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    //! Channel that delivers a message after a certain amount of time.
    //!
    //! Messages cannot be sent into this kind of channel; they are materialized on demand.

    use std::sync::atomic::{AtomicBool, Ordering};
    use std::thread;
    use std::time::{Duration, Instant};

    use context::Context;
    use err::{RecvTimeoutError, TryRecvError};
    use select::{Operation, SelectHandle, Token};
    use utils;

    /// Result of a receive operation.
    pub type AfterToken = Option<Instant>;

    /// Channel that delivers a message after a certain amount of time.
    pub struct Channel {
        /// The instant at which the message will be delivered.
        delivery_time: Instant,

        /// `true` if the message has been received.
        received: AtomicBool,
    }

    impl Channel {
        /// Creates a channel that delivers a message after a certain duration of time.
        #[inline]
        pub fn new(dur: Duration) -> Self {
            Channel {
                delivery_time: Instant::now() + dur,
                received: AtomicBool::new(false),
            }
        }

        /// Attempts to receive a message without blocking.
        #[inline]
        pub fn try_recv(&self) -> Result<Instant, TryRecvError> {
            // We use relaxed ordering because this is just an optional optimistic check.
            if self.received.load(Ordering::Relaxed) {
                // The message has already been received.
                return Err(TryRecvError::Empty);
            }

            if Instant::now() < self.delivery_time {
                // The message was not delivered yet.
                return Err(TryRecvError::Empty);
            }

            // Try receiving the message if it is still available.
            if !self.received.swap(true, Ordering::SeqCst) {
                // Success! Return delivery time as the message.
                Ok(self.delivery_time)
            } else {
                // The message was already received.
                Err(TryRecvError::Empty)
            }
        }

        /// Receives a message from the channel.
        #[inline]
        pub fn recv(&self, deadline: Option<Instant>) -> Result<Instant, RecvTimeoutError> {
            // We use relaxed ordering because this is just an optional optimistic check.
            if self.received.load(Ordering::Relaxed) {
                // The message has already been received.
                utils::sleep_until(deadline);
                return Err(RecvTimeoutError::Timeout);
            }

            // Wait until the message is received or the deadline is reached.
            loop {
                let now = Instant::now();

                // Check if we can receive the next message.
                if now >= self.delivery_time {
                    break;
                }

                // Check if the deadline has been reached.
                if let Some(d) = deadline {
                    if now >= d {
                        return Err(RecvTimeoutError::Timeout);
                    }

                    thread::sleep(self.delivery_time.min(d) - now);
                } else {
                    thread::sleep(self.delivery_time - now);
                }
            }

            // Try receiving the message if it is still available.
            if !self.received.swap(true, Ordering::SeqCst) {
                // Success! Return the message, which is the instant at which it was delivered.
                Ok(self.delivery_time)
            } else {
                // The message was already received. Block forever.
                utils::sleep_until(None);
                unreachable!()
            }
        }

        /// Reads a message from the channel.
        #[inline]
        pub unsafe fn read(&self, token: &mut Token) -> Result<Instant, ()> {
            token.after.ok_or(())
        }

        /// Returns `true` if the channel is empty.
        #[inline]
        pub fn is_empty(&self) -> bool {
            // We use relaxed ordering because this is just an optional optimistic check.
            if self.received.load(Ordering::Relaxed) {
                return true;
            }

            // If the delivery time hasn't been reached yet, the channel is empty.
            if Instant::now() < self.delivery_time {
                return true;
            }

            // The delivery time has been reached. The channel is empty only if the message has already
            // been received.
            self.received.load(Ordering::SeqCst)
        }

        /// Returns `true` if the channel is full.
        #[inline]
        pub fn is_full(&self) -> bool {
            !self.is_empty()
        }

        /// Returns the number of messages in the channel.
        #[inline]
        pub fn len(&self) -> usize {
            if self.is_empty() {
                0
            } else {
                1
            }
        }

        /// Returns the capacity of the channel.
        #[inline]
        pub fn capacity(&self) -> Option<usize> {
            Some(1)
        }
    }

    impl SelectHandle for Channel {
        #[inline]
        fn try_select(&self, token: &mut Token) -> bool {
            match self.try_recv() {
                Ok(msg) => {
                    token.after = Some(msg);
                    true
                }
                Err(TryRecvError::Disconnected) => {
                    token.after = None;
                    true
                }
                Err(TryRecvError::Empty) => false,
            }
        }

        #[inline]
        fn deadline(&self) -> Option<Instant> {
            // We use relaxed ordering because this is just an optional optimistic check.
            if self.received.load(Ordering::Relaxed) {
                None
            } else {
                Some(self.delivery_time)
            }
        }

        #[inline]
        fn register(&self, _oper: Operation, _cx: &Context) -> bool {
            self.is_ready()
        }

        #[inline]
        fn unregister(&self, _oper: Operation) {}

        #[inline]
        fn accept(&self, token: &mut Token, _cx: &Context) -> bool {
            self.try_select(token)
        }

        #[inline]
        fn is_ready(&self) -> bool {
            !self.is_empty()
        }

        #[inline]
        fn watch(&self, _oper: Operation, _cx: &Context) -> bool {
            self.is_ready()
        }

        #[inline]
        fn unwatch(&self, _oper: Operation) {}
    }

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