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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.
     */

    // http://www.1024cores.net/home/lock-free-algorithms/queues/unbounded-spsc-queue

    //! A single-producer single-consumer concurrent queue
    //!
    //! This module contains the implementation of an SPSC queue which can be used
    //! concurrently between two threads. This data structure is safe to use and
    //! enforces the semantics that there is one pusher and one popper.

    use alloc::boxed::Box;
    use core::ptr;
    use core::cell::UnsafeCell;

    use sync::atomic::{AtomicPtr, AtomicUsize, Ordering};

    // Node within the linked list queue of messages to send
    struct Node<T> {
        // FIXME: this could be an uninitialized T if we're careful enough, and
        //      that would reduce memory usage (and be a bit faster).
        //      is it worth it?
        value: Option<T>,           // nullable for re-use of nodes
        next: AtomicPtr<Node<T>>,   // next node in the queue
    }

    /// The single-producer single-consumer queue. This structure is not cloneable,
    /// but it can be safely shared in an Arc if it is guaranteed that there
    /// is only one popper and one pusher touching the queue at any one point in
    /// time.
    pub struct Queue<T> {
        // consumer fields
        tail: UnsafeCell<*mut Node<T>>, // where to pop from
        tail_prev: AtomicPtr<Node<T>>, // where to pop from

        // producer fields
        head: UnsafeCell<*mut Node<T>>,      // where to push to
        first: UnsafeCell<*mut Node<T>>,     // where to get new nodes from
        tail_copy: UnsafeCell<*mut Node<T>>, // between first/tail

        // Cache maintenance fields. Additions and subtractions are stored
        // separately in order to allow them to use nonatomic addition/subtraction.
        cache_bound: usize,
        cache_additions: AtomicUsize,
        cache_subtractions: AtomicUsize,
    }

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

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

    impl<T> Node<T> {
        fn new() -> *mut Node<T> {
            Box::into_raw(box Node {
                value: None,
                next: AtomicPtr::new(ptr::null_mut::<Node<T>>()),
            })
        }
    }

    impl<T> Queue<T> {
        /// Creates a new queue.
        ///
        /// This is unsafe as the type system doesn't enforce a single
        /// consumer-producer relationship. It also allows the consumer to `pop`
        /// items while there is a `peek` active due to all methods having a
        /// non-mutable receiver.
        ///
        /// # Arguments
        ///
        ///   * `bound` - This queue implementation is implemented with a linked
        ///               list, and this means that a push is always a malloc. In
        ///               order to amortize this cost, an internal cache of nodes is
        ///               maintained to prevent a malloc from always being
        ///               necessary. This bound is the limit on the size of the
        ///               cache (if desired). If the value is 0, then the cache has
        ///               no bound. Otherwise, the cache will never grow larger than
        ///               `bound` (although the queue itself could be much larger.
        pub unsafe fn new(bound: usize) -> Queue<T> {
            let n1 = Node::new();
            let n2 = Node::new();
            (*n1).next.store(n2, Ordering::Relaxed);
            Queue {
                tail: UnsafeCell::new(n2),
                tail_prev: AtomicPtr::new(n1),
                head: UnsafeCell::new(n2),
                first: UnsafeCell::new(n1),
                tail_copy: UnsafeCell::new(n1),
                cache_bound: bound,
                cache_additions: AtomicUsize::new(0),
                cache_subtractions: AtomicUsize::new(0),
            }
        }

        /// Pushes a new value onto this queue. Note that to use this function
        /// safely, it must be externally guaranteed that there is only one pusher.
        pub fn push(&self, t: T) {
            unsafe {
                // Acquire a node (which either uses a cached one or allocates a new
                // one), and then append this to the 'head' node.
                let n = self.alloc();
                assert!((*n).value.is_none());
                (*n).value = Some(t);
                (*n).next.store(ptr::null_mut(), Ordering::Relaxed);
                (**self.head.get()).next.store(n, Ordering::Release);
                *self.head.get() = n;
            }
        }

        unsafe fn alloc(&self) -> *mut Node<T> {
            // First try to see if we can consume the 'first' node for our uses.
            // We try to avoid as many atomic instructions as possible here, so
            // the addition to cache_subtractions is not atomic (plus we're the
            // only one subtracting from the cache).
            if *self.first.get() != *self.tail_copy.get() {
                if self.cache_bound > 0 {
                    let b = self.cache_subtractions.load(Ordering::Relaxed);
                    self.cache_subtractions.store(b + 1, Ordering::Relaxed);
                }
                let ret = *self.first.get();
                *self.first.get() = (*ret).next.load(Ordering::Relaxed);
                return ret;
            }
            // If the above fails, then update our copy of the tail and try
            // again.
            *self.tail_copy.get() = self.tail_prev.load(Ordering::Acquire);
            if *self.first.get() != *self.tail_copy.get() {
                if self.cache_bound > 0 {
                    let b = self.cache_subtractions.load(Ordering::Relaxed);
                    self.cache_subtractions.store(b + 1, Ordering::Relaxed);
                }
                let ret = *self.first.get();
                *self.first.get() = (*ret).next.load(Ordering::Relaxed);
                return ret;
            }
            // If all of that fails, then we have to allocate a new node
            // (there's nothing in the node cache).
            Node::new()
        }

        /// Attempts to pop a value from this queue. Remember that to use this type
        /// safely you must ensure that there is only one popper at a time.
        pub fn pop(&self) -> Option<T> {
            unsafe {
                // The `tail` node is not actually a used node, but rather a
                // sentinel from where we should start popping from. Hence, look at
                // tail's next field and see if we can use it. If we do a pop, then
                // the current tail node is a candidate for going into the cache.
                let tail = *self.tail.get();
                let next = (*tail).next.load(Ordering::Acquire);
                if next.is_null() { return None }
                assert!((*next).value.is_some());
                let ret = (*next).value.take();

                *self.tail.get() = next;
                if self.cache_bound == 0 {
                    self.tail_prev.store(tail, Ordering::Release);
                } else {
                    // FIXME: this is dubious with overflow.
                    let additions = self.cache_additions.load(Ordering::Relaxed);
                    let subtractions = self.cache_subtractions.load(Ordering::Relaxed);
                    let size = additions - subtractions;

                    if size < self.cache_bound {
                        self.tail_prev.store(tail, Ordering::Release);
                        self.cache_additions.store(additions + 1, Ordering::Relaxed);
                    } else {
                        (*self.tail_prev.load(Ordering::Relaxed))
                              .next.store(next, Ordering::Relaxed);
                        // We have successfully erased all references to 'tail', so
                        // now we can safely drop it.
                        let _: Box<Node<T>> = Box::from_raw(tail);
                    }
                }
                ret
            }
        }

        /// Attempts to peek at the head of the queue, returning `None` if the queue
        /// has no data currently
        ///
        /// # Warning
        /// The reference returned is invalid if it is not used before the consumer
        /// pops the value off the queue. If the producer then pushes another value
        /// onto the queue, it will overwrite the value pointed to by the reference.
        pub fn peek(&self) -> Option<&mut T> {
            // This is essentially the same as above with all the popping bits
            // stripped out.
            unsafe {
                let tail = *self.tail.get();
                let next = (*tail).next.load(Ordering::Acquire);
                if next.is_null() { None } else { (*next).value.as_mut() }
            }
        }
    }

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

    #[cfg(all(test, not(target_os = "emscripten")))]
    mod tests {
        use sync::Arc;
        use super::Queue;
        use thread;
        use sync::mpsc::channel;

        #[test]
        fn smoke() {
            unsafe {
                let queue = Queue::new(0);
                queue.push(1);
                queue.push(2);
                assert_eq!(queue.pop(), Some(1));
                assert_eq!(queue.pop(), Some(2));
                assert_eq!(queue.pop(), None);
                queue.push(3);
                queue.push(4);
                assert_eq!(queue.pop(), Some(3));
                assert_eq!(queue.pop(), Some(4));
                assert_eq!(queue.pop(), None);
            }
        }

        #[test]
        fn peek() {
            unsafe {
                let queue = Queue::new(0);
                queue.push(vec![1]);

                // Ensure the borrowchecker works
                match queue.peek() {
                    Some(vec) => {
                        assert_eq!(&*vec, &[1]);
                    },
                    None => unreachable!()
                }

                match queue.pop() {
                    Some(vec) => {
                        assert_eq!(&*vec, &[1]);
                    },
                    None => unreachable!()
                }
            }
        }

        #[test]
        fn drop_full() {
            unsafe {
                let q: Queue<Box<_>> = Queue::new(0);
                q.push(box 1);
                q.push(box 2);
            }
        }

        #[test]
        fn smoke_bound() {
            unsafe {
                let q = Queue::new(0);
                q.push(1);
                q.push(2);
                assert_eq!(q.pop(), Some(1));
                assert_eq!(q.pop(), Some(2));
                assert_eq!(q.pop(), None);
                q.push(3);
                q.push(4);
                assert_eq!(q.pop(), Some(3));
                assert_eq!(q.pop(), Some(4));
                assert_eq!(q.pop(), None);
            }
        }

        #[test]
        fn stress() {
            unsafe {
                stress_bound(0);
                stress_bound(1);
            }

            unsafe fn stress_bound(bound: usize) {
                let q = Arc::new(Queue::new(bound));

                let (tx, rx) = channel();
                let q2 = q.clone();
                let _t = thread::spawn(move|| {
                    for _ in 0..100000 {
                        loop {
                            match q2.pop() {
                                Some(1) => break,
                                Some(_) => panic!(),
                                None => {}
                            }
                        }
                    }
                    tx.send(()).unwrap();
                });
                for _ in 0..100000 {
                    q.push(1);
                }
                rx.recv().unwrap();
            }
        }
    }

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