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    //! A multi-producer, single-consumer, futures-aware, FIFO queue with back
    //! pressure, for use communicating between tasks on the same thread.
    //!
    //! These queues are the same as those in `futures::sync`, except they're not
    //! intended to be sent across threads.

    use std::any::Any;
    use std::cell::RefCell;
    use std::collections::VecDeque;
    use std::error::Error;
    use std::fmt;
    use std::mem;
    use std::rc::{Rc, Weak};

    use task::{self, Task};
    use future::Executor;
    use sink::SendAll;
    use resultstream::{self, Results};
    use unsync::oneshot;
    use {Async, AsyncSink, Future, Poll, StartSend, Sink, Stream};

    /// Creates a bounded in-memory channel with buffered storage.
    ///
    /// This method creates concrete implementations of the `Stream` and `Sink`
    /// traits which can be used to communicate a stream of values between tasks
    /// with backpressure. The channel capacity is exactly `buffer`. On average,
    /// sending a message through this channel performs no dynamic allocation.
    pub fn channel<T>(buffer: usize) -> (Sender<T>, Receiver<T>) {
        channel_(Some(buffer))
    }

    fn channel_<T>(buffer: Option<usize>) -> (Sender<T>, Receiver<T>) {
        let shared = Rc::new(RefCell::new(Shared {
            buffer: VecDeque::new(),
            capacity: buffer,
            blocked_senders: VecDeque::new(),
            blocked_recv: None,
        }));
        let sender = Sender { shared: Rc::downgrade(&shared) };
        let receiver = Receiver { state: State::Open(shared) };
        (sender, receiver)
    }

    #[derive(Debug)]
    struct Shared<T> {
        buffer: VecDeque<T>,
        capacity: Option<usize>,
        blocked_senders: VecDeque<Task>,
        blocked_recv: Option<Task>,
    }

    /// The transmission end of a channel.
    ///
    /// This is created by the `channel` function.
    #[derive(Debug)]
    pub struct Sender<T> {
        shared: Weak<RefCell<Shared<T>>>,
    }

    impl<T> Sender<T> {
        fn do_send(&self, msg: T) -> StartSend<T, SendError<T>> {
            let shared = match self.shared.upgrade() {
                Some(shared) => shared,
                None => return Err(SendError(msg)), // receiver was dropped
            };
            let mut shared = shared.borrow_mut();

            match shared.capacity {
                Some(capacity) if shared.buffer.len() == capacity => {
                    shared.blocked_senders.push_back(task::current());
                    Ok(AsyncSink::NotReady(msg))
                }
                _ => {
                    shared.buffer.push_back(msg);
                    if let Some(task) = shared.blocked_recv.take() {
                        task.notify();
                    }
                    Ok(AsyncSink::Ready)
                }
            }
        }
    }

    impl<T> Clone for Sender<T> {
        fn clone(&self) -> Self {
            Sender { shared: self.shared.clone() }
        }
    }

    impl<T> Sink for Sender<T> {
        type SinkItem = T;
        type SinkError = SendError<T>;

        fn start_send(&mut self, msg: T) -> StartSend<T, SendError<T>> {
            self.do_send(msg)
        }

        fn poll_complete(&mut self) -> Poll<(), SendError<T>> {
            Ok(Async::Ready(()))
        }

        fn close(&mut self) -> Poll<(), SendError<T>> {
            Ok(Async::Ready(()))
        }
    }

    impl<T> Drop for Sender<T> {
        fn drop(&mut self) {
            let shared = match self.shared.upgrade() {
                Some(shared) => shared,
                None => return,
            };
            // The number of existing `Weak` indicates if we are possibly the last
            // `Sender`. If we are the last, we possibly must notify a blocked
            // `Receiver`. `self.shared` is always one of the `Weak` to this shared
            // data. Therefore the smallest possible Rc::weak_count(&shared) is 1.
            if Rc::weak_count(&shared) == 1 {
                if let Some(task) = shared.borrow_mut().blocked_recv.take() {
                    // Wake up receiver as its stream has ended
                    task.notify();
                }
            }
        }
    }

    /// The receiving end of a channel which implements the `Stream` trait.
    ///
    /// This is created by the `channel` function.
    #[derive(Debug)]
    pub struct Receiver<T> {
        state: State<T>,
    }

    /// Possible states of a receiver. We're either Open (can receive more messages)
    /// or we're closed with a list of messages we have left to receive.
    #[derive(Debug)]
    enum State<T> {
        Open(Rc<RefCell<Shared<T>>>),
        Closed(VecDeque<T>),
    }

    impl<T> Receiver<T> {
        /// Closes the receiving half
        ///
        /// This prevents any further messages from being sent on the channel while
        /// still enabling the receiver to drain messages that are buffered.
        pub fn close(&mut self) {
            let (blockers, items) = match self.state {
                State::Open(ref state) => {
                    let mut state = state.borrow_mut();
                    let items = mem::replace(&mut state.buffer, VecDeque::new());
                    let blockers = mem::replace(&mut state.blocked_senders, VecDeque::new());
                    (blockers, items)
                }
                State::Closed(_) => return,
            };
            self.state = State::Closed(items);
            for task in blockers {
                task.notify();
            }
        }
    }

    impl<T> Stream for Receiver<T> {
        type Item = T;
        type Error = ();

        fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
            let me = match self.state {
                State::Open(ref mut me) => me,
                State::Closed(ref mut items) => {
                    return Ok(Async::Ready(items.pop_front()))
                }
            };

            if let Some(shared) = Rc::get_mut(me) {
                // All senders have been dropped, so drain the buffer and end the
                // stream.
                return Ok(Async::Ready(shared.borrow_mut().buffer.pop_front()));
            }

            let mut shared = me.borrow_mut();
            if let Some(msg) = shared.buffer.pop_front() {
                if let Some(task) = shared.blocked_senders.pop_front() {
                    drop(shared);
                    task.notify();
                }
                Ok(Async::Ready(Some(msg)))
            } else {
                shared.blocked_recv = Some(task::current());
                Ok(Async::NotReady)
            }
        }
    }

    impl<T> Drop for Receiver<T> {
        fn drop(&mut self) {
            self.close();
        }
    }

    /// The transmission end of an unbounded channel.
    ///
    /// This is created by the `unbounded` function.
    #[derive(Debug)]
    pub struct UnboundedSender<T>(Sender<T>);

    impl<T> Clone for UnboundedSender<T> {
        fn clone(&self) -> Self {
            UnboundedSender(self.0.clone())
        }
    }

    impl<T> Sink for UnboundedSender<T> {
        type SinkItem = T;
        type SinkError = SendError<T>;

        fn start_send(&mut self, msg: T) -> StartSend<T, SendError<T>> {
            self.0.start_send(msg)
        }
        fn poll_complete(&mut self) -> Poll<(), SendError<T>> {
            Ok(Async::Ready(()))
        }
        fn close(&mut self) -> Poll<(), SendError<T>> {
            Ok(Async::Ready(()))
        }
    }

    impl<'a, T> Sink for &'a UnboundedSender<T> {
        type SinkItem = T;
        type SinkError = SendError<T>;

        fn start_send(&mut self, msg: T) -> StartSend<T, SendError<T>> {
            self.0.do_send(msg)
        }

        fn poll_complete(&mut self) -> Poll<(), SendError<T>> {
            Ok(Async::Ready(()))
        }

        fn close(&mut self) -> Poll<(), SendError<T>> {
            Ok(Async::Ready(()))
        }
    }

    impl<T> UnboundedSender<T> {
        /// Sends the provided message along this channel.
        ///
        /// This is an unbounded sender, so this function differs from `Sink::send`
        /// by ensuring the return type reflects that the channel is always ready to
        /// receive messages.
        #[deprecated(note = "renamed to `unbounded_send`")]
        #[doc(hidden)]
        pub fn send(&self, msg: T) -> Result<(), SendError<T>> {
            self.unbounded_send(msg)
        }

        /// Sends the provided message along this channel.
        ///
        /// This is an unbounded sender, so this function differs from `Sink::send`
        /// by ensuring the return type reflects that the channel is always ready to
        /// receive messages.
        pub fn unbounded_send(&self, msg: T) -> Result<(), SendError<T>> {
            let shared = match self.0.shared.upgrade() {
                Some(shared) => shared,
                None => return Err(SendError(msg)),
            };
            let mut shared = shared.borrow_mut();
            shared.buffer.push_back(msg);
            if let Some(task) = shared.blocked_recv.take() {
                drop(shared);
                task.notify();
            }
            Ok(())
        }
    }

    /// The receiving end of an unbounded channel.
    ///
    /// This is created by the `unbounded` function.
    #[derive(Debug)]
    pub struct UnboundedReceiver<T>(Receiver<T>);

    impl<T> UnboundedReceiver<T> {
        /// Closes the receiving half
        ///
        /// This prevents any further messages from being sent on the channel while
        /// still enabling the receiver to drain messages that are buffered.
        pub fn close(&mut self) {
            self.0.close();
        }
    }

    impl<T> Stream for UnboundedReceiver<T> {
        type Item = T;
        type Error = ();

        fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
            self.0.poll()
        }
    }

    /// Creates an unbounded in-memory channel with buffered storage.
    ///
    /// Identical semantics to `channel`, except with no limit to buffer size.
    pub fn unbounded<T>() -> (UnboundedSender<T>, UnboundedReceiver<T>) {
        let (send, recv) = channel_(None);
        (UnboundedSender(send), UnboundedReceiver(recv))
    }

    /// Error type for sending, used when the receiving end of a channel is
    /// dropped
    pub struct SendError<T>(T);

    impl<T> fmt::Debug for SendError<T> {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_tuple("SendError")
                .field(&"...")
                .finish()
        }
    }

    impl<T> fmt::Display for SendError<T> {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            write!(fmt, "send failed because receiver is gone")
        }
    }

    impl<T: Any> Error for SendError<T> {
        fn description(&self) -> &str {
            "send failed because receiver is gone"
        }
    }

    impl<T> SendError<T> {
        /// Returns the message that was attempted to be sent but failed.
        pub fn into_inner(self) -> T {
            self.0
        }
    }

    /// Handle returned from the `spawn` function.
    ///
    /// This handle is a stream that proxies a stream on a separate `Executor`.
    /// Created through the `mpsc::spawn` function, this handle will produce
    /// the same values as the proxied stream, as they are produced in the executor,
    /// and uses a limited buffer to exert back-pressure on the remote stream.
    ///
    /// If this handle is dropped, then the stream will no longer be polled and is
    /// scheduled to be dropped.
    pub struct SpawnHandle<Item, Error> {
        inner: Receiver<Result<Item, Error>>,
        _cancel_tx: oneshot::Sender<()>,
    }

    /// Type of future which `Executor` instances must be able to execute for `spawn`.
    pub struct Execute<S: Stream> {
        inner: SendAll<Sender<Result<S::Item, S::Error>>, Results<S, SendError<Result<S::Item, S::Error>>>>,
        cancel_rx: oneshot::Receiver<()>,
    }

    /// Spawns a `stream` onto the instance of `Executor` provided, `executor`,
    /// returning a handle representing the remote stream.
    ///
    /// The `stream` will be canceled if the `SpawnHandle` is dropped.
    ///
    /// The `SpawnHandle` returned is a stream that is a proxy for `stream` itself.
    /// When `stream` has additional items available, then the `SpawnHandle`
    /// will have those same items available.
    ///
    /// At most `buffer + 1` elements will be buffered at a time. If the buffer
    /// is full, then `stream` will stop progressing until more space is available.
    /// This allows the `SpawnHandle` to exert backpressure on the `stream`.
    ///
    /// # Panics
    ///
    /// This function will panic if `executor` is unable spawn a `Future` containing
    /// the entirety of the `stream`.
    pub fn spawn<S, E>(stream: S, executor: &E, buffer: usize) -> SpawnHandle<S::Item, S::Error>
        where S: Stream,
              E: Executor<Execute<S>>
    {
        let (cancel_tx, cancel_rx) = oneshot::channel();
        let (tx, rx) = channel(buffer);
        executor.execute(Execute {
            inner: tx.send_all(resultstream::new(stream)),
            cancel_rx: cancel_rx,
        }).expect("failed to spawn stream");
        SpawnHandle {
            inner: rx,
            _cancel_tx: cancel_tx,
        }
    }

    /// Spawns a `stream` onto the instance of `Executor` provided, `executor`,
    /// returning a handle representing the remote stream, with unbounded buffering.
    ///
    /// The `stream` will be canceled if the `SpawnHandle` is dropped.
    ///
    /// The `SpawnHandle` returned is a stream that is a proxy for `stream` itself.
    /// When `stream` has additional items available, then the `SpawnHandle`
    /// will have those same items available.
    ///
    /// An unbounded buffer is used, which means that values will be buffered as
    /// fast as `stream` can produce them, without any backpressure. Therefore, if
    /// `stream` is an infinite stream, it can use an unbounded amount of memory, and
    /// potentially hog CPU resources. In particular, if `stream` is infinite
    /// and doesn't ever yield (by returning `Async::NotReady` from `poll`), it
    /// will result in an infinite loop.
    ///
    /// # Panics
    ///
    /// This function will panic if `executor` is unable spawn a `Future` containing
    /// the entirety of the `stream`.
    pub fn spawn_unbounded<S,E>(stream: S, executor: &E) -> SpawnHandle<S::Item, S::Error>
        where S: Stream,
              E: Executor<Execute<S>>
    {
        let (cancel_tx, cancel_rx) = oneshot::channel();
        let (tx, rx) = channel_(None);
        executor.execute(Execute {
            inner: tx.send_all(resultstream::new(stream)),
            cancel_rx: cancel_rx,
        }).expect("failed to spawn stream");
        SpawnHandle {
            inner: rx,
            _cancel_tx: cancel_tx,
        }
    }

    impl<I, E> Stream for SpawnHandle<I, E> {
        type Item = I;
        type Error = E;

        fn poll(&mut self) -> Poll<Option<I>, E> {
            match self.inner.poll() {
                Ok(Async::Ready(Some(Ok(t)))) => Ok(Async::Ready(Some(t.into()))),
                Ok(Async::Ready(Some(Err(e)))) => Err(e),
                Ok(Async::Ready(None)) => Ok(Async::Ready(None)),
                Ok(Async::NotReady) => Ok(Async::NotReady),
                Err(_) => unreachable!("mpsc::Receiver should never return Err"),
            }
        }
    }

    impl<I, E> fmt::Debug for SpawnHandle<I, E> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("SpawnHandle")
                .finish()
        }
    }

    impl<S: Stream> Future for Execute<S> {
        type Item = ();
        type Error = ();

        fn poll(&mut self) -> Poll<(), ()> {
            match self.cancel_rx.poll() {
                Ok(Async::NotReady) => (),
                _ => return Ok(Async::Ready(())),
            }
            match self.inner.poll() {
                Ok(Async::NotReady) => Ok(Async::NotReady),
                _ => Ok(Async::Ready(()))
            }
        }
    }

    impl<S: Stream> fmt::Debug for Execute<S> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Execute")
             .finish()
        }
    }

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