import 'dart:async';

import 'package:connectivity_plus/connectivity_plus.dart';

import 'social_service.dart';
import 'social_settings.dart';

/// Opens a session against the given relays. Injectable so the connection logic
/// can be unit-tested without a real relay.
typedef SessionOpener = Future<SocialSession> Function(List<String> relays);

/// ONE shared relay connection per identity, reused by every social feature
/// (offers, messaging, trust, profile, the inbox listener) instead of each
/// opening its own `RelayPool`. Fewer sockets, less battery, one place to manage
/// reconnection.
///
/// Lazily connects on first [session] call; reconnects when the network returns
/// and drops the session when it goes away, announcing each change on [sessions]
/// so long-lived consumers (the inbox listener) can re-subscribe. Callers must
/// NOT close the session they get — this owns its lifecycle. Recreated on an
/// identity switch (the old one is disposed).
class SocialConnection {
  SocialConnection({
    required SocialService social,
    required SocialSettings settings,
    SessionOpener? open,
    Stream<bool>? online,
    List<Duration>? retrySchedule,
  })  : _settings = settings,
        _open = open ?? social.openSession,
        _online = online,
        _retrySchedule = retrySchedule ?? _defaultRetrySchedule;

  /// Backoff for self-retries after a failed attempt while started and not
  /// knowingly offline (the fresh-install case: online the whole time, first
  /// connect fails, so no connectivity change ever retriggers a connect).
  static const _defaultRetrySchedule = [
    Duration(seconds: 5),
    Duration(seconds: 15),
    Duration(seconds: 45),
    Duration(seconds: 90),
  ];

  final SocialSettings _settings;
  final SessionOpener _open;
  final Stream<bool>? _online;
  final List<Duration> _retrySchedule;

  final _sessions = StreamController<SocialSession?>.broadcast();
  SocialSession? _current;
  Future<SocialSession?>? _pending;
  StreamSubscription<bool>? _onlineSub;
  bool _disposed = false;
  bool _started = false;
  bool _knownOffline = false;
  Timer? _retryTimer;
  int _retryIndex = 0;

  /// Emits the live session on each (re)connect, and null when it drops.
  Stream<SocialSession?> get sessions => _sessions.stream;

  /// The current shared session, or null if not connected right now.
  SocialSession? get current => _current;

  /// Begins watching connectivity (reconnect on regain, drop when offline) and
  /// attempts an initial connect. Called at startup when sharing is already on,
  /// and again the moment someone joins the sharing side — hence the guard, so
  /// a second call never stacks a second connectivity subscription.
  void start() {
    if (_started || _disposed) return;
    _started = true;
    _onlineSub = (_online ?? _connectivityOnline()).listen((isOnline) {
      _knownOffline = !isOnline;
      if (!isOnline) {
        _cancelRetry();
        _drop();
      } else if (_current == null) {
        _retryIndex = 0; // fresh network — start the backoff over
        unawaited(session());
      }
    });
    unawaited(session()); // initial attempt (also connects if already online)
  }

  /// The shared session, connecting on first use. Returns null when offline or
  /// no relay is reachable; the feature then degrades gracefully.
  Future<SocialSession?> session() {
    if (_current != null) return Future.value(_current);
    return _pending ??= _connect();
  }

  Future<SocialSession?> _connect() async {
    try {
      final relays = await _settings.relayUrls();
      if (relays.isEmpty) return null;
      final s = await _open(relays);
      if (_disposed) {
        await s.close();
        return null;
      }
      _current = s;
      _retryIndex = 0;
      _cancelRetry();
      _sessions.add(s);
      return s;
    } catch (_) {
      _scheduleRetry(); // unreachable — retry with backoff (see below)
      return null;
    } finally {
      _pending = null;
    }
  }

  /// After a failed attempt, retries by itself while started and not knowingly
  /// offline — a connectivity change may never come if the device was online
  /// all along. Un-started connections (one-shot [session] callers, tests)
  /// never leave a timer behind.
  void _scheduleRetry() {
    if (!_started || _disposed || _knownOffline || _current != null) return;
    _cancelRetry();
    final i = _retryIndex < _retrySchedule.length
        ? _retryIndex
        : _retrySchedule.length - 1;
    _retryIndex++;
    _retryTimer = Timer(_retrySchedule[i], () {
      if (_disposed || _current != null) return;
      unawaited(session());
    });
  }

  void _cancelRetry() {
    _retryTimer?.cancel();
    _retryTimer = null;
  }

  void _drop() {
    final s = _current;
    _current = null;
    if (s != null) {
      unawaited(s.close());
      if (!_sessions.isClosed) _sessions.add(null);
    }
  }

  /// Goes offline for good until [start] is called again: stops watching
  /// connectivity, cancels any pending retry and tears the live session down.
  /// This is what "turn sharing off" must do — before it existed, clearing the
  /// server list only took effect on the next launch, because the already-open
  /// session was never closed. Unlike [dispose] the object stays usable.
  Future<void> stop() async {
    _started = false;
    _cancelRetry();
    await _onlineSub?.cancel();
    _onlineSub = null;
    _drop();
  }

  Future<void> dispose() async {
    _disposed = true;
    _cancelRetry();
    await _onlineSub?.cancel();
    _onlineSub = null;
    _drop();
    if (!_sessions.isClosed) await _sessions.close();
  }

  static Stream<bool> _connectivityOnline() =>
      Connectivity().onConnectivityChanged.map((results) =>
          results.any((r) => r != ConnectivityResult.none));
}