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 '../api/marianumconnect/auth/token_storage.dart';
import '../push/push_secure_storage.dart';
import '../utils/exponential_backoff.dart';
import '../utils/random_id.dart';
import 'account_codec.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.
///
/// Several accounts can be signed in; the frozen [SessionKeys] slots, the MC
/// token and the group-keychain app passwords always hold the *active* one
/// (native code and the background isolate read only those). Inactive
/// accounts are parked in a per-account vault entry.
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 const _indexKey = 'accounts_index';
  static String _vaultKey(String id) => 'account_vault_$id';
  static const _tokenStorage = MarianumConnectTokenStorage();

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

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

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

  Session? get current => _current;

  /// Every signed-in account and which one is active.
  final ValueNotifier<AccountIndex> accounts = ValueNotifier(
    AccountIndex.empty,
  );

  AccountEntry? get activeAccount => accounts.value.active;

  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());

  /// Makes [session] the active account, replacing the active slots
  /// completely. An account signed in before keeps its entry; call
  /// [stashActive] first so the previously active one stays switchable.
  Future<void> signIn(Session session) async {
    await _writeActive(session);
    await _saveIndex(
      accounts.value.activate(
        session,
        newId: randomHexId(bytes: 8),
        now: _now(),
      ),
    );
    if (!_loaded.isCompleted) _loaded.complete();
  }

  Future<void> _writeActive(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;
  }

  /// Wipes the active slots and forgets the active account. Other accounts
  /// stay in their vaults; see [activate].
  Future<AccountEntry?> signOut() async {
    final removed = activeAccount;
    _loaded = Completer();
    _current = null;
    await Future.wait([
      for (final field in _sessionFields) _secureStorage.delete(key: field),
      _writeGroupSecret(SessionKeys.appPassword, null),
      _writeGroupSecret(SessionKeys.appPasswordTalk, null),
      if (removed != null) _secureStorage.delete(key: _vaultKey(removed.id)),
    ]);
    if (removed != null) await _saveIndex(accounts.value.remove(removed.id));
    return removed;
  }

  /// Parks the active account (session, app passwords, MC token) in its vault
  /// so the slots can be taken over by another account.
  Future<void> stashActive() async {
    final session = _current;
    final entry = activeAccount;
    if (session == null || entry == null) return;
    final fields = {
      ...sessionVaultFields(session),
      ...await _tokenStorage.readAll(),
    };
    await _secureStorage.write(
      key: _vaultKey(entry.id),
      value: encodeVault(fields),
    );
  }

  /// Loads the vault of [id] into the active slots. The active account must
  /// have been stashed before, or its session is lost.
  Future<void> activate(String id) async {
    final fields = decodeVault(await _secureStorage.read(key: _vaultKey(id)));
    final session = decodeSession(fields);
    if (session == null) throw StateError('No stored session for account $id');
    await _writeActive(session);
    await _tokenStorage.writeAll(fields);
    await _saveIndex(accounts.value.select(id, now: _now()));
    if (!_loaded.isCompleted) _loaded.complete();
  }

  /// Remembers the real name of the active account for the account list.
  Future<void> setDisplayName(String displayName) async {
    final entry = activeAccount;
    if (entry == null || entry.displayName == displayName) return;
    await _saveIndex(accounts.value.rename(entry.id, displayName));
  }

  /// Session and MC token of an inactive account, e.g. to revoke them.
  Future<(Session?, Map<String, String>)> readVault(String id) async {
    final fields = decodeVault(await _secureStorage.read(key: _vaultKey(id)));
    return (decodeSession(fields), fields);
  }

  /// Drops an inactive account without touching the active slots.
  Future<void> forget(String id) async {
    await _secureStorage.delete(key: _vaultKey(id));
    await _saveIndex(accounts.value.remove(id));
  }

  Future<void> _saveIndex(AccountIndex index) async {
    accounts.value = index;
    await _secureStorage.write(key: _indexKey, value: index.encode());
  }

  static int _now() => DateTime.now().millisecondsSinceEpoch;

  /// 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);
    await _loadIndex();
    if (!_loaded.isCompleted) _loaded.complete();
  }

  Future<void> _loadIndex() async {
    var index = AccountIndex.decode(await _secureStorage.read(key: _indexKey));
    final session = _current;
    if (session != null) {
      if (index.active == null ||
          index.matching(session)?.id != index.activeId) {
        // First start after the update: the existing account keeps its data
        // in the un-namespaced storage.
        index = index.activate(
          session,
          newId: randomHexId(bytes: 8),
          namespace: index.accounts.isEmpty ? '' : null,
          now: _now(),
        );
        await _secureStorage.write(key: _indexKey, value: index.encode());
      }
      accounts.value = index;
      return;
    }
    accounts.value = index.remove(index.activeId ?? '');
    // Interrupted switch or sign-out: fall back to another stored account
    // instead of showing the login screen.
    final fallback = accounts.value.mostRecentExcept(null);
    if (fallback != null) await activate(fallback.id);
  }

  // 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);
    }
  }
}