/// The seam between the recording pipeline and whatever produces GPS fixes.
///
/// This exists for three reasons, in order of importance:
///
///  1. **Testability.** The pipeline is the riskiest code in the app and must be
///     verifiable without a device, an emulator, or a real ride.
///  2. **Swappability.** `geolocator` is the free choice. If a real iOS ride shows it
///     losing fixes when the OS suspends a stationary app, moving to
///     `flutter_background_geolocation` becomes one new implementation of this interface
///     rather than a rewrite of the engine.
///  3. **Platform honesty.** Android and iOS keep a process alive by genuinely different
///     mechanisms. Confining that difference behind this interface keeps it out of the
///     pipeline entirely.
library;

import 'dart:async';

/// A single platform-neutral GPS fix.
///
/// Deliberately **not** `TrackPoint`: a fix has no trip or segment identity. Those ids
/// are stamped on by the recorder at the moment of creation, which is what makes a pause
/// safe — see the recording engine.
///
/// `speedMps` is metres per second, as every platform reports it. Conversion to km/h and
/// the noise-floor sanitisation happen once, in the engine, via the ported `Telemetry`
/// functions.
class LocationFix {
  const LocationFix({
    required this.timestamp,
    required this.latitude,
    required this.longitude,
    required this.speedMps,
    required this.altitudeM,
    required this.accuracyM,
    required this.bearingDeg,
  });

  final int timestamp;
  final double latitude;
  final double longitude;
  final double speedMps;
  final double altitudeM;
  final double accuracyM;
  final double bearingDeg;

  @override
  String toString() =>
      'LocationFix($latitude, $longitude, ${speedMps}m/s, ±${accuracyM}m)';
}

/// Why location is unavailable, when it is.
enum LocationFailure {
  /// The user denied the permission, or has not granted it yet.
  permissionDenied,

  /// Denied permanently — only a trip to system settings will fix it.
  permissionDeniedForever,

  /// Location services are switched off device-wide.
  serviceDisabled,
}

class LocationException implements Exception {
  const LocationException(this.failure, [this.message]);

  final LocationFailure failure;
  final String? message;

  @override
  String toString() => 'LocationException($failure${message == null ? '' : ': $message'})';
}

/// A source of GPS fixes.
///
/// Implementations must guarantee that [fixes] never throws mid-stream for a transient
/// problem — the recorder treats a closed stream as "recording has stopped", which is a
/// user-visible event. Transient errors belong in logs, not in the stream.
abstract class LocationSource {
  /// Fixes, at roughly 1–2 Hz while recording.
  ///
  /// Nothing is emitted until [start] has been called.
  Stream<LocationFix> get fixes;

  /// Begins delivering fixes, requesting permission if needed.
  ///
  /// Throws [LocationException] if permission or the device service is unavailable.
  /// On Android this is also where the foreground service is raised; on iOS it is where
  /// background updates are enabled.
  Future<void> start();

  /// Stops delivering fixes and releases whatever the platform was holding — the
  /// foreground service and wake lock on Android, background updates on iOS.
  ///
  /// Must be idempotent: the recorder calls it on pause, stop, and discard, and those can
  /// arrive in any order after a process restart.
  Future<void> stop();

  Future<void> dispose();
}

/// An in-memory [LocationSource] for tests.
///
/// Lets the whole recording pipeline — batching, segment stamping, accumulation,
/// persistence, pause boundaries — be exercised on the Dart VM with no device.
class FakeLocationSource implements LocationSource {
  final _controller = StreamController<LocationFix>.broadcast();

  var _started = false;
  var _startCalls = 0;
  var _stopCalls = 0;

  bool get isRunning => _started;
  int get startCalls => _startCalls;
  int get stopCalls => _stopCalls;

  /// Set to make [start] throw, so permission handling can be tested.
  LocationException? failOnStart;

  @override
  Stream<LocationFix> get fixes => _controller.stream;

  @override
  Future<void> start() async {
    _startCalls++;
    final failure = failOnStart;
    if (failure != null) throw failure;
    _started = true;
  }

  @override
  Future<void> stop() async {
    _stopCalls++;
    _started = false;
  }

  @override
  Future<void> dispose() => _controller.close();

  /// Emits a fix as though the platform had produced it.
  ///
  /// Silently ignored while stopped, mirroring the real thing: a platform that has been
  /// told to stop does not keep delivering.
  void emit(LocationFix fix) {
    if (!_started) return;
    _controller.add(fix);
  }

  /// Convenience for building a plausible fix without spelling out every field.
  void emitAt({
    required int timestamp,
    double latitude = 51.0,
    double longitude = -114.0,
    double speedMps = 11.111111111111112, // 40 km/h
    double altitudeM = 1000.0,
    double accuracyM = 5.0,
    double bearingDeg = 0.0,
  }) =>
      emit(LocationFix(
        timestamp: timestamp,
        latitude: latitude,
        longitude: longitude,
        speedMps: speedMps,
        altitudeM: altitudeM,
        accuracyM: accuracyM,
        bearingDeg: bearingDeg,
      ));
}