  static String speedUnit(UnitSystem u) => u == UnitSystem.metric ? 'm/s' : 'mph';

  /// The other system's equivalent beside a metric setting: `≈ 394 ft` / `≈ 120 m`.
  static String altHint(int metres, UnitSystem u) =>
      u == UnitSystem.metric ? '≈ ${(metres * feetPerMetre).round()} ft' : '${(metres * feetPerMetre).round()} ft';
}