/// Rolling 5 km fuel economy calculated from the ECU fuel rate or MAF.
///
/// The ECU's engine-fuel-rate PID (`5E`) is preferred because it uses the same
/// injection data as the car's trip computer and works for petrol and diesel.
/// When that PID is unavailable, MAF and petrol stoichiometric constants
/// (AFR 14.7, 745 g/L) are used as a fallback. The fallback is deliberately an
/// estimate; the dashboard presents it as a trend, not a measurement.
class ObdEconomy {
  static const windowKm = 5.0;
  static const minimumKm = 0.5;
  static const _maxGap = Duration(seconds: 30);
  static const _afr = 14.7;
  static const _fuelDensityGramsPerLitre = 745.0;
  static const _maxPlausibleFuelRateLph = 200.0;

  final List<_Segment> _segments = [];
  DateTime? _lastAt;
  bool _hasPrevious = false;

  double? get litresPer100Km {
    if (_segments.isEmpty) return null;
    var km = 0.0;
    var litres = 0.0;
    for (final segment in _segments) {
      km += segment.km;
      litres += segment.litres;
    }
    if (km < minimumKm || km <= 0) return null;
    return (100 * litres / km).clamp(0.0, 99.9);
  }

  double get accumulatedKm {
    return _segments.fold(0, (total, segment) => total + segment.km);
  }

  void reset() {
    _segments.clear();
    _lastAt = null;
    _hasPrevious = false;
  }

  void add({
    required DateTime at,
    required double? speedKmh,
    required double? mafGps,
    required double? throttlePercent,
    double? fuelRateLph,
  }) {
    final previousAt = _lastAt;
    _lastAt = at;

    // A supported PID can briefly return a placeholder or a corrupt frame.
    // Ignore implausible values and fall back to MAF for this sample.
    final directFuelRate =
        fuelRateLph != null &&
            fuelRateLph >= 0 &&
            fuelRateLph <= _maxPlausibleFuelRateLph
        ? fuelRateLph
        : null;

    if (speedKmh == null || (directFuelRate == null && mafGps == null)) {
      _hasPrevious = false;
      return;
    }
    if (previousAt == null ||
        !_hasPrevious ||
        at.difference(previousAt) > _maxGap ||
        at.isBefore(previousAt)) {
      if (previousAt != null && at.difference(previousAt) > _maxGap) {
        reset();
      }
      _lastAt = at;
      _hasPrevious = true;
      return;
    }

    final deltaSeconds = at.difference(previousAt).inMilliseconds / 1000.0;
    if (deltaSeconds <= 0) return;
    final double litresPerHour;
    if (directFuelRate != null) {
      litresPerHour = directFuelRate;
    } else {
      final fuelCut =
          throttlePercent != null && throttlePercent < 1 && speedKmh > 10;
      litresPerHour = fuelCut
          ? 0.0
          : (mafGps! / _afr) * 3600 / _fuelDensityGramsPerLitre;
    }
    final km = speedKmh * deltaSeconds / 3600;
    if (km > 0) {
      _segments.add(
        _Segment(km: km, litres: litresPerHour * deltaSeconds / 3600),
      );
      _trim();
    }
    _lastAt = at;
    _hasPrevious = true;
  }

  void _trim() {
    var km = accumulatedKm;
    while (km > windowKm && _segments.isNotEmpty) {
      km -= _segments.removeAt(0).km;
    }
  }
}

class _Segment {
  const _Segment({required this.km, required this.litres});

  final double km;
  final double litres;
}