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

import 'package:fforte/l10n/app_localizations.dart';
import 'package:fforte/services/notification_service.dart';
import 'package:flutter/material.dart';
import 'package:geolocator/geolocator.dart';
import 'package:latlong2/latlong.dart';
import 'package:shared_preferences/shared_preferences.dart';

/// Service that runs the geolocator service to track the device's position
/// Uses the Singleton design pattern to ensure only one tracking instance exists
/// This service is essential for excursion tracking functionality
///
/// Start tracking via [startTracking]
/// Control tracking via [pauseTracking], [stopTracking], and [resumeTracking]
class TrackingService {
  // Singleton implementation
  static TrackingService? _instance;

  factory TrackingService() {
    _instance ??= TrackingService._internal();
    return _instance!;
  }

  TrackingService._internal();

  /// Reset all tracking values for a fresh start
  static void resetInstance() {
    if (_instance != null) {
      _instance!.dispose();
      _instance = null;
    }
  }

  // Core tracking variables
  // Stores tracked GPS coordinates
  List<LatLng> pathList = [];
  // Stores GPS accuracy values
  List<double> accuracyList = [];
  // Timer for periodic tracking
  Timer? _positionTimer;
  // Current tracking status
  bool isTracking = false;
  // Context for UI interactions
  BuildContext? _lastContext;
  // Stream controllers for position and stats updates
  final _positionController = StreamController<Position>.broadcast();
  final _statsController = StreamController<TrackingStats>.broadcast();

  // Stream getters
  Stream<Position> get positionStream$ => _positionController.stream;
  Stream<TrackingStats> get statsStream$ => _statsController.stream;

  // Last measured GPS accuracy for display
  double? currentAccuracy;

  /// Calculate median accuracy from a list of accuracy values
  /// This is preferred over mean because initial accuracy values can be very high (~9000m)
  double _calculateMedianAccuracy(List<double> accuracies) {
    if (accuracies.isEmpty) return 0;
    if (accuracies.length == 1) return accuracies.first;

    // Copy list to preserve original data
    var sorted = List<double>.from(accuracies)..sort();

    // Calculate median
    if (sorted.length % 2 == 0) {
      int midIndex = sorted.length ~/ 2;
      return (sorted[midIndex - 1] + sorted[midIndex]) / 2;
    } else {
      return sorted[sorted.length ~/ 2];
    }
  }

  /// Start position tracking
  /// - Initializes high-accuracy GPS tracking
  /// - Sets up periodic position updates
  /// - Shows tracking notification
  Future<void> startTracking(BuildContext context) async {
    if (isTracking) return;
    
    // Configure high-accuracy GPS settings
    final LocationSettings locationSettings =
        LocationSettings(accuracy: LocationAccuracy.high);

    _lastContext = context;
    
    // Initialize and show tracking notification
    await NotificationService().initNotification();
    if (context.mounted) {
      NotificationService().showNotification(
        title: AppLocalizations.of(context)!.trackingRunningInBackground,
      );
    }

    // Load tracking interval from settings
    final prefs = await SharedPreferences.getInstance();
    final intervalSeconds = prefs.getInt('trackingInterval') ?? 60;

    // Set up periodic position updates
    _positionTimer =
        Timer.periodic(Duration(seconds: intervalSeconds), (_) async {
      try {
        final Position position = await Geolocator.getCurrentPosition(
            locationSettings: locationSettings);

        // Store position and accuracy data
        pathList.add(LatLng(position.latitude, position.longitude));
        accuracyList.add(position.accuracy);
        currentAccuracy = position.accuracy;
        _positionController.add(position);
        _updateStats();
      } catch (e) {
        // Handle errors with notification
        NotificationService().deleteNotification();
        NotificationService().showNotification(title: "ERROR: $e");
      }
    });

    // Get initial position immediately
    try {
      final Position position = await Geolocator.getCurrentPosition(
          locationSettings: locationSettings);

      pathList.add(LatLng(position.latitude, position.longitude));
      accuracyList.add(position.accuracy);
      currentAccuracy = position.accuracy;
      _positionController.add(position);
      _updateStats();
    } catch (e) {
      NotificationService().deleteNotification();
      NotificationService().showNotification(title: "ERROR: $e");
    }

    isTracking = true;
  }

  // Last calculated tracking statistics
  TrackingStats? _lastStats;
  TrackingStats? get currentStats => _lastStats;

  /// Update tracking statistics
  /// Calculates:
  /// - Current GPS accuracy
  /// - Average accuracy
  /// - Total distance traveled
  void _updateStats() {
    if (pathList.isEmpty) {
      _lastStats = TrackingStats(
          currentAccuracy: currentAccuracy ?? 0,
          averageAccuracy: 0,
          totalDistanceMeters: 0);
      _statsController.add(_lastStats!);
      return;
    }

    // Calculate total distance
    double totalDistance = 0;
    for (int i = 1; i < pathList.length; i++) {
      totalDistance += _calculateDistance(
          pathList[i - 1].latitude,
          pathList[i - 1].longitude,
          pathList[i].latitude,
          pathList[i].longitude);
    }

    double medianAccuracy = _calculateMedianAccuracy(accuracyList);

    _lastStats = TrackingStats(
        currentAccuracy: currentAccuracy ?? 0,
        averageAccuracy: medianAccuracy,
        totalDistanceMeters: totalDistance);
    _statsController.add(_lastStats!);
  }

  /// Request a manual stats update
  void requestStatsUpdate() {
    _updateStats();
  }

  /// Calculate distance between two GPS coordinates using the Haversine formula
  /// @return Distance in meters
  double _calculateDistance(
      double lat1, double lon1, double lat2, double lon2) {
    const double earthRadius = 6371000; // Earth radius in meters

    // Convert coordinates to radians
    double lat1Rad = lat1 * math.pi / 180;
    double lat2Rad = lat2 * math.pi / 180;
    double deltaLat = (lat2 - lat1) * math.pi / 180;
    double deltaLon = (lon2 - lon1) * math.pi / 180;

    // Haversine formula calculation
    double a = math.sin(deltaLat / 2) * math.sin(deltaLat / 2) +
        math.cos(lat1Rad) *
            math.cos(lat2Rad) *
            math.sin(deltaLon / 2) *
            math.sin(deltaLon / 2);

    double c = 2 * math.atan2(math.sqrt(a), math.sqrt(1 - a));
    return earthRadius * c;
  }

  /// Temporarily pause tracking
  void pauseTracking() {
    _positionTimer?.cancel();
    isTracking = false;
  }

  /// Resume paused tracking
  void resumeTracking() {
    if (!isTracking && _lastContext != null) {
      startTracking(_lastContext!);
    }
    isTracking = true;
  }

  /// Stop tracking completely and clear current state
  void stopTracking() {
    _positionTimer?.cancel();
    NotificationService().deleteNotification();
    isTracking = false;
    accuracyList.clear();
    currentAccuracy = null;
    _lastContext = null;
  }

  /// Clear all recorded position data
  void clearPath() {
    pathList.clear();
    accuracyList.clear();
    currentAccuracy = null;
    _updateStats();
  }

  /// Convert tracked path to string format
  /// Format: "latitude,longitude;latitude,longitude;..."
  String getPathAsString() {
    return pathList.map((pos) => "${pos.latitude},${pos.longitude}").join(";");
  }

  /// Clean up resources
  void dispose() {
    stopTracking();
    _positionController.close();
    _statsController.close();
  }
}

/// Data class for tracking statistics
/// Contains:
/// - Current GPS accuracy
/// - Average accuracy
/// - Total distance in meters
class TrackingStats {
  final double currentAccuracy;
  final double averageAccuracy;
  final double totalDistanceMeters;

  TrackingStats({
    required this.currentAccuracy,
    required this.averageAccuracy,
    required this.totalDistanceMeters,
  });
}