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

import 'package:crypto/crypto.dart';
import 'package:dio/dio.dart';

import '../config/app_config.dart';
import 'oidc_service.dart';

/// Web implementation of OIDC service using browser redirect flow.
///
/// Uses Authorization Code flow with PKCE for secure authentication.
/// On web, we can't use flutter_appauth, so we implement the flow manually
/// using browser redirects and URL parsing.
class OidcServiceWeb implements OidcService {
  OidcServiceWeb({Dio? dio}) : _dio = dio ?? Dio();

  final Dio _dio;

  /// OIDC scopes to request.
  static const _scopes = ['openid', 'profile', 'email', 'offline_access'];

  /// Redirect URI for web.
  static String get _redirectUri => '${AppConfig.webBaseUrl}/callback';

  // PKCE state stored during auth flow (in-memory for single-page app)
  static String? _codeVerifier;
  static String? _state;

  /// Get the authorization URL to redirect the browser to.
  ///
  /// Returns a URL that the browser should navigate to for authentication.
  /// The [codeVerifier] and [state] are stored for later verification.
  Future<String> getAuthorizationUrl() async {
    // Fetch OIDC discovery document
    final discovery = await _fetchDiscovery();
    final authEndpoint = discovery['authorization_endpoint'] as String;

    // Generate PKCE code verifier and challenge
    _codeVerifier = _generateCodeVerifier();
    final codeChallenge = _generateCodeChallenge(_codeVerifier!);

    // Generate state for CSRF protection
    _state = _generateRandomString(32);

    // Build authorization URL
    final params = {
      'client_id': AppConfig.authClientId,
      'redirect_uri': _redirectUri,
      'response_type': 'code',
      'scope': _scopes.join(' '),
      'code_challenge': codeChallenge,
      'code_challenge_method': 'S256',
      'state': _state,
    };

    final uri = Uri.parse(authEndpoint).replace(queryParameters: params);
    developer.log('Authorization URL: $uri', name: 'oidc_web');
    return uri.toString();
  }

  /// Exchange authorization code for tokens.
  ///
  /// Call this after the browser redirects back with the authorization code.
  /// [code] is the authorization code from the callback URL.
  /// [state] is the state parameter from the callback URL (verified for CSRF).
  Future<OidcTokens> exchangeCode(String code, String state) async {
    // Verify state matches
    if (_state == null || state != _state) {
      throw OidcException('State mismatch - possible CSRF attack');
    }

    if (_codeVerifier == null) {
      throw OidcException('No code verifier - flow not started properly');
    }

    try {
      // Fetch token endpoint from discovery
      final discovery = await _fetchDiscovery();
      final tokenEndpoint = discovery['token_endpoint'] as String;

      developer.log('Exchanging code at: $tokenEndpoint', name: 'oidc_web');

      // Exchange code for tokens
      final response = await _dio.post<Map<String, dynamic>>(
        tokenEndpoint,
        data: {
          'grant_type': 'authorization_code',
          'client_id': AppConfig.authClientId,
          'redirect_uri': _redirectUri,
          'code': code,
          'code_verifier': _codeVerifier,
        },
        options: Options(
          contentType: Headers.formUrlEncodedContentType,
        ),
      );

      final data = response.data!;
      developer.log('Token exchange successful', name: 'oidc_web');

      // Clear stored PKCE state
      _codeVerifier = null;
      _state = null;

      return OidcTokens(
        accessToken: data['access_token'] as String,
        refreshToken: data['refresh_token'] as String?,
        expiresAt: DateTime.now().add(
          Duration(seconds: data['expires_in'] as int? ?? 3600),
        ),
        idToken: data['id_token'] as String?,
      );
    } on DioException catch (e) {
      developer.log('Token exchange failed: $e', name: 'oidc_web');
      throw OidcException('Token exchange failed: ${e.message}');
    } finally {
      // Clear PKCE state on error too
      _codeVerifier = null;
      _state = null;
    }
  }

  /// Not used on web - use [getAuthorizationUrl] and [exchangeCode] instead.
  @override
  Future<OidcTokens> signIn() async {
    throw OidcException(
      'signIn() not supported on web. Use getAuthorizationUrl() and exchangeCode() instead.',
    );
  }

  /// Refresh the access token using a refresh token.
  @override
  Future<OidcTokens> refreshToken(String refreshToken) async {
    try {
      final discovery = await _fetchDiscovery();
      final tokenEndpoint = discovery['token_endpoint'] as String;

      final response = await _dio.post<Map<String, dynamic>>(
        tokenEndpoint,
        data: {
          'grant_type': 'refresh_token',
          'client_id': AppConfig.authClientId,
          'refresh_token': refreshToken,
        },
        options: Options(
          contentType: Headers.formUrlEncodedContentType,
        ),
      );

      final data = response.data!;

      return OidcTokens(
        accessToken: data['access_token'] as String,
        refreshToken: data['refresh_token'] as String? ?? refreshToken,
        expiresAt: DateTime.now().add(
          Duration(seconds: data['expires_in'] as int? ?? 3600),
        ),
        idToken: data['id_token'] as String?,
      );
    } on DioException catch (e) {
      throw OidcException('Token refresh failed: ${e.message}');
    }
  }

  /// Fetch OIDC discovery document.
  Future<Map<String, dynamic>> _fetchDiscovery() async {
    final response = await _dio.get<Map<String, dynamic>>(
      AppConfig.authDiscoveryUrl,
    );
    return response.data!;
  }

  /// Generate a random code verifier for PKCE.
  String _generateCodeVerifier() {
    return _generateRandomString(64);
  }

  /// Generate code challenge from verifier using S256.
  String _generateCodeChallenge(String verifier) {
    final bytes = utf8.encode(verifier);
    final digest = sha256.convert(bytes);
    return base64Url.encode(digest.bytes).replaceAll('=', '');
  }

  /// Generate a random string of given length.
  String _generateRandomString(int length) {
    const chars =
        'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-._~';
    final random = Random.secure();
    return List.generate(length, (_) => chars[random.nextInt(chars.length)])
        .join();
  }
}