// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'dart:async';

import '../stream_channel.dart';

/// Allows the caller to force a channel to disconnect.
///
/// When [disconnect] is called, the channel (or channels) transformed by this
/// transformer will act as though the remote end had disconnected—the stream
/// will emit a done event, and the sink will ignore future inputs. The inner
/// sink will also be closed to notify the remote end of the disconnection.
///
/// If a channel is transformed after the [disconnect] has been called, it will
/// be disconnected immediately.
class Disconnector<T> implements StreamChannelTransformer<T, T> {
  /// Whether [disconnect] has been called.
  bool get isDisconnected => _isDisconnected;
  var _isDisconnected = false;

  /// The sinks for transformed channels.
  ///
  /// Note that we assume that transformed channels provide the stream channel
  /// guarantees. This allows us to only track sinks, because we know closing
  /// the underlying sink will cause the stream to emit a done event.
  final _sinks = <_DisconnectorSink<T>>[];

  /// Disconnects all channels that have been transformed.
  void disconnect() {
    _isDisconnected = true;
    for (var sink in _sinks) {
      sink._disconnect();
    }
    _sinks.clear();
  }

  StreamChannel<T> bind(StreamChannel<T> channel) {
    return channel.changeSink((innerSink) {
      var sink = new _DisconnectorSink<T>(innerSink);

      if (_isDisconnected) {
        sink._disconnect();
      } else {
        _sinks.add(sink);
      }

      return sink;
    });
  }
}

/// A sink wrapper that can force a disconnection.
class _DisconnectorSink<T> implements StreamSink<T> {
  /// The inner sink.
  final StreamSink<T> _inner;

  Future get done => _inner.done;

  /// Whether [Disconnector.disconnect] has been called.
  var _isDisconnected = false;

  /// Whether the user has called [close].
  var _closed = false;

  /// The subscription to the stream passed to [addStream], if a stream is
  /// currently being added.
  StreamSubscription<T> _addStreamSubscription;

  /// The completer for the future returned by [addStream], if a stream is
  /// currently being added.
  Completer _addStreamCompleter;

  /// Whether we're currently adding a stream with [addStream].
  bool get _inAddStream => _addStreamSubscription != null;

  _DisconnectorSink(this._inner);

  void add(T data) {
    if (_closed) throw new StateError("Cannot add event after closing.");
    if (_inAddStream) {
      throw new StateError("Cannot add event while adding stream.");
    }
    if (_isDisconnected) return;

    _inner.add(data);
  }

  void addError(error, [StackTrace stackTrace]) {
    if (_closed) throw new StateError("Cannot add event after closing.");
    if (_inAddStream) {
      throw new StateError("Cannot add event while adding stream.");
    }
    if (_isDisconnected) return;

    _inner.addError(error, stackTrace);
  }

  Future addStream(Stream<T> stream) {
    if (_closed) throw new StateError("Cannot add stream after closing.");
    if (_inAddStream) {
      throw new StateError("Cannot add stream while adding stream.");
    }
    if (_isDisconnected) return new Future.value();

    _addStreamCompleter = new Completer.sync();
    _addStreamSubscription = stream.listen(
        _inner.add,
        onError: _inner.addError,
        onDone: _addStreamCompleter.complete);
    return _addStreamCompleter.future.then((_) {
      _addStreamCompleter = null;
      _addStreamSubscription = null;
    });
  }

  Future close() {
    if (_inAddStream) {
      throw new StateError("Cannot close sink while adding stream.");
    }

    _closed = true;
    return _inner.close();
  }

  /// Disconnects this sink.
  ///
  /// This closes the underlying sink and stops forwarding events.
  void _disconnect() {
    _isDisconnected = true;
    _inner.close();

    if (!_inAddStream) return;
    _addStreamCompleter.complete(_addStreamSubscription.cancel());
    _addStreamCompleter = null;
    _addStreamSubscription = null;
  }
}