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

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

import 'api_exception.dart';
import 'app_config.dart';
import 'session_store.dart';

/// One decoded response: the `data` payload plus the envelope's metadata.
class ApiResponse {
  const ApiResponse({
    this.data,
    this.etag,
    this.total,
    this.nextCursor,
    this.notModified = false,
    this.requestId,
  });

  /// The `data` member of the envelope. `null` on a 304 or a 204.
  final dynamic data;

  /// `ETag` from the response, to be replayed as `If-None-Match`.
  final String? etag;

  /// Total rows behind a filtered list, when the endpoint sends one.
  final int? total;

  /// Keyset cursor for the next page. Null on the last page.
  final String? nextCursor;

  /// True when the server answered `304` — the caller keeps what it has.
  final bool notModified;

  final String? requestId;

  Map<String, dynamic> get map =>
      data is Map<String, dynamic> ? data as Map<String, dynamic> : const {};

  /// The rows of a list endpoint.
  ///
  /// `data` is the list itself on most of this surface, but a paged endpoint
  /// wraps it — `data: { items: [...], nextCursor }` — and the contract does
  /// not say which of the two spellings a given route uses. Reading through the
  /// wrapper costs one lookup; guessing wrong shows the customer an empty
  /// Orders tab and no error at all, which is the failure that cannot be
  /// diagnosed from a screenshot.
  List<Map<String, dynamic>> get rows =>
      (_list ?? const []).whereType<Map<String, dynamic>>().toList();

  List<dynamic>? get _list {
    final payload = data;
    if (payload is List) return payload;
    if (payload is Map<String, dynamic>) {
      for (final key in _listKeys) {
        final value = payload[key];
        if (value is List) return value;
      }
    }
    return null;
  }

  static const List<String> _listKeys = [
    'items',
    'results',
    'rows',
    'data',
    'bookings',
    'orders',
    'states',
    'districts',
    'slots',
    'places',
    'locations',
  ];
}

/// The app's only HTTP caller.
///
/// Everything the contract asks for on the wire lives here and nowhere else:
/// the `data` envelope, the nine error codes, bearer auth with silent refresh,
/// the client identity headers, idempotency keys, `ETag` revalidation, keyset
/// paging and the retry policy. A screen that wanted to add a header would have
/// to come through this class, which is the point.
class ApiClient {
  ApiClient({
    http.Client? httpClient,
    SessionStore? sessions,
    this.onSessionLost,
  }) : _http = httpClient ?? http.Client(),
       _sessions = sessions ?? SecureSessionStore();

  final http.Client _http;
  final SessionStore _sessions;

  /// Called when a refresh fails and the session is dropped, so [AppState] can
  /// return the customer to sign-in rather than leaving them on a dead screen.
  /// Settable because the state that handles it is built after this client.
  void Function()? onSessionLost;

  SessionStore get sessions => _sessions;

  Session? _session;
  bool _restored = false;

  /// Single-flight refresh: ten calls hitting 401 together must produce one
  /// refresh, not ten — the token rotates, so the other nine would be replaying
  /// a token the server has already revoked and would kill the whole chain.
  Future<Session?>? _refreshing;

  final Random _rng = Random();

  // ------------------------------------------------------------------ session

  Future<Session?> currentSession() async {
    if (!_restored) {
      _session = await _sessions.read();
      _restored = true;
    }
    return _session;
  }

  Future<void> adoptSession(Session session) async {
    _session = session;
    _restored = true;
    await _sessions.write(session);
  }

  Future<void> dropSession() async {
    _session = null;
    _restored = true;
    await _sessions.clear();
  }

  // ------------------------------------------------------------------ requests

  Future<ApiResponse> get(
    String path, {
    Map<String, String>? query,
    String? ifNoneMatch,
    bool authenticated = true,
    Duration? timeout,
  }) => _send(
    'GET',
    path,
    query: query,
    ifNoneMatch: ifNoneMatch,
    authenticated: authenticated,
    timeout: timeout,
    // A GET carries no side effect, so replaying one is always safe.
    retries: 2,
  );

  Future<ApiResponse> post(
    String path, {
    Object? body,
    Map<String, String>? query,
    String? idempotencyKey,
    bool authenticated = true,
    Duration? timeout,
  }) => _send(
    'POST',
    path,
    body: body,
    query: query,
    idempotencyKey: idempotencyKey,
    authenticated: authenticated,
    timeout: timeout ?? AppConfig.writeTimeout,
  );

  Future<ApiResponse> patch(
    String path, {
    Object? body,
    bool authenticated = true,
  }) => _send('PATCH', path, body: body, authenticated: authenticated);

  Future<ApiResponse> put(
    String path, {
    Object? body,
    bool authenticated = true,
  }) => _send('PUT', path, body: body, authenticated: authenticated);

  Future<ApiResponse> delete(String path, {bool authenticated = true}) =>
      _send('DELETE', path, authenticated: authenticated);

  /// A fresh idempotency key. Held by the caller across retries of the *same*
  /// intent — a new key means a new booking.
  String newIdempotencyKey() {
    const alphabet = 'abcdefghijklmnopqrstuvwxyz0123456789';
    final now = DateTime.now().microsecondsSinceEpoch.toRadixString(36);
    final salt = List.generate(
      10,
      (_) => alphabet[_rng.nextInt(alphabet.length)],
    ).join();
    return '$now-$salt';
  }

  // ------------------------------------------------------------------ internals

  Future<ApiResponse> _send(
    String method,
    String path, {
    Object? body,
    Map<String, String>? query,
    String? ifNoneMatch,
    String? idempotencyKey,
    bool authenticated = true,
    Duration? timeout,
    int retries = 0,
    bool allowRefresh = true,
  }) async {
    final uri = Uri.parse(
      AppConfig.url(path),
    ).replace(queryParameters: (query?.isEmpty ?? true) ? null : query);

    ApiException? lastTransient;

    for (var attempt = 0; attempt <= retries; attempt++) {
      if (attempt > 0) {
        await Future<void>.delayed(Duration(milliseconds: 300 * attempt * attempt));
      }
      try {
        final response = await _once(
          method,
          uri,
          body: body,
          ifNoneMatch: ifNoneMatch,
          idempotencyKey: idempotencyKey,
          authenticated: authenticated,
          timeout: timeout,
        );
        return response;
      } on ApiException catch (e) {
        // 401 once, then refresh and try again — but never on the refresh call
        // itself, which would recurse.
        if (e.isAuthFailure && authenticated && allowRefresh) {
          final refreshed = await _refreshSession();
          if (refreshed != null) {
            return _send(
              method,
              path,
              body: body,
              query: query,
              ifNoneMatch: ifNoneMatch,
              idempotencyKey: idempotencyKey,
              authenticated: authenticated,
              timeout: timeout,
              retries: 0,
              allowRefresh: false,
            );
          }
          await dropSession();
          onSessionLost?.call();
          rethrow;
        }
        if (!e.isTransient || attempt == retries) rethrow;
        lastTransient = e;
      }
    }
    throw lastTransient ??
        ApiException(ApiException.network, 'We could not reach Doormile');
  }

  Future<ApiResponse> _once(
    String method,
    Uri uri, {
    Object? body,
    String? ifNoneMatch,
    String? idempotencyKey,
    required bool authenticated,
    Duration? timeout,
  }) async {
    final headers = <String, String>{
      'Accept': 'application/json',
      'X-Client': AppConfig.clientHeader,
      'X-Platform': AppConfig.platformHeader,
      if (body != null) 'Content-Type': 'application/json',
      'If-None-Match': ?ifNoneMatch,
      'Idempotency-Key': ?idempotencyKey,
    };

    if (authenticated) {
      final session = await currentSession();
      if (session != null) {
        // Refresh proactively rather than spending a 401 to discover it.
        final usable = session.isExpired
            ? (await _refreshSession() ?? session)
            : session;
        headers['Authorization'] = 'Bearer ${usable.accessToken}';
      }
    }

    final request = http.Request(method, uri)..headers.addAll(headers);
    if (body != null) request.body = jsonEncode(body);

    http.Response response;
    try {
      final streamed = await _http
          .send(request)
          .timeout(timeout ?? AppConfig.requestTimeout);
      response = await http.Response.fromStream(streamed);
    } on TimeoutException {
      throw ApiException(
        ApiException.network,
        'Doormile took too long to answer',
      );
    } on SocketException {
      throw ApiException(ApiException.network, 'We could not reach Doormile');
    } on http.ClientException {
      throw ApiException(ApiException.network, 'We could not reach Doormile');
    }

    return _decode(response, uri);
  }

  ApiResponse _decode(http.Response response, Uri uri) {
    final requestId = response.headers['x-request-id'];
    final status = response.statusCode;

    if (status == 304) {
      return ApiResponse(
        notModified: true,
        etag: response.headers['etag'],
        requestId: requestId,
      );
    }

    dynamic decoded;
    if (response.body.isNotEmpty) {
      try {
        decoded = jsonDecode(response.body);
      } catch (_) {
        // An HTML error page or a truncated body. The contract forbids both,
        // so treat it as the server being unreachable rather than guessing.
        _log(uri, status, requestId, 'unparseable body');
        throw ApiException(
          status >= 500 ? ApiException.serverError : ApiException.network,
          'Something went wrong',
          status,
          requestId,
        );
      }
    }

    final envelope = decoded is Map<String, dynamic> ? decoded : const {};

    if (status >= 200 && status < 300) {
      // `success: false` with a 200 would be a contract violation, but reading
      // it costs nothing and beats rendering a failure as data.
      if (envelope['success'] == false) {
        throw _errorFrom(envelope, status, requestId, uri);
      }
      final payload = envelope.containsKey('data') ? envelope['data'] : decoded;
      // Keyset metadata sits beside `data` on some routes and inside it on the
      // paged ones. Both are read, envelope first.
      final inner = payload is Map<String, dynamic>
          ? payload
          : const <String, dynamic>{};
      return ApiResponse(
        data: payload,
        etag: response.headers['etag'],
        total: (envelope['total'] ?? inner['total']) is num
            ? ((envelope['total'] ?? inner['total']) as num).toInt()
            : null,
        nextCursor:
            (envelope['nextCursor'] ?? inner['nextCursor']) as String?,
        requestId: requestId,
      );
    }

    throw _errorFrom(envelope, status, requestId, uri);
  }

  ApiException _errorFrom(
    Map<dynamic, dynamic> envelope,
    int status,
    String? requestId,
    Uri uri,
  ) {
    final error = envelope['error'];
    final serverCode = (error is Map && error['code'] is String)
        ? error['code'] as String
        : '';
    final code = ApiException.normalise(
      serverCode,
      () => _codeForStatus(status),
    );
    final message = envelope['message'] is String &&
            (envelope['message'] as String).trim().isNotEmpty
        ? envelope['message'] as String
        : (error is Map && error['message'] is String
              ? error['message'] as String
              : _messageForStatus(status));
    _log(uri, status, requestId, serverCode.isEmpty ? code : serverCode);
    return ApiException(code, message, status, requestId, serverCode);
  }

  static String _codeForStatus(int status) => switch (status) {
    400 => ApiException.invalid,
    401 => ApiException.unauthorized,
    403 => ApiException.forbidden,
    404 => ApiException.notFound,
    409 => ApiException.conflict,
    422 => ApiException.unserviceable,
    429 => ApiException.rateLimited,
    _ => status >= 500 ? ApiException.serverError : ApiException.invalid,
  };

  static String _messageForStatus(int status) => switch (status) {
    401 => 'Please sign in again',
    403 => 'You do not have access to this',
    404 => 'We could not find that',
    409 => 'That is no longer available',
    429 => 'Too many attempts. Try again in a minute',
    _ => 'Something went wrong',
  };

  void _log(Uri uri, int status, String? requestId, String detail) {
    debugPrint(
      '[API] $status ${uri.path} · $detail'
      '${requestId == null ? '' : ' · req $requestId'}',
    );
  }

  // ------------------------------------------------------------------ refresh

  /// Rotates the session. Returns null when the refresh itself failed, which
  /// means the chain is dead and the customer must sign in again.
  Future<Session?> _refreshSession() {
    final inFlight = _refreshing;
    if (inFlight != null) return inFlight;

    final future = _doRefresh();
    _refreshing = future;
    return future.whenComplete(() => _refreshing = null);
  }

  Future<Session?> _doRefresh() async {
    final session = await currentSession();
    if (session == null) return null;
    try {
      final response = await _send(
        'POST',
        '/auth/refresh',
        body: {'refreshToken': session.refreshToken},
        authenticated: false,
        allowRefresh: false,
      );
      final data = response.map;
      final access = data['accessToken'] as String?;
      if (access == null || access.isEmpty) return null;
      final rotated = Session(
        accessToken: access,
        // A server that does not rotate returns no new refresh token; keeping
        // the old one is then correct. If it does rotate, this is the only
        // copy that still works, so it is persisted before anything uses it.
        refreshToken: (data['refreshToken'] as String?) ?? session.refreshToken,
        expiresAt: DateTime.now().add(
          Duration(seconds: (data['expiresIn'] as num?)?.toInt() ?? 3600),
        ),
        customer: session.customer,
      );
      await adoptSession(rotated);
      return rotated;
    } on ApiException catch (e) {
      debugPrint('[API] refresh failed: $e');
      return null;
    }
  }

  void close() => _http.close();
}