import 'dart:io';
import 'dart:convert';
import 'package:sqflite/sqflite.dart';
import 'package:path/path.dart' as path;
import 'package:path_provider/path_provider.dart';
import 'package:http/http.dart' as http;
import 'models/item.dart';

/// Service for local storage and caching operations.
/// 
/// This service provides:
/// - CRUD operations for items stored in a local SQLite database
/// - Image caching functionality that downloads and stores images locally
/// 
/// The service is designed to be modular and does not depend on UI components.
/// All methods are easily mockable for testing and integration with higher-level modules.
class LocalStorageService {
  /// Database instance (null until initialized).
  Database? _database;

  /// Path to the cache directory for images.
  Directory? _cacheDirectory;

  /// Map to track cache status (in-memory cache for faster lookups).
  final Map<String, bool> _cacheStatus = {};

  /// Optional database path for testing (null uses default).
  final String? _testDbPath;

  /// Optional cache directory path for testing (null uses default).
  final Directory? _testCacheDir;

  /// Creates a LocalStorageService instance.
  /// 
  /// [testDbPath] - Optional database path for testing.
  /// [testCacheDir] - Optional cache directory for testing.
  LocalStorageService({
    String? testDbPath,
    Directory? testCacheDir,
  })  : _testDbPath = testDbPath,
        _testCacheDir = testCacheDir;

  /// Initializes the database and cache directory.
  /// 
  /// Must be called before using any other methods.
  /// 
  /// Throws [Exception] if initialization fails.
  Future<void> initialize() async {
    try {
      // Initialize database
      final dbPath = _testDbPath ?? await _getDatabasePath();
      _database = await openDatabase(
        dbPath,
        version: 1,
        onCreate: _onCreate,
      );

      // Initialize cache directory
      if (_testCacheDir != null) {
        _cacheDirectory = _testCacheDir;
      } else {
        final appDir = await getApplicationDocumentsDirectory();
        _cacheDirectory = Directory(path.join(appDir.path, 'image_cache'));
      }
      
      if (!await _cacheDirectory!.exists()) {
        await _cacheDirectory!.create(recursive: true);
      }
    } catch (e) {
      throw Exception('Failed to initialize LocalStorageService: $e');
    }
  }

  /// Creates the database schema if it doesn't exist.
  Future<void> _onCreate(Database db, int version) async {
    await db.execute('''
      CREATE TABLE items (
        id TEXT PRIMARY KEY,
        data TEXT NOT NULL,
        created_at INTEGER NOT NULL,
        updated_at INTEGER NOT NULL
      )
    ''');
  }

  /// Converts Map to JSON string for database storage.
  String _dataToJson(Map<String, dynamic> data) {
    return jsonEncode(data);
  }

  /// Converts JSON string from database to Map.
  Map<String, dynamic> _jsonToData(String jsonString) {
    return jsonDecode(jsonString) as Map<String, dynamic>;
  }

  /// Gets the path to the database file.
  Future<String> _getDatabasePath() async {
    final documentsDirectory = await getApplicationDocumentsDirectory();
    return path.join(documentsDirectory.path, 'local_storage.db');
  }

  /// Ensures the database is initialized.
  /// 
  /// Throws [Exception] if database is not initialized.
  void _ensureInitialized() {
    if (_database == null) {
      throw Exception('LocalStorageService not initialized. Call initialize() first.');
    }
  }

  /// Inserts a new item into the database.
  /// 
  /// [item] - The item to insert.
  /// 
  /// Throws [Exception] if insertion fails or if item with same ID already exists.
  Future<void> insertItem(Item item) async {
    _ensureInitialized();
    try {
      final map = item.toMap();
      map['data'] = _dataToJson(map['data'] as Map<String, dynamic>);
      await _database!.insert(
        'items',
        map,
        conflictAlgorithm: ConflictAlgorithm.replace,
      );
    } catch (e) {
      throw Exception('Failed to insert item: $e');
    }
  }

  /// Retrieves an item by its ID.
  /// 
  /// [id] - The unique identifier of the item.
  /// 
  /// Returns the [Item] if found, null otherwise.
  /// Throws [Exception] if retrieval fails.
  Future<Item?> getItem(String id) async {
    _ensureInitialized();
    try {
      final List<Map<String, dynamic>> maps = await _database!.query(
        'items',
        where: 'id = ?',
        whereArgs: [id],
        limit: 1,
      );

      if (maps.isEmpty) {
        return null;
      }

      // Parse data from JSON string
      final row = maps.first;
      final dataMap = _jsonToData(row['data'] as String);
      
      return Item(
        id: row['id'] as String,
        data: dataMap,
        createdAt: row['created_at'] as int,
        updatedAt: row['updated_at'] as int,
      );
    } catch (e) {
      throw Exception('Failed to get item: $e');
    }
  }

  /// Retrieves all items from the database.
  /// 
  /// Returns a list of all [Item] instances, ordered by creation date (newest first).
  /// Returns empty list if no items exist.
  /// Throws [Exception] if retrieval fails.
  Future<List<Item>> getAllItems() async {
    _ensureInitialized();
    try {
      final List<Map<String, dynamic>> maps = await _database!.query(
        'items',
        orderBy: 'created_at DESC',
      );

      return maps.map((row) {
        final dataMap = _jsonToData(row['data'] as String);
        return Item(
          id: row['id'] as String,
          data: dataMap,
          createdAt: row['created_at'] as int,
          updatedAt: row['updated_at'] as int,
        );
      }).toList();
    } catch (e) {
      throw Exception('Failed to get all items: $e');
    }
  }

  /// Deletes an item by its ID.
  /// 
  /// [id] - The unique identifier of the item to delete.
  /// 
  /// Throws [Exception] if deletion fails.
  Future<void> deleteItem(String id) async {
    _ensureInitialized();
    try {
      final deleted = await _database!.delete(
        'items',
        where: 'id = ?',
        whereArgs: [id],
      );

      if (deleted == 0) {
        throw Exception('Item with id $id not found');
      }
    } catch (e) {
      throw Exception('Failed to delete item: $e');
    }
  }

  /// Updates an existing item in the database.
  /// 
  /// [item] - The item with updated data. The ID must exist in the database.
  /// 
  /// Throws [Exception] if update fails or if item doesn't exist.
  Future<void> updateItem(Item item) async {
    _ensureInitialized();
    try {
      // Update the updated_at timestamp
      final updatedItem = item.copyWith(
        updatedAt: DateTime.now().millisecondsSinceEpoch,
      );

      final map = updatedItem.toMap();
      map['data'] = _dataToJson(map['data'] as Map<String, dynamic>);
      
      final updated = await _database!.update(
        'items',
        map,
        where: 'id = ?',
        whereArgs: [item.id],
      );

      if (updated == 0) {
        throw Exception('Item with id ${item.id} not found');
      }
    } catch (e) {
      throw Exception('Failed to update item: $e');
    }
  }

  /// Gets a cached image file, downloading it if not available in cache.
  /// 
  /// [url] - The URL of the image to fetch.
  /// 
  /// Returns a [File] pointing to the cached image.
  /// 
  /// Behavior:
  /// - If image exists in cache, returns the cached file immediately.
  /// - If image is not cached, downloads it, stores in cache, and returns the file.
  /// 
  /// Throws [Exception] if download fails or cache directory is not initialized.
  Future<File> getCachedImage(String url) async {
    if (_cacheDirectory == null) {
      throw Exception('Cache directory not initialized. Call initialize() first.');
    }

    // Generate cache file name from URL (using hash to avoid invalid file names)
    final urlHash = url.hashCode.toString();
    final cacheFile = File(path.join(_cacheDirectory!.path, urlHash));

    // Check if file exists in cache
    if (await cacheFile.exists()) {
      _cacheStatus[url] = true;
      return cacheFile;
    }

    // Download and cache the image
    try {
      final response = await http.get(Uri.parse(url));
      
      if (response.statusCode != 200) {
        throw Exception('Failed to download image: HTTP ${response.statusCode}');
      }

      await cacheFile.writeAsBytes(response.bodyBytes);
      _cacheStatus[url] = true;
      
      return cacheFile;
    } catch (e) {
      throw Exception('Failed to cache image: $e');
    }
  }

  /// Clears all cached images.
  /// 
  /// Throws [Exception] if cache directory is not initialized.
  Future<void> clearImageCache() async {
    if (_cacheDirectory == null) {
      throw Exception('Cache directory not initialized. Call initialize() first.');
    }

    try {
      if (await _cacheDirectory!.exists()) {
        await for (final entity in _cacheDirectory!.list()) {
          if (entity is File) {
            await entity.delete();
          }
        }
      }
      _cacheStatus.clear();
    } catch (e) {
      throw Exception('Failed to clear image cache: $e');
    }
  }

  /// Closes the database connection.
  /// 
  /// Should be called when the service is no longer needed.
  Future<void> close() async {
    await _database?.close();
    _database = null;
  }
}