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    //! Bounded channel based on a preallocated array.
    //!
    //! This flavor has a fixed, positive capacity.
    //!
    //! The implementation is based on Dmitry Vyukov's bounded MPMC queue.
    //!
    //! Source:
    //!   - http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue
    //!   - https://docs.google.com/document/d/1yIAYmbvL3JxOKOjuCyon7JhW4cSv1wy5hC0ApeGMV9s/pub
    //!
    //! Copyright & License:
    //!   - Copyright (c) 2010-2011 Dmitry Vyukov
    //!   - Simplified BSD License and Apache License, Version 2.0
    //!   - http://www.1024cores.net/home/code-license

    use std::cell::UnsafeCell;
    use std::marker::PhantomData;
    use std::mem;
    use std::ptr;
    use std::sync::atomic::{self, AtomicUsize, Ordering};
    use std::time::Instant;

    use crossbeam_utils::{Backoff, CachePadded};

    use context::Context;
    use err::{RecvTimeoutError, SendTimeoutError, TryRecvError, TrySendError};
    use select::{Operation, SelectHandle, Selected, Token};
    use waker::SyncWaker;

    /// A slot in a channel.
    struct Slot<T> {
        /// The current stamp.
        stamp: AtomicUsize,

        /// The message in this slot.
        msg: UnsafeCell<T>,
    }

    /// The token type for the array flavor.
    pub struct ArrayToken {
        /// Slot to read from or write to.
        slot: *const u8,

        /// Stamp to store into the slot after reading or writing.
        stamp: usize,
    }

    impl Default for ArrayToken {
        #[inline]
        fn default() -> Self {
            ArrayToken {
                slot: ptr::null(),
                stamp: 0,
            }
        }
    }

    /// Bounded channel based on a preallocated array.
    pub struct Channel<T> {
        /// The head of the channel.
        ///
        /// This value is a "stamp" consisting of an index into the buffer, a mark bit, and a lap, but
        /// packed into a single `usize`. The lower bits represent the index, while the upper bits
        /// represent the lap. The mark bit in the head is always zero.
        ///
        /// Messages are popped from the head of the channel.
        head: CachePadded<AtomicUsize>,

        /// The tail of the channel.
        ///
        /// This value is a "stamp" consisting of an index into the buffer, a mark bit, and a lap, but
        /// packed into a single `usize`. The lower bits represent the index, while the upper bits
        /// represent the lap. The mark bit indicates that the channel is disconnected.
        ///
        /// Messages are pushed into the tail of the channel.
        tail: CachePadded<AtomicUsize>,

        /// The buffer holding slots.
        buffer: *mut Slot<T>,

        /// The channel capacity.
        cap: usize,

        /// A stamp with the value of `{ lap: 1, mark: 0, index: 0 }`.
        one_lap: usize,

        /// If this bit is set in the tail, that means the channel is disconnected.
        mark_bit: usize,

        /// Senders waiting while the channel is full.
        senders: SyncWaker,

        /// Receivers waiting while the channel is empty and not disconnected.
        receivers: SyncWaker,

        /// Indicates that dropping a `Channel<T>` may drop values of type `T`.
        _marker: PhantomData<T>,
    }

    impl<T> Channel<T> {
        /// Creates a bounded channel of capacity `cap`.
        ///
        /// # Panics
        ///
        /// Panics if the capacity is not in the range `1 ..= usize::max_value() / 4`.
        pub fn with_capacity(cap: usize) -> Self {
            assert!(cap > 0, "capacity must be positive");

            // Make sure there are at least two most significant bits: one to encode laps and one more
            // to indicate that the channel is disconnected. If we can't reserve two bits, then panic.
            // In that case, the buffer is likely too large to allocate anyway.
            let cap_limit = usize::max_value() / 4;
            assert!(
                cap <= cap_limit,
                "channel capacity is too large: {} > {}",
                cap,
                cap_limit
            );

            // Compute constants `mark_bit` and `one_lap`.
            let mark_bit = (cap + 1).next_power_of_two();
            let one_lap = mark_bit * 2;

            // Head is initialized to `{ lap: 0, mark: 0, index: 0 }`.
            let head = 0;
            // Tail is initialized to `{ lap: 0, mark: 0, index: 0 }`.
            let tail = 0;

            // Allocate a buffer of `cap` slots.
            let buffer = {
                let mut v = Vec::<Slot<T>>::with_capacity(cap);
                let ptr = v.as_mut_ptr();
                mem::forget(v);
                ptr
            };

            // Initialize stamps in the slots.
            for i in 0..cap {
                unsafe {
                    // Set the stamp to `{ lap: 0, mark: 0, index: i }`.
                    let slot = buffer.add(i);
                    ptr::write(&mut (*slot).stamp, AtomicUsize::new(i));
                }
            }

            Channel {
                buffer,
                cap,
                one_lap,
                mark_bit,
                head: CachePadded::new(AtomicUsize::new(head)),
                tail: CachePadded::new(AtomicUsize::new(tail)),
                senders: SyncWaker::new(),
                receivers: SyncWaker::new(),
                _marker: PhantomData,
            }
        }

        /// Returns a receiver handle to the channel.
        pub fn receiver(&self) -> Receiver<T> {
            Receiver(self)
        }

        /// Returns a sender handle to the channel.
        pub fn sender(&self) -> Sender<T> {
            Sender(self)
        }

        /// Attempts to reserve a slot for sending a message.
        fn start_send(&self, token: &mut Token) -> bool {
            let backoff = Backoff::new();
            let mut tail = self.tail.load(Ordering::Relaxed);

            loop {
                // Check if the channel is disconnected.
                if tail & self.mark_bit != 0 {
                    token.array.slot = ptr::null();
                    token.array.stamp = 0;
                    return true;
                }

                // Deconstruct the tail.
                let index = tail & (self.mark_bit - 1);
                let lap = tail & !(self.one_lap - 1);

                // Inspect the corresponding slot.
                let slot = unsafe { &*self.buffer.add(index) };
                let stamp = slot.stamp.load(Ordering::Acquire);

                // If the tail and the stamp match, we may attempt to push.
                if tail == stamp {
                    let new_tail = if index + 1 < self.cap {
                        // Same lap, incremented index.
                        // Set to `{ lap: lap, mark: 0, index: index + 1 }`.
                        tail + 1
                    } else {
                        // One lap forward, index wraps around to zero.
                        // Set to `{ lap: lap.wrapping_add(1), mark: 0, index: 0 }`.
                        lap.wrapping_add(self.one_lap)
                    };

                    // Try moving the tail.
                    match self
                        .tail
                        .compare_exchange_weak(tail, new_tail, Ordering::SeqCst, Ordering::Relaxed)
                    {
                        Ok(_) => {
                            // Prepare the token for the follow-up call to `write`.
                            token.array.slot = slot as *const Slot<T> as *const u8;
                            token.array.stamp = tail + 1;
                            return true;
                        }
                        Err(t) => {
                            tail = t;
                            backoff.spin();
                        }
                    }
                } else if stamp.wrapping_add(self.one_lap) == tail + 1 {
                    atomic::fence(Ordering::SeqCst);
                    let head = self.head.load(Ordering::Relaxed);

                    // If the head lags one lap behind the tail as well...
                    if head.wrapping_add(self.one_lap) == tail {
                        // ...then the channel is full.
                        return false;
                    }

                    backoff.spin();
                    tail = self.tail.load(Ordering::Relaxed);
                } else {
                    // Snooze because we need to wait for the stamp to get updated.
                    backoff.snooze();
                    tail = self.tail.load(Ordering::Relaxed);
                }
            }
        }

        /// Writes a message into the channel.
        pub unsafe fn write(&self, token: &mut Token, msg: T) -> Result<(), T> {
            // If there is no slot, the channel is disconnected.
            if token.array.slot.is_null() {
                return Err(msg);
            }

            let slot: &Slot<T> = &*(token.array.slot as *const Slot<T>);

            // Write the message into the slot and update the stamp.
            slot.msg.get().write(msg);
            slot.stamp.store(token.array.stamp, Ordering::Release);

            // Wake a sleeping receiver.
            self.receivers.notify();
            Ok(())
        }

        /// Attempts to reserve a slot for receiving a message.
        fn start_recv(&self, token: &mut Token) -> bool {
            let backoff = Backoff::new();
            let mut head = self.head.load(Ordering::Relaxed);

            loop {
                // Deconstruct the head.
                let index = head & (self.mark_bit - 1);
                let lap = head & !(self.one_lap - 1);

                // Inspect the corresponding slot.
                let slot = unsafe { &*self.buffer.add(index) };
                let stamp = slot.stamp.load(Ordering::Acquire);

                // If the the stamp is ahead of the head by 1, we may attempt to pop.
                if head + 1 == stamp {
                    let new = if index + 1 < self.cap {
                        // Same lap, incremented index.
                        // Set to `{ lap: lap, mark: 0, index: index + 1 }`.
                        head + 1
                    } else {
                        // One lap forward, index wraps around to zero.
                        // Set to `{ lap: lap.wrapping_add(1), mark: 0, index: 0 }`.
                        lap.wrapping_add(self.one_lap)
                    };

                    // Try moving the head.
                    match self
                        .head
                        .compare_exchange_weak(head, new, Ordering::SeqCst, Ordering::Relaxed)
                    {
                        Ok(_) => {
                            // Prepare the token for the follow-up call to `read`.
                            token.array.slot = slot as *const Slot<T> as *const u8;
                            token.array.stamp = head.wrapping_add(self.one_lap);
                            return true;
                        }
                        Err(h) => {
                            head = h;
                            backoff.spin();
                        }
                    }
                } else if stamp == head {
                    atomic::fence(Ordering::SeqCst);
                    let tail = self.tail.load(Ordering::Relaxed);

                    // If the tail equals the head, that means the channel is empty.
                    if (tail & !self.mark_bit) == head {
                        // If the channel is disconnected...
                        if tail & self.mark_bit != 0 {
                            // ...then receive an error.
                            token.array.slot = ptr::null();
                            token.array.stamp = 0;
                            return true;
                        } else {
                            // Otherwise, the receive operation is not ready.
                            return false;
                        }
                    }

                    backoff.spin();
                    head = self.head.load(Ordering::Relaxed);
                } else {
                    // Snooze because we need to wait for the stamp to get updated.
                    backoff.snooze();
                    head = self.head.load(Ordering::Relaxed);
                }
            }
        }

        /// Reads a message from the channel.
        pub unsafe fn read(&self, token: &mut Token) -> Result<T, ()> {
            if token.array.slot.is_null() {
                // The channel is disconnected.
                return Err(());
            }

            let slot: &Slot<T> = &*(token.array.slot as *const Slot<T>);

            // Read the message from the slot and update the stamp.
            let msg = slot.msg.get().read();
            slot.stamp.store(token.array.stamp, Ordering::Release);

            // Wake a sleeping sender.
            self.senders.notify();
            Ok(msg)
        }

        /// Attempts to send a message into the channel.
        pub fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
            let token = &mut Token::default();
            if self.start_send(token) {
                unsafe { self.write(token, msg).map_err(TrySendError::Disconnected) }
            } else {
                Err(TrySendError::Full(msg))
            }
        }

        /// Sends a message into the channel.
        pub fn send(&self, msg: T, deadline: Option<Instant>) -> Result<(), SendTimeoutError<T>> {
            let token = &mut Token::default();
            loop {
                // Try sending a message several times.
                let backoff = Backoff::new();
                loop {
                    if self.start_send(token) {
                        let res = unsafe { self.write(token, msg) };
                        return res.map_err(SendTimeoutError::Disconnected);
                    }

                    if backoff.is_completed() {
                        break;
                    } else {
                        backoff.snooze();
                    }
                }

                Context::with(|cx| {
                    // Prepare for blocking until a receiver wakes us up.
                    let oper = Operation::hook(token);
                    self.senders.register(oper, cx);

                    // Has the channel become ready just now?
                    if !self.is_full() || self.is_disconnected() {
                        let _ = cx.try_select(Selected::Aborted);
                    }

                    // Block the current thread.
                    let sel = cx.wait_until(deadline);

                    match sel {
                        Selected::Waiting => unreachable!(),
                        Selected::Aborted | Selected::Disconnected => {
                            self.senders.unregister(oper).unwrap();
                        }
                        Selected::Operation(_) => {}
                    }
                });

                if let Some(d) = deadline {
                    if Instant::now() >= d {
                        return Err(SendTimeoutError::Timeout(msg));
                    }
                }
            }
        }

        /// Attempts to receive a message without blocking.
        pub fn try_recv(&self) -> Result<T, TryRecvError> {
            let token = &mut Token::default();

            if self.start_recv(token) {
                unsafe { self.read(token).map_err(|_| TryRecvError::Disconnected) }
            } else {
                Err(TryRecvError::Empty)
            }
        }

        /// Receives a message from the channel.
        pub fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> {
            let token = &mut Token::default();
            loop {
                // Try receiving a message several times.
                let backoff = Backoff::new();
                loop {
                    if self.start_recv(token) {
                        let res = unsafe { self.read(token) };
                        return res.map_err(|_| RecvTimeoutError::Disconnected);
                    }

                    if backoff.is_completed() {
                        break;
                    } else {
                        backoff.snooze();
                    }
                }

                Context::with(|cx| {
                    // Prepare for blocking until a sender wakes us up.
                    let oper = Operation::hook(token);
                    self.receivers.register(oper, cx);

                    // Has the channel become ready just now?
                    if !self.is_empty() || self.is_disconnected() {
                        let _ = cx.try_select(Selected::Aborted);
                    }

                    // Block the current thread.
                    let sel = cx.wait_until(deadline);

                    match sel {
                        Selected::Waiting => unreachable!(),
                        Selected::Aborted | Selected::Disconnected => {
                            self.receivers.unregister(oper).unwrap();
                            // If the channel was disconnected, we still have to check for remaining
                            // messages.
                        }
                        Selected::Operation(_) => {}
                    }
                });

                if let Some(d) = deadline {
                    if Instant::now() >= d {
                        return Err(RecvTimeoutError::Timeout);
                    }
                }
            }
        }

        /// Returns the current number of messages inside the channel.
        pub fn len(&self) -> usize {
            loop {
                // Load the tail, then load the head.
                let tail = self.tail.load(Ordering::SeqCst);
                let head = self.head.load(Ordering::SeqCst);

                // If the tail didn't change, we've got consistent values to work with.
                if self.tail.load(Ordering::SeqCst) == tail {
                    let hix = head & (self.mark_bit - 1);
                    let tix = tail & (self.mark_bit - 1);

                    return if hix < tix {
                        tix - hix
                    } else if hix > tix {
                        self.cap - hix + tix
                    } else if (tail & !self.mark_bit) == head {
                        0
                    } else {
                        self.cap
                    };
                }
            }
        }

        /// Returns the capacity of the channel.
        pub fn capacity(&self) -> Option<usize> {
            Some(self.cap)
        }

        /// Disconnects the channel and wakes up all blocked receivers.
        pub fn disconnect(&self) {
            let tail = self.tail.fetch_or(self.mark_bit, Ordering::SeqCst);

            if tail & self.mark_bit == 0 {
                self.senders.disconnect();
                self.receivers.disconnect();
            }
        }

        /// Returns `true` if the channel is disconnected.
        pub fn is_disconnected(&self) -> bool {
            self.tail.load(Ordering::SeqCst) & self.mark_bit != 0
        }

        /// Returns `true` if the channel is empty.
        pub fn is_empty(&self) -> bool {
            let head = self.head.load(Ordering::SeqCst);
            let tail = self.tail.load(Ordering::SeqCst);

            // Is the tail equal to the head?
            //
            // Note: If the head changes just before we load the tail, that means there was a moment
            // when the channel was not empty, so it is safe to just return `false`.
            (tail & !self.mark_bit) == head
        }

        /// Returns `true` if the channel is full.
        pub fn is_full(&self) -> bool {
            let tail = self.tail.load(Ordering::SeqCst);
            let head = self.head.load(Ordering::SeqCst);

            // Is the head lagging one lap behind tail?
            //
            // Note: If the tail changes just before we load the head, that means there was a moment
            // when the channel was not full, so it is safe to just return `false`.
            head.wrapping_add(self.one_lap) == tail & !self.mark_bit
        }
    }

    impl<T> Drop for Channel<T> {
        fn drop(&mut self) {
            // Get the index of the head.
            let hix = self.head.load(Ordering::Relaxed) & (self.mark_bit - 1);

            // Loop over all slots that hold a message and drop them.
            for i in 0..self.len() {
                // Compute the index of the next slot holding a message.
                let index = if hix + i < self.cap {
                    hix + i
                } else {
                    hix + i - self.cap
                };

                unsafe {
                    self.buffer.add(index).drop_in_place();
                }
            }

            // Finally, deallocate the buffer, but don't run any destructors.
            unsafe {
                Vec::from_raw_parts(self.buffer, 0, self.cap);
            }
        }
    }

    /// Receiver handle to a channel.
    pub struct Receiver<'a, T: 'a>(&'a Channel<T>);

    /// Sender handle to a channel.
    pub struct Sender<'a, T: 'a>(&'a Channel<T>);

    impl<'a, T> SelectHandle for Receiver<'a, T> {
        fn try_select(&self, token: &mut Token) -> bool {
            self.0.start_recv(token)
        }

        fn deadline(&self) -> Option<Instant> {
            None
        }

        fn register(&self, oper: Operation, cx: &Context) -> bool {
            self.0.receivers.register(oper, cx);
            self.is_ready()
        }

        fn unregister(&self, oper: Operation) {
            self.0.receivers.unregister(oper);
        }

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

        fn is_ready(&self) -> bool {
            !self.0.is_empty() || self.0.is_disconnected()
        }

        fn watch(&self, oper: Operation, cx: &Context) -> bool {
            self.0.receivers.watch(oper, cx);
            self.is_ready()
        }

        fn unwatch(&self, oper: Operation) {
            self.0.receivers.unwatch(oper);
        }
    }

    impl<'a, T> SelectHandle for Sender<'a, T> {
        fn try_select(&self, token: &mut Token) -> bool {
            self.0.start_send(token)
        }

        fn deadline(&self) -> Option<Instant> {
            None
        }

        fn register(&self, oper: Operation, cx: &Context) -> bool {
            self.0.senders.register(oper, cx);
            self.is_ready()
        }

        fn unregister(&self, oper: Operation) {
            self.0.senders.unregister(oper);
        }

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

        fn is_ready(&self) -> bool {
            !self.0.is_full() || self.0.is_disconnected()
        }

        fn watch(&self, oper: Operation, cx: &Context) -> bool {
            self.0.senders.watch(oper, cx);
            self.is_ready()
        }

        fn unwatch(&self, oper: Operation) {
            self.0.senders.unwatch(oper);
        }
    }

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