import 'dart:convert';
import 'dart:io';
import 'dart:math';

import 'package:crypto/crypto.dart';
import 'package:drift_postgres/drift_postgres.dart';
import 'package:injectable/injectable.dart';
import 'package:mnemo_cards_backend/database/database.dart';
import 'package:mnemo_cards_common_backend/mnemo_cards_common_backend.dart';

/// Service for JWT token generation and verification
///
/// Generates access tokens (short-lived) and refresh tokens (long-lived)
@lazySingleton
class JwtService {
  // Read secrets from environment variables
  static final String _jwtSecret = Platform.environment['JWT_SECRET'] ?? 
      'dev-jwt-secret-change-me-in-production';
  static final String _jwtRefreshSecret = Platform.environment['JWT_REFRESH_SECRET'] ?? 
      'dev-refresh-secret-change-me-in-production';
  
  static const int _accessTokenExpirySeconds = 3600; // 1 hour
  static const int _refreshTokenExpirySeconds = 2592000; // 30 days

  final AppDatabase _db;

  JwtService(this._db);

  /// Generate access and refresh tokens for a user
  Future<JwtTokens> generateTokens(UserModel user) async {
    final now = DateTime.now();
    final jti = _generateJti();

    // Access token payload
    final accessTokenPayload = {
      'sub': user.id.toString(),
      'iat': now.millisecondsSinceEpoch ~/ 1000,
      'exp': (now.millisecondsSinceEpoch ~/ 1000) + _accessTokenExpirySeconds,
      'type': 'access',
      'userId': user.id,
    };

    // Refresh token payload
    final refreshTokenPayload = {
      'sub': user.id.toString(),
      'iat': now.millisecondsSinceEpoch ~/ 1000,
      'exp': (now.millisecondsSinceEpoch ~/ 1000) + _refreshTokenExpirySeconds,
      'type': 'refresh',
      'userId': user.id,
      'jti': jti, // JWT ID for token invalidation
    };

    // Create JWT tokens with proper HMAC-SHA256 signing
    final accessToken = _createSimpleJwt(accessTokenPayload);
    final refreshToken = _createSimpleJwt(refreshTokenPayload);

    // Store refresh token for invalidation
    final userId = user.id;
    if (userId == null) {
      throw Exception('Cannot create tokens for user without ID');
    }
    final expiresAt = now.add(Duration(seconds: _refreshTokenExpirySeconds));
    await _storeRefreshToken(jti, userId, now, expiresAt);

    return JwtTokens(
      accessToken: accessToken,
      refreshToken: refreshToken,
      expiresIn: _accessTokenExpirySeconds,
    );
  }

  // Store last verification failure reason for debugging
  String? lastVerificationError;

  /// Verify access token and return user ID
  String? verifyAccessToken(String token, {bool debug = false}) {
    lastVerificationError = null;
    try {
      final payload = _verifySimpleJwt(token, debug: debug);
      if (payload == null) {
        lastVerificationError = lastVerificationError ?? 'Signature verification failed';
        return null;
      }

      // Check if token is expired
      final exp = payload['exp'] as int?;
      if (exp != null) {
        final expiryTime = DateTime.fromMillisecondsSinceEpoch(exp * 1000);
        if (DateTime.now().isAfter(expiryTime)) {
          lastVerificationError = 'Token expired at $expiryTime';
          return null;
        }
      }

      // Check token type
      if (payload['type'] != 'access') {
        lastVerificationError = 'Wrong token type: ${payload['type']}';
        return null;
      }

      return payload['userId']?.toString();
    } catch (e) {
      lastVerificationError = 'Exception: $e';
      return null;
    }
  }

  /// Verify refresh token and return user ID
  Future<String?> verifyRefreshToken(String token) async {
    try {
      final payload = _verifySimpleJwt(token);
      if (payload == null) return null;

      // Check if token is expired
      final exp = payload['exp'] as int?;
      if (exp != null) {
        final expiryTime = DateTime.fromMillisecondsSinceEpoch(exp * 1000);
        if (DateTime.now().isAfter(expiryTime)) {
          return null;
        }
      }

      // Check token type
      if (payload['type'] != 'refresh') {
        return null;
      }

      // Check if token is blacklisted
      final jti = payload['jti'] as String?;
      if (jti != null) {
        final isBlacklisted = await _isTokenBlacklisted(jti);
        if (isBlacklisted) {
          return null;
        }
      }

      return payload['userId']?.toString();
    } catch (e) {
      return null;
    }
  }

  /// Create a simple JWT (base64 encoded JSON)
  /// Note: This is a simplified implementation
  /// For production, use a proper JWT library with proper signing
  String _createSimpleJwt(Map<String, dynamic> payload) {
    final header = {'alg': 'HS256', 'typ': 'JWT'};
    final headerB64 = base64UrlEncode(utf8.encode(jsonEncode(header)));
    final payloadB64 = base64UrlEncode(utf8.encode(jsonEncode(payload)));

    // Use different secrets for access and refresh tokens
    final isRefreshToken = payload['type'] == 'refresh';
    final secret = isRefreshToken ? _jwtRefreshSecret : _jwtSecret;

    // Create signature with HMAC-SHA256
    final signatureInput = '$headerB64.$payloadB64';
    final signature = _hmacSha256(utf8.encode(signatureInput), secret);
    final signatureB64 = base64UrlEncode(signature);

    return '$headerB64.$payloadB64.$signatureB64';
  }

  /// Verify a simple JWT
  Map<String, dynamic>? _verifySimpleJwt(String token, {bool debug = false}) {
    try {
      final parts = token.split('.');
      if (parts.length != 3) {
        lastVerificationError = 'Invalid token format: ${parts.length} parts';
        return null;
      }

      final headerB64 = parts[0];
      final payloadB64 = parts[1];
      final signatureB64 = parts[2];

      // Decode payload first to determine token type
      final payloadJson = utf8.decode(base64Url.decode(payloadB64));
      final payload = jsonDecode(payloadJson) as Map<String, dynamic>;

      // Use different secrets for access and refresh tokens
      final isRefreshToken = payload['type'] == 'refresh';
      final secret = isRefreshToken ? _jwtRefreshSecret : _jwtSecret;

      // Verify signature with appropriate secret
      final signatureInput = '$headerB64.$payloadB64';
      final expectedSignature = _hmacSha256(utf8.encode(signatureInput), secret);
      final expectedSignatureB64 = base64UrlEncode(expectedSignature);

      if (signatureB64 != expectedSignatureB64) {
        lastVerificationError = 'Signature mismatch: got ${signatureB64.substring(0, 10)}..., '
            'expected ${expectedSignatureB64.substring(0, 10)}...';
        return null; // Invalid signature
      }

      return payload;
    } catch (e) {
      lastVerificationError = 'Parse exception: $e';
      return null;
    }
  }

  /// Extract payload from JWT without verification (for extracting jti during logout)
  Map<String, dynamic>? extractPayload(String token) {
    try {
      final parts = token.split('.');
      if (parts.length != 3) return null;

      final payloadB64 = parts[1];
      final payloadJson = utf8.decode(base64Url.decode(payloadB64));
      return jsonDecode(payloadJson) as Map<String, dynamic>;
    } catch (e) {
      return null;
    }
  }

  /// HMAC-SHA256 implementation using crypto package
  List<int> _hmacSha256(List<int> data, String key) {
    final keyBytes = utf8.encode(key);
    final hmac = Hmac(sha256, keyBytes);
    final digest = hmac.convert(data);
    return digest.bytes;
  }

  String _generateJti() {
    final random = Random();
    return base64UrlEncode(
      List<int>.generate(16, (_) => random.nextInt(256)),
    );
  }

  Future<void> _storeRefreshToken(
    String jti,
    String userId,
    DateTime createdAt,
    DateTime expiresAt,
  ) async {
    await _db.transaction(() async {
      // Delete old token if exists (shouldn't happen due to unique index, but be safe)
      final existing = await _db.userDao.getRefreshTokenByJti(jti);
      if (existing != null) {
        await _db.userDao.revokeRefreshToken(existing.id);
      }

      // Store new token
      await _db.userDao.createRefreshToken(
        RefreshTokensCompanion.insert(
          jti: jti,
          userId: userId,
          expiresAt: PgDateTime(expiresAt), // required field - raw DateTime
          // createdAt and isBlacklisted use defaults from table
        ),
      );
    });
  }

  Future<bool> _isTokenBlacklisted(String jti) async {
    final token = await _db.userDao.getRefreshTokenByJti(jti);

    if (token == null) {
      return true; // Token not found, consider it invalid
    }

    // Check if expired
    if (token.expiresAt.dateTime.isBefore(DateTime.now())) {
      return true; // Expired tokens are considered invalid
    }

    return token.isBlacklisted;
  }

  /// Blacklist a refresh token (for logout)
  Future<void> blacklistRefreshToken(String jti) async {
    await _db.userDao.revokeRefreshTokenByJti(jti);
  }

  /// Clean up expired tokens (should be called periodically)
  Future<void> cleanupExpiredTokens() async {
    await _db.userDao.deleteExpiredRefreshTokens();
  }
}

/// JWT tokens result
class JwtTokens {
  final String accessToken;
  final String refreshToken;
  final int expiresIn; // seconds

  JwtTokens({
    required this.accessToken,
    required this.refreshToken,
    required this.expiresIn,
  });
}