import 'dart:async';

class TodoRepository {
  TodoRepository({
    required DatabaseService databaseService,
    required ApiClientService apiClientService,
  })  : _databaseService = databaseService,
        _apiClientService = apiClientService;

  final DatabaseService _databaseService;
  final ApiClientService _apiClientService;

  /// Yields local data immediately, then fetches remote data, updates local, and yields fresh data.
  Stream<List<Todo>> observeTodos() async* {
    // 1. Yield local cached data first
    final localTodos = await _databaseService.getAllTodos();
    if (localTodos.isNotEmpty) {
      yield localTodos.map((model) => Todo.fromDbModel(model)).toList();
    }

    try {
      // 2. Fetch fresh data from API
      final remoteTodos = await _apiClientService.fetchTodos();
      
      // 3. Update local database
      await _databaseService.replaceAllTodos(remoteTodos);
      
      // 4. Yield fresh data
      yield remoteTodos.map((model) => Todo.fromApiModel(model)).toList();
    } on Exception catch (e) {
      // Handle network errors (UI will still have local data)
      // Log error or yield a specific error state if required
    }
  }

  /// Offline-first write: Save locally, then attempt remote sync.
  Future<void> createTodo(Todo todo) async {
    final dbModel = todo.toDbModel().copyWith(isSynced: false);
    
    // 1. Save locally immediately
    await _databaseService.insertTodo(dbModel);

    try {
      // 2. Attempt remote sync
      final apiModel = await _apiClientService.postTodo(todo.toApiModel());
      
      // 3. Mark as synced locally
      await _databaseService.updateTodo(
        dbModel.copyWith(id: apiModel.id, isSynced: true)
      );
    } on Exception catch (_) {
      // Leave as isSynced: false for background sync task to pick up later
    }
  }
}

class DatabaseService {
  static const String _tableName = 'todos';
  static const String _colId = 'id';
  static const String _colTask = 'task';
  static const String _colIsSynced = 'is_synced';

  Database? _database;

  Future<void> open() async {
    if (_database != null) return;
    
    final dbPath = join(await getDatabasesPath(), 'app_database.db');
    _database = await openDatabase(
      dbPath,
      version: 1,
      onCreate: (db, version) {
        return db.execute(
          'CREATE TABLE $_tableName('
          '$_colId INTEGER PRIMARY KEY AUTOINCREMENT, '
          '$_colTask TEXT, '
          '$_colIsSynced INTEGER)'
        );
      },
    );
  }

  Future<void> updateTodo(TodoDbModel todo) async {
    await _database!.update(
      _tableName,
      todo.toMap(),
      where: '$_colId = ?',
      whereArgs: [todo.id], // Prevents SQL injection
    );
  }
}