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    //! 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.

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

    #[cfg(all(test, not(target_os = "emscripten")))]
    mod tests;

    use core::cell::UnsafeCell;
    use core::ptr;

    use crate::boxed::Box;
    use crate::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};

    use super::cache_aligned::CacheAligned;

    // 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
        cached: bool,             // This node goes into the node cache
        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, ProducerAddition = (), ConsumerAddition = ()> {
        // consumer fields
        consumer: CacheAligned<Consumer<T, ConsumerAddition>>,

        // producer fields
        producer: CacheAligned<Producer<T, ProducerAddition>>,
    }

    struct Consumer<T, Addition> {
        tail: UnsafeCell<*mut Node<T>>, // where to pop from
        tail_prev: AtomicPtr<Node<T>>,  // where to pop from
        cache_bound: usize,             // maximum cache size
        cached_nodes: AtomicUsize,      // number of nodes marked as cacheable
        addition: Addition,
    }

    struct Producer<T, Addition> {
        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
        addition: Addition,
    }

    unsafe impl<T: Send, P: Send + Sync, C: Send + Sync> Send for Queue<T, P, C> {}

    unsafe impl<T: Send, P: Send + Sync, C: Send + Sync> Sync for Queue<T, P, C> {}

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

    impl<T, ProducerAddition, ConsumerAddition> Queue<T, ProducerAddition, ConsumerAddition> {
        /// Creates a new queue. With given additional elements in the producer and
        /// consumer portions of the queue.
        ///
        /// Due to the performance implications of cache-contention,
        /// we wish to keep fields used mainly by the producer on a separate cache
        /// line than those used by the consumer.
        /// Since cache lines are usually 64 bytes, it is unreasonably expensive to
        /// allocate one for small fields, so we allow users to insert additional
        /// fields into the cache lines already allocated by this for the producer
        /// and consumer.
        ///
        /// 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 with_additions(
            bound: usize,
            producer_addition: ProducerAddition,
            consumer_addition: ConsumerAddition,
        ) -> Self {
            let n1 = Node::new();
            let n2 = Node::new();
            (*n1).next.store(n2, Ordering::Relaxed);
            Queue {
                consumer: CacheAligned::new(Consumer {
                    tail: UnsafeCell::new(n2),
                    tail_prev: AtomicPtr::new(n1),
                    cache_bound: bound,
                    cached_nodes: AtomicUsize::new(0),
                    addition: consumer_addition,
                }),
                producer: CacheAligned::new(Producer {
                    head: UnsafeCell::new(n2),
                    first: UnsafeCell::new(n1),
                    tail_copy: UnsafeCell::new(n1),
                    addition: producer_addition,
                }),
            }
        }

        /// 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.producer.head.get()).next.store(n, Ordering::Release);
                *(&self.producer.head).get() = n;
            }
        }

        unsafe fn alloc(&self) -> *mut Node<T> {
            // First try to see if we can consume the 'first' node for our uses.
            if *self.producer.first.get() != *self.producer.tail_copy.get() {
                let ret = *self.producer.first.get();
                *self.producer.0.first.get() = (*ret).next.load(Ordering::Relaxed);
                return ret;
            }
            // If the above fails, then update our copy of the tail and try
            // again.
            *self.producer.0.tail_copy.get() = self.consumer.tail_prev.load(Ordering::Acquire);
            if *self.producer.first.get() != *self.producer.tail_copy.get() {
                let ret = *self.producer.first.get();
                *self.producer.0.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.consumer.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.consumer.0.tail.get() = next;
                if self.consumer.cache_bound == 0 {
                    self.consumer.tail_prev.store(tail, Ordering::Release);
                } else {
                    let cached_nodes = self.consumer.cached_nodes.load(Ordering::Relaxed);
                    if cached_nodes < self.consumer.cache_bound && !(*tail).cached {
                        self.consumer.cached_nodes.store(cached_nodes, Ordering::Relaxed);
                        (*tail).cached = true;
                    }

                    if (*tail).cached {
                        self.consumer.tail_prev.store(tail, Ordering::Release);
                    } else {
                        (*self.consumer.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.consumer.tail.get();
                let next = (*tail).next.load(Ordering::Acquire);
                if next.is_null() { None } else { (*next).value.as_mut() }
            }
        }

        pub fn producer_addition(&self) -> &ProducerAddition {
            &self.producer.addition
        }

        pub fn consumer_addition(&self) -> &ConsumerAddition {
            &self.consumer.addition
        }
    }

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