import 'dart:async';
import 'dart:convert';

import 'package:http/http.dart' as http;

import '../../domain/entities/pos_session.dart';

/// Raised when the back office refuses or cannot answer a sign-in.
///
/// Carries a message meant to be shown to whoever is standing at the till, so
/// it is written for them rather than for a log: what happened, and what they
/// can do about it.
class PosAuthException implements Exception {
  const PosAuthException(this.message, {this.isCredentialFailure = false});

  final String message;

  /// Whether the details were wrong, as opposed to the back office being
  /// unreachable. The till reacts differently: a bad password is worth
  /// re-typing, an unreachable server is worth waiting for.
  final bool isCredentialFailure;

  @override
  String toString() => message;
}

/// Signs a terminal in against the back office.
///
/// Talks to the same `app_users` accounts as the web console, so a manager who
/// can open the back office can open the till with the same details — one
/// account store means deactivating a leaver closes both doors at once.
///
/// ```
/// POST {base}/login
/// { "authname": "…", "password": "…", "terminal_id": "T5EDD" }
/// ```
///
/// answered with `{ code, status, details: { token, store_id, locations, … } }`.
class PosAuthApi {
  PosAuthApi({required this.baseUrl, http.Client? client})
      : _client = client ?? http.Client();

  final String baseUrl;
  final http.Client _client;

  /// Generous, because this runs on a shop's connection while somebody watches.
  /// Short enough that a dead endpoint is reported rather than hung on.
  static const _timeout = Duration(seconds: 20);

  /// Exchanges credentials for a session.
  ///
  /// [locationId] is only meaningful for an account entitled to several
  /// outlets: it says which one this terminal is standing in. It is a request,
  /// not an assertion — the back office checks it against what the account may
  /// actually reach, and that check is the whole point of the endpoint.
  Future<PosSession> login({
    required String authname,
    required String password,
    String? terminalId,
    String? deviceId,
    int? locationId,
    int? configId,
  }) async {
    if (baseUrl.isEmpty) {
      throw const PosAuthException(
        'This terminal has no back office configured. Set the endpoint in '
        'Settings → Connectivity & sync.',
      );
    }

    final body = <String, Object?>{
      'authname': authname.trim(),
      'password': password,
      if (terminalId != null && terminalId.isNotEmpty) 'terminal_id': terminalId,
      if (deviceId != null && deviceId.isNotEmpty) 'device_id': deviceId,
      if (locationId != null && locationId > 0) 'location_id': locationId,
      // Sent only when known. The backend infers it when absent, and a shop
      // has no way to find out what its configid is.
      if (configId != null && configId > 0) 'configid': configId,
    };

    final http.Response response;
    try {
      response = await _client
          .post(
            Uri.parse('$baseUrl/login'),
            headers: const {'Content-Type': 'application/json'},
            body: jsonEncode(body),
          )
          .timeout(_timeout);
    } on TimeoutException {
      throw const PosAuthException(
        'The back office did not answer in time. Check the connection and try '
        'again.',
      );
    } on Object {
      throw const PosAuthException(
        'Could not reach the back office. Check the connection and try again.',
      );
    }

    Map<String, Object?> decoded;
    try {
      decoded = jsonDecode(response.body) as Map<String, Object?>;
    } on Object {
      throw PosAuthException(
        'The back office answered with something this terminal could not read '
        '(HTTP ${response.statusCode}).',
      );
    }

    if (response.statusCode != 200) {
      throw PosAuthException(
        (decoded['message'] as String?) ??
            'Sign-in was refused (HTTP ${response.statusCode}).',
        // 401 is a wrong email or password; 403 is a real account that may not
        // open this till. Only the first is worth re-typing.
        isCredentialFailure: response.statusCode == 401,
      );
    }

    final details = decoded['details'];
    if (details is! Map<String, Object?>) {
      throw const PosAuthException(
        'The back office accepted the sign-in but returned no session.',
      );
    }

    final session = PosSession.fromJson(details);

    // A session with no token cannot authenticate anything, and one with no
    // outlet cannot bill. Refused here rather than being saved and failing
    // later against every request, which would be much harder to diagnose.
    if (session.token.isEmpty) {
      throw const PosAuthException(
        'The back office returned a session with no token.',
      );
    }
    if (session.locationId <= 0) {
      throw const PosAuthException(
        'This account is not attached to an outlet, so it cannot open a till.',
      );
    }

    return session;
  }

  void dispose() => _client.close();
}