import 'dart:async';
import 'dart:developer';
import 'dart:io';

import 'package:flutter/foundation.dart';
import 'package:flutter_secure_storage/flutter_secure_storage.dart';
import 'package:shared_preferences/shared_preferences.dart';

import '../push/push_secure_storage.dart';
import '../utils/exponential_backoff.dart';
import 'nextcloud_credentials.dart';
import 'session.dart';
import 'session_codec.dart';

/// Owns the active [Session] and its persistence. One instance per isolate;
/// the widget background isolate reads the same keychain.
class SessionManager {
  // `first_unlock` so a background launch on a locked device (silent push,
  // BGAppRefresh) can still read the session. Items written by older versions
  // carry the plugin default `unlocked` and are invisible to this instance
  // until _migrateKeychainAccessibility moved them over.
  static const FlutterSecureStorage _secureStorage = FlutterSecureStorage(
    iOptions: IOSOptions(accessibility: KeychainAccessibility.first_unlock),
  );
  static const FlutterSecureStorage _legacySecureStorage = FlutterSecureStorage(
    iOptions: IOSOptions(accessibility: KeychainAccessibility.unlocked),
  );
  static const List<String> _sessionFields = [
    SessionKeys.kind,
    SessionKeys.username,
    SessionKeys.password,
    SessionKeys.guardianEmail,
    SessionKeys.demo,
    SessionKeys.loginFlow,
  ];

  static final SessionManager _instance = SessionManager._();
  factory SessionManager() => _instance;

  SessionManager._() {
    unawaited(_loadWithRetry());
  }

  Completer<void> _loaded = Completer();
  Session? _current;

  Session? get current => _current;

  bool get isSignedIn => _current != null;

  bool get isDemo => _current?.isDemo ?? false;

  /// Whether the session has a Nextcloud identity (Talk, Files, NC push).
  bool get hasNextcloud => _current?.nextcloud != null;

  /// True once the stored session has been read (or given up on).
  bool get isLoaded => _loaded.isCompleted;

  /// Resolves once the stored session is known. After [signOut] it stays
  /// pending until the next sign-in.
  Future<Session?> waitForLoad() async {
    await _loaded.future;
    return _current;
  }

  /// Stops waiting for the stored session; the app then behaves as signed
  /// out. The keychain entries stay untouched so a later start can still
  /// restore the session.
  void abandonLoad() {
    if (!_loaded.isCompleted) _loaded.complete();
  }

  /// Bumped when the server rejects a token that cannot be renewed silently
  /// (passwordless accounts). The app confirms via [SessionValidator] before
  /// signing out, so a transient 401 does not cost the session.
  final ValueNotifier<int> unauthorizedSignal = ValueNotifier(0);

  void reportUnauthorized() => unauthorizedSignal.value++;

  NextcloudCredentials requireNextcloud() =>
      _current?.nextcloud ?? (throw const NextcloudUnavailableException());

  /// Replaces any stored session completely; no prior [signOut] needed.
  Future<void> signIn(Session session) async {
    await Future.wait([
      for (final MapEntry(:key, :value) in encodeSessionFields(session).entries)
        _writeSecret(key, value),
      _writeGroupSecret(
        SessionKeys.appPassword,
        session.nextcloud?.appPassword,
      ),
      _writeGroupSecret(
        SessionKeys.appPasswordTalk,
        session.nextcloud?.appPasswordTalk,
      ),
    ]);
    _current = session;
    if (!_loaded.isCompleted) _loaded.complete();
  }

  Future<void> signOut() async {
    _loaded = Completer();
    _current = null;
    await Future.wait([
      for (final field in _sessionFields) _secureStorage.delete(key: field),
      _writeGroupSecret(SessionKeys.appPassword, null),
      _writeGroupSecret(SessionKeys.appPasswordTalk, null),
    ]);
  }

  /// Persists a freshly minted Nextcloud app password; from then on every
  /// Nextcloud call authenticates with it instead of the real password.
  Future<void> setAppPassword(String appPassword) async {
    _updateNextcloud((nc) => nc.copyWith(appPassword: () => appPassword));
    await _writeGroupSecret(SessionKeys.appPassword, appPassword);
  }

  Future<void> clearAppPassword() async {
    _updateNextcloud((nc) => nc.copyWith(appPassword: () => null));
    await _writeGroupSecret(SessionKeys.appPassword, null);
  }

  /// Adopts an app password obtained via Login Flow v2 and switches the
  /// account into flow mode. A previously stored Talk app password belonged
  /// to the old session era and is dropped — the second (optional) flow pass
  /// stores a fresh one via [setAppPasswordTalk].
  Future<void> setLoginFlow(String appPassword) async {
    await setAppPassword(appPassword);
    await clearAppPasswordTalk();
    _updateNextcloud((nc) => nc.copyWith(usesLoginFlow: true));
    await _secureStorage.write(key: SessionKeys.loginFlow, value: 'true');
  }

  Future<void> setAppPasswordTalk(String appPassword) async {
    _updateNextcloud((nc) => nc.copyWith(appPasswordTalk: () => appPassword));
    await _writeGroupSecret(SessionKeys.appPasswordTalk, appPassword);
  }

  Future<void> clearAppPasswordTalk() async {
    _updateNextcloud((nc) => nc.copyWith(appPasswordTalk: () => null));
    await _writeGroupSecret(SessionKeys.appPasswordTalk, null);
  }

  void _updateNextcloud(
    NextcloudCredentials Function(NextcloudCredentials) update,
  ) {
    final session = _current;
    if (session is CredentialSession) {
      _current = session.withNextcloud(update(session.nextcloud));
    }
  }

  Future<void> _writeSecret(String key, String? value) => value == null
      ? _secureStorage.delete(key: key)
      : _secureStorage.write(key: key, value: value);

  // App passwords live in the push-shared (group-scoped) keystore so the iOS
  // Notification Service Extension can authenticate Nextcloud calls too. That
  // keystore may be unavailable (entitlement not provisioned); the in-memory
  // copy still serves this session.
  Future<void> _writeGroupSecret(String key, String? value) async {
    try {
      if (value == null) {
        await pushSecureStorage.delete(key: key);
      } else {
        await pushSecureStorage.write(key: key, value: value);
      }
    } on Object {
      // ignore — see above
    }
  }

  /// iOS keychain reads fail while protected data is unavailable (app launch
  /// racing the unlock, background wake on a locked device). Without a retry
  /// the completer never resolved and the app stayed on the launch screen.
  Future<void> _loadWithRetry() async {
    for (var attempt = 1; !_loaded.isCompleted; attempt++) {
      try {
        await _migrateAndLoad();
        return;
      } catch (e, s) {
        log('Session load failed (attempt $attempt): $e', stackTrace: s);
        await Future<void>.delayed(exponentialBackoff(attempt));
      }
    }
  }

  Future<void> _migrateAndLoad() async {
    await _migrateFromLegacyStorage();
    await _migrateKeychainAccessibility();
    // On the startup critical path (and every background wake): read in
    // parallel instead of one keychain round-trip after the other.
    final values = await Future.wait(
      _sessionFields.map((field) => _secureStorage.read(key: field)),
    );
    final raw = Map<String, String?>.fromIterables(_sessionFields, values);
    try {
      final (appPassword, appPasswordTalk) = await (
        pushSecureStorage.read(key: SessionKeys.appPassword),
        pushSecureStorage.read(key: SessionKeys.appPasswordTalk),
      ).wait;
      raw[SessionKeys.appPassword] = appPassword;
      raw[SessionKeys.appPasswordTalk] = appPasswordTalk;
    } on Object {
      // Group keystore unavailable: fall back to the real password.
    }
    _current = decodeSession(raw);
    if (!_loaded.isCompleted) _loaded.complete();
  }

  // Move credentials from the old SharedPreferences plain-text storage into the
  // platform's secure keystore. Run once per install and clear the legacy keys.
  Future<void> _migrateFromLegacyStorage() async {
    final prefs = await SharedPreferences.getInstance();
    final legacyUsername = prefs.getString(SessionKeys.username);
    final legacyPassword = prefs.getString(SessionKeys.password);
    if (legacyUsername == null || legacyPassword == null) return;

    final hasSecure =
        (await _secureStorage.read(key: SessionKeys.username)) != null;
    if (!hasSecure) {
      await _secureStorage.write(
        key: SessionKeys.username,
        value: legacyUsername,
      );
      await _secureStorage.write(
        key: SessionKeys.password,
        value: legacyPassword,
      );
    }
    await prefs.remove(SessionKeys.username);
    await prefs.remove(SessionKeys.password);
  }

  Future<void> _migrateKeychainAccessibility() async {
    if (!Platform.isIOS) return;
    final legacyValues = await Future.wait(
      _sessionFields.map((field) => _legacySecureStorage.read(key: field)),
    );
    for (final (i, field) in _sessionFields.indexed) {
      final value = legacyValues[i];
      if (value == null) continue;
      // Same account+service: the legacy item has to go before the re-add.
      await _legacySecureStorage.delete(key: field);
      await _secureStorage.write(key: field, value: value);
    }
  }
}