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    /* Copyright (c) 2010-2011 Dmitry Vyukov. All rights reserved.
     * Redistribution and use in source and binary forms, with or without
     * modification, are permitted provided that the following conditions are met:
     *
     *    1. Redistributions of source code must retain the above copyright notice,
     *       this list of conditions and the following disclaimer.
     *
     *    2. Redistributions in binary form must reproduce the above copyright
     *       notice, this list of conditions and the following disclaimer in the
     *       documentation and/or other materials provided with the distribution.
     *
     * THIS SOFTWARE IS PROVIDED BY DMITRY VYUKOV "AS IS" AND ANY EXPRESS OR IMPLIED
     * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
     * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
     * SHALL DMITRY VYUKOV OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
     * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
     * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
     * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
     * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
     * OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
     * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     *
     * The views and conclusions contained in the software and documentation are
     * those of the authors and should not be interpreted as representing official
     * policies, either expressed or implied, of Dmitry Vyukov.
     */

    //! A mostly lock-free multi-producer, single consumer queue.
    //!
    //! This module contains an implementation of a concurrent MPSC queue. This
    //! queue can be used to share data between threads, and is also used as the
    //! building block of channels in rust.
    //!
    //! Note that the current implementation of this queue has a caveat of the `pop`
    //! method, and see the method for more information about it. Due to this
    //! caveat, this queue may not be appropriate for all use-cases.

    // http://www.1024cores.net/home/lock-free-algorithms
    //                         /queues/non-intrusive-mpsc-node-based-queue

    // NOTE: this implementation is lifted from the standard library and only
    //       slightly modified

    pub use self::PopResult::*;
    use std::prelude::v1::*;

    use std::cell::UnsafeCell;
    use std::ptr;
    use std::sync::atomic::{AtomicPtr, Ordering};

    /// A result of the `pop` function.
    pub enum PopResult<T> {
        /// Some data has been popped
        Data(T),
        /// The queue is empty
        Empty,
        /// The queue is in an inconsistent state. Popping data should succeed, but
        /// some pushers have yet to make enough progress in order allow a pop to
        /// succeed. It is recommended that a pop() occur "in the near future" in
        /// order to see if the sender has made progress or not
        Inconsistent,
    }

    #[derive(Debug)]
    struct Node<T> {
        next: AtomicPtr<Node<T>>,
        value: Option<T>,
    }

    /// The multi-producer single-consumer structure. This is not cloneable, but it
    /// may be safely shared so long as it is guaranteed that there is only one
    /// popper at a time (many pushers are allowed).
    #[derive(Debug)]
    pub struct Queue<T> {
        head: AtomicPtr<Node<T>>,
        tail: UnsafeCell<*mut Node<T>>,
    }

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

    impl<T> Node<T> {
        unsafe fn new(v: Option<T>) -> *mut Node<T> {
            Box::into_raw(Box::new(Node {
                next: AtomicPtr::new(ptr::null_mut()),
                value: v,
            }))
        }
    }

    impl<T> Queue<T> {
        /// Creates a new queue that is safe to share among multiple producers and
        /// one consumer.
        pub fn new() -> Queue<T> {
            let stub = unsafe { Node::new(None) };
            Queue {
                head: AtomicPtr::new(stub),
                tail: UnsafeCell::new(stub),
            }
        }

        /// Pushes a new value onto this queue.
        pub fn push(&self, t: T) {
            unsafe {
                let n = Node::new(Some(t));
                let prev = self.head.swap(n, Ordering::AcqRel);
                (*prev).next.store(n, Ordering::Release);
            }
        }

        /// Pops some data from this queue.
        ///
        /// Note that the current implementation means that this function cannot
        /// return `Option<T>`. It is possible for this queue to be in an
        /// inconsistent state where many pushes have succeeded and completely
        /// finished, but pops cannot return `Some(t)`. This inconsistent state
        /// happens when a pusher is preempted at an inopportune moment.
        ///
        /// This inconsistent state means that this queue does indeed have data, but
        /// it does not currently have access to it at this time.
        ///
        /// This function is unsafe because only one thread can call it at a time.
        pub unsafe fn pop(&self) -> PopResult<T> {
            let tail = *self.tail.get();
            let next = (*tail).next.load(Ordering::Acquire);

            if !next.is_null() {
                *self.tail.get() = next;
                assert!((*tail).value.is_none());
                assert!((*next).value.is_some());
                let ret = (*next).value.take().unwrap();
                drop(Box::from_raw(tail));
                return Data(ret);
            }

            if self.head.load(Ordering::Acquire) == tail {Empty} else {Inconsistent}
        }
    }

    impl<T> Drop for Queue<T> {
        fn drop(&mut self) {
            unsafe {
                let mut cur = *self.tail.get();
                while !cur.is_null() {
                    let next = (*cur).next.load(Ordering::Relaxed);
                    drop(Box::from_raw(cur));
                    cur = next;
                }
            }
        }
    }

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