import 'dart:async';
import 'dart:math' as math;

import 'package:flutter/foundation.dart';
import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:geolocator/geolocator.dart';

import '../services/location_platform_service.dart';

enum UserLocationIssue {
  servicesDisabled,
  permissionDenied,
  permissionDeniedForever,
  unavailable,
}

/// Состояние геолокации: позиция или причина отсутствия.
/// Запрашивается один раз, кешируется до перезапуска провайдера.
class UserLocation {
  final Position? position;
  final String? error; // null -- всё ок, иначе причина
  final UserLocationIssue? issue;
  final DateTime? updatedAt;

  const UserLocation({this.position, this.error, this.issue, this.updatedAt});

  bool get hasPosition => position != null;
  bool get needsAppSettings =>
      issue == UserLocationIssue.permissionDeniedForever;
  bool get needsLocationSettings => issue == UserLocationIssue.servicesDisabled;
  bool get canRequestPermission => issue == UserLocationIssue.permissionDenied;

  /// Расстояние в км до точки. Возвращает null если нет позиции
  /// или у цели нет координат.
  double? distanceToKm(double? lat, double? lon) {
    if (position == null || lat == null || lon == null) return null;
    return calculateDistanceKm(
      position!.latitude,
      position!.longitude,
      lat,
      lon,
    );
  }
}

final userLocationProvider =
    NotifierProvider<UserLocationNotifier, UserLocation>(
      () => UserLocationNotifier(),
    );

final locationPlatformServiceProvider = Provider<LocationPlatformService>(
  (ref) => DeviceLocationPlatformService(),
);

class UserLocationNotifier extends Notifier<UserLocation> {
  StreamSubscription<Position>? _sub;
  int _watchers = 0;

  @override
  UserLocation build() {
    ref.onDispose(() {
      _sub?.cancel();
      _sub = null;
    });
    return const UserLocation();
  }

  /// Запросить текущую позицию. Первый вызов проверяет пермишены
  /// и отдаёт lastKnown мгновенно (если есть).
  Future<void> fetch() async {
    if (state.hasPosition) return;
    await refresh();
  }

  /// Начать непрерывное отслеживание. Вызывать из initState экрана.
  /// Ref-counted: несколько экранов могут вызвать startWatching,
  /// стрим остановится только когда все вызовут stopWatching.
  Future<void> startWatching() async {
    _watchers++;
    await _startWatchingIfPossible();
  }

  Future<void> ensureWatchingStarted() async {
    if (_watchers == 0 || _sub != null) {
      return;
    }
    await _startWatchingIfPossible();
  }

  Future<void> _startWatchingIfPossible() async {
    final locationService = ref.read(locationPlatformServiceProvider);
    if (_sub != null) return;

    final permissionState = await _ensurePermission(requestIfDenied: false);
    if (!permissionState.isGranted) {
      state = permissionState.toLocation();
      return;
    }

    if (!state.hasPosition) {
      try {
        final last = await locationService.getLastKnownPosition();
        if (last != null) {
          state = _fromPosition(last);
        }
      } catch (_) {}
    }

    const settings = LocationSettings(
      accuracy: LocationAccuracy.low,
      distanceFilter: 20,
    );

    _sub = locationService
        .getPositionStream(locationSettings: settings)
        .listen(
          (pos) => state = _fromPosition(pos),
          onError: (e) {
            debugPrint('Ошибка стрима геолокации: $e');
            state = UserLocation(
              error: 'Не удалось отслеживать позицию: $e',
              issue: UserLocationIssue.unavailable,
              updatedAt: state.updatedAt,
            );
          },
        );
  }

  /// Остановить отслеживание. Вызывать из dispose экрана.
  void stopWatching() {
    _watchers = (_watchers - 1).clamp(0, 999);
    if (_watchers == 0) {
      _sub?.cancel();
      _sub = null;
    }
  }

  Future<void> refresh() async {
    final locationService = ref.read(locationPlatformServiceProvider);
    final permissionState = await _ensurePermission(requestIfDenied: false);
    if (!permissionState.isGranted) {
      state = permissionState.toLocation();
      return;
    }

    try {
      final last = await locationService.getLastKnownPosition();
      if (last != null) {
        state = _fromPosition(last);
        return;
      }

      final pos = await locationService.getCurrentPosition(
        locationSettings: const LocationSettings(
          accuracy: LocationAccuracy.low,
          timeLimit: Duration(seconds: 10),
        ),
      );
      state = _fromPosition(pos);
    } catch (e) {
      state = UserLocation(
        error: 'Не удалось получить позицию: $e',
        issue: UserLocationIssue.unavailable,
        updatedAt: state.updatedAt,
      );
    }
  }

  Future<void> requestPermission() async {
    final locationService = ref.read(locationPlatformServiceProvider);
    await locationService.requestPermission();
    if (_watchers > 0 && _sub == null) {
      await _startWatchingIfPossible();
      return;
    }
    await refresh();
  }

  Future<void> openAppSettings() async {
    await ref.read(locationPlatformServiceProvider).openAppSettings();
  }

  Future<void> openLocationSettings() async {
    await ref.read(locationPlatformServiceProvider).openLocationSettings();
  }

  /// Проверить сервис и пермишены. Возвращает null если всё ок,
  /// иначе строку с причиной ошибки.
  Future<_LocationPermissionState> _ensurePermission({
    required bool requestIfDenied,
  }) async {
    final locationService = ref.read(locationPlatformServiceProvider);

    if (!await locationService.isLocationServiceEnabled()) {
      return const _LocationPermissionState(
        issue: UserLocationIssue.servicesDisabled,
        message: 'Геолокация выключена',
      );
    }

    var perm = await locationService.checkPermission();
    if (perm == LocationPermission.denied && requestIfDenied) {
      perm = await locationService.requestPermission();
    }
    if (perm == LocationPermission.denied) {
      return const _LocationPermissionState(
        issue: UserLocationIssue.permissionDenied,
        message: 'Нет разрешения на геолокацию',
      );
    }
    if (perm == LocationPermission.deniedForever) {
      return const _LocationPermissionState(
        issue: UserLocationIssue.permissionDeniedForever,
        message: 'Доступ к геолокации запрещён навсегда',
      );
    }
    return const _LocationPermissionState();
  }

  UserLocation _fromPosition(Position position) {
    return UserLocation(position: position, updatedAt: position.timestamp);
  }
}

class _LocationPermissionState {
  final UserLocationIssue? issue;
  final String? message;

  const _LocationPermissionState({this.issue, this.message});

  bool get isGranted => issue == null;

  UserLocation toLocation() {
    return UserLocation(error: message, issue: issue);
  }
}

double calculateDistanceKm(double lat1, double lon1, double lat2, double lon2) {
  const earthRadiusKm = 6371.0;
  final dLat = _degToRad(lat2 - lat1);
  final dLon = _degToRad(lon2 - lon1);
  final a =
      math.sin(dLat / 2) * math.sin(dLat / 2) +
      math.cos(_degToRad(lat1)) *
          math.cos(_degToRad(lat2)) *
          math.sin(dLon / 2) *
          math.sin(dLon / 2);
  final c = 2 * math.atan2(math.sqrt(a), math.sqrt(1 - a));
  return earthRadiusKm * c;
}

double _degToRad(double deg) => deg * (math.pi / 180);

/// Форматирование расстояния в человекочитаемый вид
String formatDistance(double km) {
  if (km < 1.0) {
    return '${(km * 1000).round()} м';
  } else if (km < 10.0) {
    return '${km.toStringAsFixed(1)} км';
  } else {
    return '${km.round()} км';
  }
}