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    //! A multi-producer, single-consumer, futures-aware, FIFO queue with back pressure.
    //!
    //! A channel can be used as a communication primitive between tasks running on
    //! `futures-rs` executors. Channel creation provides `Receiver` and `Sender`
    //! handles. `Receiver` implements `Stream` and allows a task to read values
    //! out of the channel. If there is no message to read from the channel, the
    //! current task will be notified when a new value is sent. `Sender` implements
    //! the `Sink` trait and allows a task to send messages into the channel. If
    //! the channel is at capacity, then send will be rejected and the task will be
    //! notified when additional capacity is available.
    //!
    //! # Disconnection
    //!
    //! When all `Sender` handles have been dropped, it is no longer possible to
    //! send values into the channel. This is considered the termination event of
    //! the stream. As such, `Sender::poll` will return `Ok(Ready(None))`.
    //!
    //! If the receiver handle is dropped, then messages can no longer be read out
    //! of the channel. In this case, a `send` will result in an error.
    //!
    //! # Clean Shutdown
    //!
    //! If the `Receiver` is simply dropped, then it is possible for there to be
    //! messages still in the channel that will not be processed. As such, it is
    //! usually desirable to perform a "clean" shutdown. To do this, the receiver
    //! will first call `close`, which will prevent any further messages to be sent
    //! into the channel. Then, the receiver consumes the channel to completion, at
    //! which point the receiver can be dropped.

    // At the core, the channel uses an atomic FIFO queue for message passing. This
    // queue is used as the primary coordination primitive. In order to enforce
    // capacity limits and handle back pressure, a secondary FIFO queue is used to
    // send parked task handles.
    //
    // The general idea is that the channel is created with a `buffer` size of `n`.
    // The channel capacity is `n + num-senders`. Each sender gets one "guaranteed"
    // slot to hold a message. This allows `Sender` to know for a fact that a send
    // will succeed *before* starting to do the actual work of sending the value.
    // Since most of this work is lock-free, once the work starts, it is impossible
    // to safely revert.
    //
    // If the sender is unable to process a send operation, then the current
    // task is parked and the handle is sent on the parked task queue.
    //
    // Note that the implementation guarantees that the channel capacity will never
    // exceed the configured limit, however there is no *strict* guarantee that the
    // receiver will wake up a parked task *immediately* when a slot becomes
    // available. However, it will almost always unpark a task when a slot becomes
    // available and it is *guaranteed* that a sender will be unparked when the
    // message that caused the sender to become parked is read out of the channel.
    //
    // The steps for sending a message are roughly:
    //
    // 1) Increment the channel message count
    // 2) If the channel is at capacity, push the task handle onto the wait queue
    // 3) Push the message onto the message queue.
    //
    // The steps for receiving a message are roughly:
    //
    // 1) Pop a message from the message queue
    // 2) Pop a task handle from the wait queue
    // 3) Decrement the channel message count.
    //
    // It's important for the order of operations on lock-free structures to happen
    // in reverse order between the sender and receiver. This makes the message
    // queue the primary coordination structure and establishes the necessary
    // happens-before semantics required for the acquire / release semantics used
    // by the queue structure.

    use std::fmt;
    use std::error::Error;
    use std::any::Any;
    use std::sync::atomic::AtomicUsize;
    use std::sync::atomic::Ordering::SeqCst;
    use std::sync::{Arc, Mutex};
    use std::thread;
    use std::usize;

    use sync::mpsc::queue::{Queue, PopResult};
    use sync::oneshot;
    use task::{self, Task};
    use future::Executor;
    use sink::SendAll;
    use resultstream::{self, Results};
    use {Async, AsyncSink, Future, Poll, StartSend, Sink, Stream};

    mod queue;

    /// The transmission end of a channel which is used to send values.
    ///
    /// This is created by the `channel` method.
    #[derive(Debug)]
    pub struct Sender<T> {
        // Channel state shared between the sender and receiver.
        inner: Arc<Inner<T>>,

        // Handle to the task that is blocked on this sender. This handle is sent
        // to the receiver half in order to be notified when the sender becomes
        // unblocked.
        sender_task: Arc<Mutex<SenderTask>>,

        // True if the sender might be blocked. This is an optimization to avoid
        // having to lock the mutex most of the time.
        maybe_parked: bool,
    }

    /// The transmission end of a channel which is used to send values.
    ///
    /// This is created by the `unbounded` method.
    #[derive(Debug)]
    pub struct UnboundedSender<T>(Sender<T>);

    trait AssertKinds: Send + Sync + Clone {}
    impl AssertKinds for UnboundedSender<u32> {}

    /// The receiving end of a channel which implements the `Stream` trait.
    ///
    /// This is a concrete implementation of a stream which can be used to represent
    /// a stream of values being computed elsewhere. This is created by the
    /// `channel` method.
    #[derive(Debug)]
    pub struct Receiver<T> {
        inner: Arc<Inner<T>>,
    }

    /// The receiving end of a channel which implements the `Stream` trait.
    ///
    /// This is a concrete implementation of a stream which can be used to represent
    /// a stream of values being computed elsewhere. This is created by the
    /// `unbounded` method.
    #[derive(Debug)]
    pub struct UnboundedReceiver<T>(Receiver<T>);

    /// Error type for sending, used when the receiving end of a channel is
    /// dropped
    #[derive(Clone, PartialEq, Eq)]
    pub struct SendError<T>(T);

    /// Error type returned from `try_send`
    #[derive(Clone, PartialEq, Eq)]
    pub struct TrySendError<T> {
        kind: TrySendErrorKind<T>,
    }

    #[derive(Clone, PartialEq, Eq)]
    enum TrySendErrorKind<T> {
        Full(T),
        Disconnected(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
        }
    }

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

    impl<T> fmt::Display for TrySendError<T> {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            if self.is_full() {
                write!(fmt, "send failed because channel is full")
            } else {
                write!(fmt, "send failed because receiver is gone")
            }
        }
    }

    impl<T: Any> Error for TrySendError<T> {
        fn description(&self) -> &str {
            if self.is_full() {
                "send failed because channel is full"
            } else {
                "send failed because receiver is gone"
            }
        }
    }

    impl<T> TrySendError<T> {
        /// Returns true if this error is a result of the channel being full
        pub fn is_full(&self) -> bool {
            use self::TrySendErrorKind::*;

            match self.kind {
                Full(_) => true,
                _ => false,
            }
        }

        /// Returns true if this error is a result of the receiver being dropped
        pub fn is_disconnected(&self) -> bool {
            use self::TrySendErrorKind::*;

            match self.kind {
                Disconnected(_) => true,
                _ => false,
            }
        }

        /// Returns the message that was attempted to be sent but failed.
        pub fn into_inner(self) -> T {
            use self::TrySendErrorKind::*;

            match self.kind {
                Full(v) | Disconnected(v) => v,
            }
        }
    }

    #[derive(Debug)]
    struct Inner<T> {
        // Max buffer size of the channel. If `None` then the channel is unbounded.
        buffer: Option<usize>,

        // Internal channel state. Consists of the number of messages stored in the
        // channel as well as a flag signalling that the channel is closed.
        state: AtomicUsize,

        // Atomic, FIFO queue used to send messages to the receiver
        message_queue: Queue<Option<T>>,

        // Atomic, FIFO queue used to send parked task handles to the receiver.
        parked_queue: Queue<Arc<Mutex<SenderTask>>>,

        // Number of senders in existence
        num_senders: AtomicUsize,

        // Handle to the receiver's task.
        recv_task: Mutex<ReceiverTask>,
    }

    // Struct representation of `Inner::state`.
    #[derive(Debug, Clone, Copy)]
    struct State {
        // `true` when the channel is open
        is_open: bool,

        // Number of messages in the channel
        num_messages: usize,
    }

    #[derive(Debug)]
    struct ReceiverTask {
        unparked: bool,
        task: Option<Task>,
    }

    // Returned from Receiver::try_park()
    enum TryPark {
        Parked,
        Closed,
        NotEmpty,
    }

    // The `is_open` flag is stored in the left-most bit of `Inner::state`
    const OPEN_MASK: usize = usize::MAX - (usize::MAX >> 1);

    // When a new channel is created, it is created in the open state with no
    // pending messages.
    const INIT_STATE: usize = OPEN_MASK;

    // The maximum number of messages that a channel can track is `usize::MAX >> 1`
    const MAX_CAPACITY: usize = !(OPEN_MASK);

    // The maximum requested buffer size must be less than the maximum capacity of
    // a channel. This is because each sender gets a guaranteed slot.
    const MAX_BUFFER: usize = MAX_CAPACITY >> 1;

    // Sent to the consumer to wake up blocked producers
    #[derive(Debug)]
    struct SenderTask {
        task: Option<Task>,
        is_parked: bool,
    }

    impl SenderTask {
        fn new() -> Self {
            SenderTask {
                task: None,
                is_parked: false,
            }
        }

        fn notify(&mut self) {
            self.is_parked = false;

            if let Some(task) = self.task.take() {
                task.notify();
            }
        }
    }

    /// Creates an in-memory channel implementation of the `Stream` trait with
    /// bounded capacity.
    ///
    /// This method creates a concrete implementation of the `Stream` trait which
    /// can be used to send values across threads in a streaming fashion. This
    /// channel is unique in that it implements back pressure to ensure that the
    /// sender never outpaces the receiver. The channel capacity is equal to
    /// `buffer + num-senders`. In other words, each sender gets a guaranteed slot
    /// in the channel capacity, and on top of that there are `buffer` "first come,
    /// first serve" slots available to all senders.
    ///
    /// The `Receiver` returned implements the `Stream` trait and has access to any
    /// number of the associated combinators for transforming the result.
    pub fn channel<T>(buffer: usize) -> (Sender<T>, Receiver<T>) {
        // Check that the requested buffer size does not exceed the maximum buffer
        // size permitted by the system.
        assert!(buffer < MAX_BUFFER, "requested buffer size too large");
        channel2(Some(buffer))
    }

    /// Creates an in-memory channel implementation of the `Stream` trait with
    /// unbounded capacity.
    ///
    /// This method creates a concrete implementation of the `Stream` trait which
    /// can be used to send values across threads in a streaming fashion. A `send`
    /// on this channel will always succeed as long as the receive half has not
    /// been closed. If the receiver falls behind, messages will be buffered
    /// internally.
    ///
    /// **Note** that the amount of available system memory is an implicit bound to
    /// the channel. Using an `unbounded` channel has the ability of causing the
    /// process to run out of memory. In this case, the process will be aborted.
    pub fn unbounded<T>() -> (UnboundedSender<T>, UnboundedReceiver<T>) {
        let (tx, rx) = channel2(None);
        (UnboundedSender(tx), UnboundedReceiver(rx))
    }

    fn channel2<T>(buffer: Option<usize>) -> (Sender<T>, Receiver<T>) {
        let inner = Arc::new(Inner {
            buffer: buffer,
            state: AtomicUsize::new(INIT_STATE),
            message_queue: Queue::new(),
            parked_queue: Queue::new(),
            num_senders: AtomicUsize::new(1),
            recv_task: Mutex::new(ReceiverTask {
                unparked: false,
                task: None,
            }),
        });

        let tx = Sender {
            inner: inner.clone(),
            sender_task: Arc::new(Mutex::new(SenderTask::new())),
            maybe_parked: false,
        };

        let rx = Receiver {
            inner: inner,
        };

        (tx, rx)
    }

    /*
     *
     * ===== impl Sender =====
     *
     */

    impl<T> Sender<T> {
        /// Attempts to send a message on this `Sender<T>` without blocking.
        ///
        /// This function, unlike `start_send`, is safe to call whether it's being
        /// called on a task or not. Note that this function, however, will *not*
        /// attempt to block the current task if the message cannot be sent.
        ///
        /// It is not recommended to call this function from inside of a future,
        /// only from an external thread where you've otherwise arranged to be
        /// notified when the channel is no longer full.
        pub fn try_send(&mut self, msg: T) -> Result<(), TrySendError<T>> {
            // If the sender is currently blocked, reject the message
            if !self.poll_unparked(false).is_ready() {
                return Err(TrySendError {
                    kind: TrySendErrorKind::Full(msg),
                });
            }

            // The channel has capacity to accept the message, so send it
            self.do_send(Some(msg), false)
                .map_err(|SendError(v)| {
                    TrySendError {
                        kind: TrySendErrorKind::Disconnected(v),
                    }
                })
        }

        // Do the send without failing
        // None means close
        fn do_send(&mut self, msg: Option<T>, do_park: bool) -> Result<(), SendError<T>> {
            // First, increment the number of messages contained by the channel.
            // This operation will also atomically determine if the sender task
            // should be parked.
            //
            // None is returned in the case that the channel has been closed by the
            // receiver. This happens when `Receiver::close` is called or the
            // receiver is dropped.
            let park_self = match self.inc_num_messages(msg.is_none()) {
                Some(park_self) => park_self,
                None => {
                    // The receiver has closed the channel. Only abort if actually
                    // sending a message. It is important that the stream
                    // termination (None) is always sent. This technically means
                    // that it is possible for the queue to contain the following
                    // number of messages:
                    //
                    //     num-senders + buffer + 1
                    //
                    if let Some(msg) = msg {
                        return Err(SendError(msg));
                    } else {
                        return Ok(());
                    }
                }
            };

            // If the channel has reached capacity, then the sender task needs to
            // be parked. This will send the task handle on the parked task queue.
            //
            // However, when `do_send` is called while dropping the `Sender`,
            // `task::current()` can't be called safely. In this case, in order to
            // maintain internal consistency, a blank message is pushed onto the
            // parked task queue.
            if park_self {
                self.park(do_park);
            }

            self.queue_push_and_signal(msg);

            Ok(())
        }

        // Do the send without parking current task.
        //
        // To be called from unbounded sender.
        fn do_send_nb(&self, msg: T) -> Result<(), SendError<T>> {
            match self.inc_num_messages(false) {
                Some(park_self) => assert!(!park_self),
                None => return Err(SendError(msg)),
            };

            self.queue_push_and_signal(Some(msg));

            Ok(())
        }

        // Push message to the queue and signal to the receiver
        fn queue_push_and_signal(&self, msg: Option<T>) {
            // Push the message onto the message queue
            self.inner.message_queue.push(msg);

            // Signal to the receiver that a message has been enqueued. If the
            // receiver is parked, this will unpark the task.
            self.signal();
        }

        // Increment the number of queued messages. Returns if the sender should
        // block.
        fn inc_num_messages(&self, close: bool) -> Option<bool> {
            let mut curr = self.inner.state.load(SeqCst);

            loop {
                let mut state = decode_state(curr);

                // The receiver end closed the channel.
                if !state.is_open {
                    return None;
                }

                // This probably is never hit? Odds are the process will run out of
                // memory first. It may be worth to return something else in this
                // case?
                assert!(state.num_messages < MAX_CAPACITY, "buffer space exhausted; \
                        sending this messages would overflow the state");

                state.num_messages += 1;

                // The channel is closed by all sender handles being dropped.
                if close {
                    state.is_open = false;
                }

                let next = encode_state(&state);
                match self.inner.state.compare_exchange(curr, next, SeqCst, SeqCst) {
                    Ok(_) => {
                        // Block if the current number of pending messages has exceeded
                        // the configured buffer size
                        let park_self = match self.inner.buffer {
                            Some(buffer) => state.num_messages > buffer,
                            None => false,
                        };

                        return Some(park_self)
                    }
                    Err(actual) => curr = actual,
                }
            }
        }

        // Signal to the receiver task that a message has been enqueued
        fn signal(&self) {
            // TODO
            // This logic can probably be improved by guarding the lock with an
            // atomic.
            //
            // Do this step first so that the lock is dropped when
            // `unpark` is called
            let task = {
                let mut recv_task = self.inner.recv_task.lock().unwrap();

                // If the receiver has already been unparked, then there is nothing
                // more to do
                if recv_task.unparked {
                    return;
                }

                // Setting this flag enables the receiving end to detect that
                // an unpark event happened in order to avoid unnecessarily
                // parking.
                recv_task.unparked = true;
                recv_task.task.take()
            };

            if let Some(task) = task {
                task.notify();
            }
        }

        fn park(&mut self, can_park: bool) {
            // TODO: clean up internal state if the task::current will fail

            let task = if can_park {
                Some(task::current())
            } else {
                None
            };

            {
                let mut sender = self.sender_task.lock().unwrap();
                sender.task = task;
                sender.is_parked = true;
            }

            // Send handle over queue
            let t = self.sender_task.clone();
            self.inner.parked_queue.push(t);

            // Check to make sure we weren't closed after we sent our task on the
            // queue
            let state = decode_state(self.inner.state.load(SeqCst));
            self.maybe_parked = state.is_open;
        }

        /// Polls the channel to determine if there is guaranteed to be capacity to send at least one
        /// item without waiting.
        ///
        /// Returns `Ok(Async::Ready(_))` if there is sufficient capacity, or returns
        /// `Ok(Async::NotReady)` if the channel is not guaranteed to have capacity. Returns
        /// `Err(SendError(_))` if the receiver has been dropped.
        ///
        /// # Panics
        ///
        /// This method will panic if called from outside the context of a task or future.
        pub fn poll_ready(&mut self) -> Poll<(), SendError<()>> {
            let state = decode_state(self.inner.state.load(SeqCst));
            if !state.is_open {
                return Err(SendError(()));
            }

            Ok(self.poll_unparked(true))
        }

        fn poll_unparked(&mut self, do_park: bool) -> Async<()> {
            // First check the `maybe_parked` variable. This avoids acquiring the
            // lock in most cases
            if self.maybe_parked {
                // Get a lock on the task handle
                let mut task = self.sender_task.lock().unwrap();

                if !task.is_parked {
                    self.maybe_parked = false;
                    return Async::Ready(())
                }

                // At this point, an unpark request is pending, so there will be an
                // unpark sometime in the future. We just need to make sure that
                // the correct task will be notified.
                //
                // Update the task in case the `Sender` has been moved to another
                // task
                task.task = if do_park {
                    Some(task::current())
                } else {
                    None
                };

                Async::NotReady
            } else {
                Async::Ready(())
            }
        }
    }

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

        fn start_send(&mut self, msg: T) -> StartSend<T, SendError<T>> {
            // If the sender is currently blocked, reject the message before doing
            // any work.
            if !self.poll_unparked(true).is_ready() {
                return Ok(AsyncSink::NotReady(msg));
            }

            // The channel has capacity to accept the message, so send it.
            self.do_send(Some(msg), true)?;

            Ok(AsyncSink::Ready)
        }

        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>> {
            self.0.do_send_nb(msg)
        }
    }

    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>> {
            self.0.poll_complete()
        }

        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_nb(msg)?;
            Ok(AsyncSink::Ready)
        }

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

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

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

    impl<T> Clone for Sender<T> {
        fn clone(&self) -> Sender<T> {
            // Since this atomic op isn't actually guarding any memory and we don't
            // care about any orderings besides the ordering on the single atomic
            // variable, a relaxed ordering is acceptable.
            let mut curr = self.inner.num_senders.load(SeqCst);

            loop {
                // If the maximum number of senders has been reached, then fail
                if curr == self.inner.max_senders() {
                    panic!("cannot clone `Sender` -- too many outstanding senders");
                }

                debug_assert!(curr < self.inner.max_senders());

                let next = curr + 1;
                let actual = self.inner.num_senders.compare_and_swap(curr, next, SeqCst);

                // The ABA problem doesn't matter here. We only care that the
                // number of senders never exceeds the maximum.
                if actual == curr {
                    return Sender {
                        inner: self.inner.clone(),
                        sender_task: Arc::new(Mutex::new(SenderTask::new())),
                        maybe_parked: false,
                    };
                }

                curr = actual;
            }
        }
    }

    impl<T> Drop for Sender<T> {
        fn drop(&mut self) {
            // Ordering between variables don't matter here
            let prev = self.inner.num_senders.fetch_sub(1, SeqCst);

            if prev == 1 {
                let _ = self.do_send(None, false);
            }
        }
    }

    /*
     *
     * ===== impl Receiver =====
     *
     */

    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 mut curr = self.inner.state.load(SeqCst);

            loop {
                let mut state = decode_state(curr);

                if !state.is_open {
                    break
                }

                state.is_open = false;

                let next = encode_state(&state);
                match self.inner.state.compare_exchange(curr, next, SeqCst, SeqCst) {
                    Ok(_) => break,
                    Err(actual) => curr = actual,
                }
            }

            // Wake up any threads waiting as they'll see that we've closed the
            // channel and will continue on their merry way.
            loop {
                match unsafe { self.inner.parked_queue.pop() } {
                    PopResult::Data(task) => {
                        task.lock().unwrap().notify();
                    }
                    PopResult::Empty => break,
                    PopResult::Inconsistent => thread::yield_now(),
                }
            }
        }

        fn next_message(&mut self) -> Async<Option<T>> {
            // Pop off a message
            loop {
                match unsafe { self.inner.message_queue.pop() } {
                    PopResult::Data(msg) => {
                        return Async::Ready(msg);
                    }
                    PopResult::Empty => {
                        // The queue is empty, return NotReady
                        return Async::NotReady;
                    }
                    PopResult::Inconsistent => {
                        // Inconsistent means that there will be a message to pop
                        // in a short time. This branch can only be reached if
                        // values are being produced from another thread, so there
                        // are a few ways that we can deal with this:
                        //
                        // 1) Spin
                        // 2) thread::yield_now()
                        // 3) task::current().unwrap() & return NotReady
                        //
                        // For now, thread::yield_now() is used, but it would
                        // probably be better to spin a few times then yield.
                        thread::yield_now();
                    }
                }
            }
        }

        // Unpark a single task handle if there is one pending in the parked queue
        fn unpark_one(&mut self) {
            loop {
                match unsafe { self.inner.parked_queue.pop() } {
                    PopResult::Data(task) => {
                        task.lock().unwrap().notify();
                        return;
                    }
                    PopResult::Empty => {
                        // Queue empty, no task to wake up.
                        return;
                    }
                    PopResult::Inconsistent => {
                        // Same as above
                        thread::yield_now();
                    }
                }
            }
        }

        // Try to park the receiver task
        fn try_park(&self) -> TryPark {
            let curr = self.inner.state.load(SeqCst);
            let state = decode_state(curr);

            // If the channel is closed, then there is no need to park.
            if !state.is_open && state.num_messages == 0 {
                return TryPark::Closed;
            }

            // First, track the task in the `recv_task` slot
            let mut recv_task = self.inner.recv_task.lock().unwrap();

            if recv_task.unparked {
                // Consume the `unpark` signal without actually parking
                recv_task.unparked = false;
                return TryPark::NotEmpty;
            }

            recv_task.task = Some(task::current());
            TryPark::Parked
        }

        fn dec_num_messages(&self) {
            let mut curr = self.inner.state.load(SeqCst);

            loop {
                let mut state = decode_state(curr);

                state.num_messages -= 1;

                let next = encode_state(&state);
                match self.inner.state.compare_exchange(curr, next, SeqCst, SeqCst) {
                    Ok(_) => break,
                    Err(actual) => curr = actual,
                }
            }
        }
    }

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

        fn poll(&mut self) -> Poll<Option<T>, ()> {
            loop {
                // Try to read a message off of the message queue.
                let msg = match self.next_message() {
                    Async::Ready(msg) => msg,
                    Async::NotReady => {
                        // There are no messages to read, in this case, attempt to
                        // park. The act of parking will verify that the channel is
                        // still empty after the park operation has completed.
                        match self.try_park() {
                            TryPark::Parked => {
                                // The task was parked, and the channel is still
                                // empty, return NotReady.
                                return Ok(Async::NotReady);
                            }
                            TryPark::Closed => {
                                // The channel is closed, there will be no further
                                // messages.
                                return Ok(Async::Ready(None));
                            }
                            TryPark::NotEmpty => {
                                // A message has been sent while attempting to
                                // park. Loop again, the next iteration is
                                // guaranteed to get the message.
                                continue;
                            }
                        }
                    }
                };

                // If there are any parked task handles in the parked queue, pop
                // one and unpark it.
                self.unpark_one();

                // Decrement number of messages
                self.dec_num_messages();

                // Return the message
                return Ok(Async::Ready(msg));
            }
        }
    }

    impl<T> Drop for Receiver<T> {
        fn drop(&mut self) {
            // Drain the channel of all pending messages
            self.close();
            while self.next_message().is_ready() {
                // ...
            }
        }
    }

    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<T>, ()> {
            self.0.poll()
        }
    }

    /// 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> {
        rx: 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 {
            rx: 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.
    ///
    /// # 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) = channel2(None);
        executor.execute(Execute {
            inner: tx.send_all(resultstream::new(stream)),
            cancel_rx: cancel_rx,
        }).expect("failed to spawn stream");
        SpawnHandle {
            rx: 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.rx.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()
        }
    }

    /*
     *
     * ===== impl Inner =====
     *
     */

    impl<T> Inner<T> {
        // The return value is such that the total number of messages that can be
        // enqueued into the channel will never exceed MAX_CAPACITY
        fn max_senders(&self) -> usize {
            match self.buffer {
                Some(buffer) => MAX_CAPACITY - buffer,
                None => MAX_BUFFER,
            }
        }
    }

    unsafe impl<T: Send> Send for Inner<T> {}
    unsafe impl<T: Send> Sync for Inner<T> {}

    /*
     *
     * ===== Helpers =====
     *
     */

    fn decode_state(num: usize) -> State {
        State {
            is_open: num & OPEN_MASK == OPEN_MASK,
            num_messages: num & MAX_CAPACITY,
        }
    }

    fn encode_state(state: &State) -> usize {
        let mut num = state.num_messages;

        if state.is_open {
            num |= OPEN_MASK;
        }

        num
    }

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  Show this help dialog
`S`
  Focus the search field
`↑`
  Move up in search results
`↓`
  Move down in search results
`↹`
  Switch tab
`⏎`
  Go to active search result
`+`
  Expand all sections
`-`
  Collapse all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g. `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g. `vec -> usize` or `* -> vec`)