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    //! An unbounded set of futures.

    use std::cell::UnsafeCell;
    use std::fmt::{self, Debug};
    use std::iter::FromIterator;
    use std::marker::PhantomData;
    use std::mem;
    use std::ptr;
    use std::sync::atomic::Ordering::{Relaxed, SeqCst, Acquire, Release, AcqRel};
    use std::sync::atomic::{AtomicPtr, AtomicBool};
    use std::sync::{Arc, Weak};
    use std::usize;

    use {task, Stream, Future, Poll, Async};
    use executor::{Notify, UnsafeNotify, NotifyHandle};
    use task_impl::{self, AtomicTask};

    /// An unbounded set of futures.
    ///
    /// This "combinator" also serves a special function in this library, providing
    /// the ability to maintain a set of futures that and manage driving them all
    /// to completion.
    ///
    /// Futures are pushed into this set and their realized values are yielded as
    /// they are ready. This structure is optimized to manage a large number of
    /// futures. Futures managed by `FuturesUnordered` will only be polled when they
    /// generate notifications. This reduces the required amount of work needed to
    /// coordinate large numbers of futures.
    ///
    /// When a `FuturesUnordered` is first created, it does not contain any futures.
    /// Calling `poll` in this state will result in `Ok(Async::Ready(None))` to be
    /// returned. Futures are submitted to the set using `push`; however, the
    /// future will **not** be polled at this point. `FuturesUnordered` will only
    /// poll managed futures when `FuturesUnordered::poll` is called. As such, it
    /// is important to call `poll` after pushing new futures.
    ///
    /// If `FuturesUnordered::poll` returns `Ok(Async::Ready(None))` this means that
    /// the set is currently not managing any futures. A future may be submitted
    /// to the set at a later time. At that point, a call to
    /// `FuturesUnordered::poll` will either return the future's resolved value
    /// **or** `Ok(Async::NotReady)` if the future has not yet completed.
    ///
    /// Note that you can create a ready-made `FuturesUnordered` via the
    /// `futures_unordered` function in the `stream` module, or you can start with an
    /// empty set with the `FuturesUnordered::new` constructor.
    #[must_use = "streams do nothing unless polled"]
    pub struct FuturesUnordered<F> {
        inner: Arc<Inner<F>>,
        len: usize,
        head_all: *const Node<F>,
    }

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

    // FuturesUnordered is implemented using two linked lists. One which links all
    // futures managed by a `FuturesUnordered` and one that tracks futures that have
    // been scheduled for polling. The first linked list is not thread safe and is
    // only accessed by the thread that owns the `FuturesUnordered` value. The
    // second linked list is an implementation of the intrusive MPSC queue algorithm
    // described by 1024cores.net.
    //
    // When a future is submitted to the set a node is allocated and inserted in
    // both linked lists. The next call to `poll` will (eventually) see this node
    // and call `poll` on the future.
    //
    // Before a managed future is polled, the current task's `Notify` is replaced
    // with one that is aware of the specific future being run. This ensures that
    // task notifications generated by that specific future are visible to
    // `FuturesUnordered`. When a notification is received, the node is scheduled
    // for polling by being inserted into the concurrent linked list.
    //
    // Each node uses an `AtomicUsize` to track it's state. The node state is the
    // reference count (the number of outstanding handles to the node) as well as a
    // flag tracking if the node is currently inserted in the atomic queue. When the
    // future is notified, it will only insert itself into the linked list if it
    // isn't currently inserted.

    #[allow(missing_debug_implementations)]
    struct Inner<T> {
        // The task using `FuturesUnordered`.
        parent: AtomicTask,

        // Head/tail of the readiness queue
        head_readiness: AtomicPtr<Node<T>>,
        tail_readiness: UnsafeCell<*const Node<T>>,
        stub: Arc<Node<T>>,
    }

    struct Node<T> {
        // The future
        future: UnsafeCell<Option<T>>,

        // Next pointer for linked list tracking all active nodes
        next_all: UnsafeCell<*const Node<T>>,

        // Previous node in linked list tracking all active nodes
        prev_all: UnsafeCell<*const Node<T>>,

        // Next pointer in readiness queue
        next_readiness: AtomicPtr<Node<T>>,

        // Queue that we'll be enqueued to when notified
        queue: Weak<Inner<T>>,

        // Whether or not this node is currently in the mpsc queue.
        queued: AtomicBool,
    }

    enum Dequeue<T> {
        Data(*const Node<T>),
        Empty,
        Inconsistent,
    }

    impl<T> FuturesUnordered<T>
        where T: Future,
    {
        /// Constructs a new, empty `FuturesUnordered`
        ///
        /// The returned `FuturesUnordered` does not contain any futures and, in this
        /// state, `FuturesUnordered::poll` will return `Ok(Async::Ready(None))`.
        pub fn new() -> FuturesUnordered<T> {
            let stub = Arc::new(Node {
                future: UnsafeCell::new(None),
                next_all: UnsafeCell::new(ptr::null()),
                prev_all: UnsafeCell::new(ptr::null()),
                next_readiness: AtomicPtr::new(ptr::null_mut()),
                queued: AtomicBool::new(true),
                queue: Weak::new(),
            });
            let stub_ptr = &*stub as *const Node<T>;
            let inner = Arc::new(Inner {
                parent: AtomicTask::new(),
                head_readiness: AtomicPtr::new(stub_ptr as *mut _),
                tail_readiness: UnsafeCell::new(stub_ptr),
                stub: stub,
            });

            FuturesUnordered {
                len: 0,
                head_all: ptr::null_mut(),
                inner: inner,
            }
        }
    }

    impl<T> FuturesUnordered<T> {
        /// Returns the number of futures contained in the set.
        ///
        /// This represents the total number of in-flight futures.
        pub fn len(&self) -> usize {
            self.len
        }

        /// Returns `true` if the set contains no futures
        pub fn is_empty(&self) -> bool {
            self.len == 0
        }

        /// Push a future into the set.
        ///
        /// This function submits the given future to the set for managing. This
        /// function will not call `poll` on the submitted future. The caller must
        /// ensure that `FuturesUnordered::poll` is called in order to receive task
        /// notifications.
        pub fn push(&mut self, future: T) {
            let node = Arc::new(Node {
                future: UnsafeCell::new(Some(future)),
                next_all: UnsafeCell::new(ptr::null_mut()),
                prev_all: UnsafeCell::new(ptr::null_mut()),
                next_readiness: AtomicPtr::new(ptr::null_mut()),
                queued: AtomicBool::new(true),
                queue: Arc::downgrade(&self.inner),
            });

            // Right now our node has a strong reference count of 1. We transfer
            // ownership of this reference count to our internal linked list
            // and we'll reclaim ownership through the `unlink` function below.
            let ptr = self.link(node);

            // We'll need to get the future "into the system" to start tracking it,
            // e.g. getting its unpark notifications going to us tracking which
            // futures are ready. To do that we unconditionally enqueue it for
            // polling here.
            self.inner.enqueue(ptr);
        }

        /// Returns an iterator that allows modifying each future in the set.
        pub fn iter_mut(&mut self) -> IterMut<T> {
            IterMut {
                node: self.head_all,
                len: self.len,
                _marker: PhantomData
            }
        }

        fn release_node(&mut self, node: Arc<Node<T>>) {
            // The future is done, try to reset the queued flag. This will prevent
            // `notify` from doing any work in the future
            let prev = node.queued.swap(true, SeqCst);

            // Drop the future, even if it hasn't finished yet. This is safe
            // because we're dropping the future on the thread that owns
            // `FuturesUnordered`, which correctly tracks T's lifetimes and such.
            unsafe {
                drop((*node.future.get()).take());
            }

            // If the queued flag was previously set then it means that this node
            // is still in our internal mpsc queue. We then transfer ownership
            // of our reference count to the mpsc queue, and it'll come along and
            // free it later, noticing that the future is `None`.
            //
            // If, however, the queued flag was *not* set then we're safe to
            // release our reference count on the internal node. The queued flag
            // was set above so all future `enqueue` operations will not actually
            // enqueue the node, so our node will never see the mpsc queue again.
            // The node itself will be deallocated once all reference counts have
            // been dropped by the various owning tasks elsewhere.
            if prev {
                mem::forget(node);
            }
        }

        /// Insert a new node into the internal linked list.
        fn link(&mut self, node: Arc<Node<T>>) -> *const Node<T> {
            let ptr = arc2ptr(node);
            unsafe {
                *(*ptr).next_all.get() = self.head_all;
                if !self.head_all.is_null() {
                    *(*self.head_all).prev_all.get() = ptr;
                }
            }

            self.head_all = ptr;
            self.len += 1;
            return ptr
        }

        /// Remove the node from the linked list tracking all nodes currently
        /// managed by `FuturesUnordered`.
        unsafe fn unlink(&mut self, node: *const Node<T>) -> Arc<Node<T>> {
            let node = ptr2arc(node);
            let next = *node.next_all.get();
            let prev = *node.prev_all.get();
            *node.next_all.get() = ptr::null_mut();
            *node.prev_all.get() = ptr::null_mut();

            if !next.is_null() {
                *(*next).prev_all.get() = prev;
            }

            if !prev.is_null() {
                *(*prev).next_all.get() = next;
            } else {
                self.head_all = next;
            }
            self.len -= 1;
            return node
        }
    }

    impl<T> Stream for FuturesUnordered<T>
        where T: Future
    {
        type Item = T::Item;
        type Error = T::Error;

        fn poll(&mut self) -> Poll<Option<T::Item>, T::Error> {
            // Ensure `parent` is correctly set.
            self.inner.parent.register();

            loop {
                let node = match unsafe { self.inner.dequeue() } {
                    Dequeue::Empty => {
                        if self.is_empty() {
                            return Ok(Async::Ready(None));
                        } else {
                            return Ok(Async::NotReady)
                        }
                    }
                    Dequeue::Inconsistent => {
                        // At this point, it may be worth yielding the thread &
                        // spinning a few times... but for now, just yield using the
                        // task system.
                        task::current().notify();
                        return Ok(Async::NotReady);
                    }
                    Dequeue::Data(node) => node,
                };

                debug_assert!(node != self.inner.stub());

                unsafe {
                    let mut future = match (*(*node).future.get()).take() {
                        Some(future) => future,

                        // If the future has already gone away then we're just
                        // cleaning out this node. See the comment in
                        // `release_node` for more information, but we're basically
                        // just taking ownership of our reference count here.
                        None => {
                            let node = ptr2arc(node);
                            assert!((*node.next_all.get()).is_null());
                            assert!((*node.prev_all.get()).is_null());
                            continue
                        }
                    };

                    // Unset queued flag... this must be done before
                    // polling. This ensures that the future gets
                    // rescheduled if it is notified **during** a call
                    // to `poll`.
                    let prev = (*node).queued.swap(false, SeqCst);
                    assert!(prev);

                    // We're going to need to be very careful if the `poll`
                    // function below panics. We need to (a) not leak memory and
                    // (b) ensure that we still don't have any use-after-frees. To
                    // manage this we do a few things:
                    //
                    // * This "bomb" here will call `release_node` if dropped
                    //   abnormally. That way we'll be sure the memory management
                    //   of the `node` is managed correctly.
                    // * The future was extracted above (taken ownership). That way
                    //   if it panics we're guaranteed that the future is
                    //   dropped on this thread and doesn't accidentally get
                    //   dropped on a different thread (bad).
                    // * We unlink the node from our internal queue to preemptively
                    //   assume it'll panic, in which case we'll want to discard it
                    //   regardless.
                    struct Bomb<'a, T: 'a> {
                        queue: &'a mut FuturesUnordered<T>,
                        node: Option<Arc<Node<T>>>,
                    }
                    impl<'a, T> Drop for Bomb<'a, T> {
                        fn drop(&mut self) {
                            if let Some(node) = self.node.take() {
                                self.queue.release_node(node);
                            }
                        }
                    }
                    let mut bomb = Bomb {
                        node: Some(self.unlink(node)),
                        queue: self,
                    };

                    // Poll the underlying future with the appropriate `notify`
                    // implementation. This is where a large bit of the unsafety
                    // starts to stem from internally. The `notify` instance itself
                    // is basically just our `Arc<Node<T>>` and tracks the mpsc
                    // queue of ready futures.
                    //
                    // Critically though `Node<T>` won't actually access `T`, the
                    // future, while it's floating around inside of `Task`
                    // instances. These structs will basically just use `T` to size
                    // the internal allocation, appropriately accessing fields and
                    // deallocating the node if need be.
                    let res = {
                        let notify = NodeToHandle(bomb.node.as_ref().unwrap());
                        task_impl::with_notify(&notify, 0, || {
                            future.poll()
                        })
                    };

                    let ret = match res {
                        Ok(Async::NotReady) => {
                            let node = bomb.node.take().unwrap();
                            *node.future.get() = Some(future);
                            bomb.queue.link(node);
                            continue
                        }
                        Ok(Async::Ready(e)) => Ok(Async::Ready(Some(e))),
                        Err(e) => Err(e),
                    };
                    return ret
                }
            }
        }
    }

    impl<T: Debug> Debug for FuturesUnordered<T> {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            write!(fmt, "FuturesUnordered {{ ... }}")
        }
    }

    impl<T> Drop for FuturesUnordered<T> {
        fn drop(&mut self) {
            // When a `FuturesUnordered` is dropped we want to drop all futures associated
            // with it. At the same time though there may be tons of `Task` handles
            // flying around which contain `Node<T>` references inside them. We'll
            // let those naturally get deallocated when the `Task` itself goes out
            // of scope or gets notified.
            unsafe {
                while !self.head_all.is_null() {
                    let head = self.head_all;
                    let node = self.unlink(head);
                    self.release_node(node);
                }
            }

            // Note that at this point we could still have a bunch of nodes in the
            // mpsc queue. None of those nodes, however, have futures associated
            // with them so they're safe to destroy on any thread. At this point
            // the `FuturesUnordered` struct, the owner of the one strong reference
            // to `Inner<T>` will drop the strong reference. At that point
            // whichever thread releases the strong refcount last (be it this
            // thread or some other thread as part of an `upgrade`) will clear out
            // the mpsc queue and free all remaining nodes.
            //
            // While that freeing operation isn't guaranteed to happen here, it's
            // guaranteed to happen "promptly" as no more "blocking work" will
            // happen while there's a strong refcount held.
        }
    }

    impl<F: Future> FromIterator<F> for FuturesUnordered<F> {
        fn from_iter<T>(iter: T) -> Self
            where T: IntoIterator<Item = F>
        {
            let mut new = FuturesUnordered::new();
            for future in iter.into_iter() {
                new.push(future);
            }
            new
        }
    }

    #[derive(Debug)]
    /// Mutable iterator over all futures in the unordered set.
    pub struct IterMut<'a, F: 'a> {
        node: *const Node<F>,
        len: usize,
        _marker: PhantomData<&'a mut FuturesUnordered<F>>
    }

    impl<'a, F> Iterator for IterMut<'a, F> {
        type Item = &'a mut F;

        fn next(&mut self) -> Option<&'a mut F> {
            if self.node.is_null() {
                return None;
            }
            unsafe {
                let future = (*(*self.node).future.get()).as_mut().unwrap();
                let next = *(*self.node).next_all.get();
                self.node = next;
                self.len -= 1;
                return Some(future);
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (self.len, Some(self.len))
        }
    }

    impl<'a, F> ExactSizeIterator for IterMut<'a, F> {}

    impl<T> Inner<T> {
        /// The enqueue function from the 1024cores intrusive MPSC queue algorithm.
        fn enqueue(&self, node: *const Node<T>) {
            unsafe {
                debug_assert!((*node).queued.load(Relaxed));

                // This action does not require any coordination
                (*node).next_readiness.store(ptr::null_mut(), Relaxed);

                // Note that these atomic orderings come from 1024cores
                let node = node as *mut _;
                let prev = self.head_readiness.swap(node, AcqRel);
                (*prev).next_readiness.store(node, Release);
            }
        }

        /// The dequeue function from the 1024cores intrusive MPSC queue algorithm
        ///
        /// Note that this unsafe as it required mutual exclusion (only one thread
        /// can call this) to be guaranteed elsewhere.
        unsafe fn dequeue(&self) -> Dequeue<T> {
            let mut tail = *self.tail_readiness.get();
            let mut next = (*tail).next_readiness.load(Acquire);

            if tail == self.stub() {
                if next.is_null() {
                    return Dequeue::Empty;
                }

                *self.tail_readiness.get() = next;
                tail = next;
                next = (*next).next_readiness.load(Acquire);
            }

            if !next.is_null() {
                *self.tail_readiness.get() = next;
                debug_assert!(tail != self.stub());
                return Dequeue::Data(tail);
            }

            if self.head_readiness.load(Acquire) as *const _ != tail {
                return Dequeue::Inconsistent;
            }

            self.enqueue(self.stub());

            next = (*tail).next_readiness.load(Acquire);

            if !next.is_null() {
                *self.tail_readiness.get() = next;
                return Dequeue::Data(tail);
            }

            Dequeue::Inconsistent
        }

        fn stub(&self) -> *const Node<T> {
            &*self.stub
        }
    }

    impl<T> Drop for Inner<T> {
        fn drop(&mut self) {
            // Once we're in the destructor for `Inner<T>` we need to clear out the
            // mpsc queue of nodes if there's anything left in there.
            //
            // Note that each node has a strong reference count associated with it
            // which is owned by the mpsc queue. All nodes should have had their
            // futures dropped already by the `FuturesUnordered` destructor above,
            // so we're just pulling out nodes and dropping their refcounts.
            unsafe {
                loop {
                    match self.dequeue() {
                        Dequeue::Empty => break,
                        Dequeue::Inconsistent => abort("inconsistent in drop"),
                        Dequeue::Data(ptr) => drop(ptr2arc(ptr)),
                    }
                }
            }
        }
    }

    #[allow(missing_debug_implementations)]
    struct NodeToHandle<'a, T: 'a>(&'a Arc<Node<T>>);

    impl<'a, T> Clone for NodeToHandle<'a, T> {
        fn clone(&self) -> Self {
            NodeToHandle(self.0)
        }
    }

    impl<'a, T> From<NodeToHandle<'a, T>> for NotifyHandle {
        fn from(handle: NodeToHandle<'a, T>) -> NotifyHandle {
            unsafe {
                let ptr = handle.0.clone();
                let ptr = mem::transmute::<Arc<Node<T>>, *mut ArcNode<T>>(ptr);
                NotifyHandle::new(hide_lt(ptr))
            }
        }
    }

    struct ArcNode<T>(PhantomData<T>);

    // We should never touch `T` on any thread other than the one owning
    // `FuturesUnordered`, so this should be a safe operation.
    unsafe impl<T> Send for ArcNode<T> {}
    unsafe impl<T> Sync for ArcNode<T> {}

    impl<T> Notify for ArcNode<T> {
        fn notify(&self, _id: usize) {
            unsafe {
                let me: *const ArcNode<T> = self;
                let me: *const *const ArcNode<T> = &me;
                let me = me as *const Arc<Node<T>>;
                Node::notify(&*me)
            }
        }
    }

    unsafe impl<T> UnsafeNotify for ArcNode<T> {
        unsafe fn clone_raw(&self) -> NotifyHandle {
            let me: *const ArcNode<T> = self;
            let me: *const *const ArcNode<T> = &me;
            let me = &*(me as *const Arc<Node<T>>);
            NodeToHandle(me).into()
        }

        unsafe fn drop_raw(&self) {
            let mut me: *const ArcNode<T> = self;
            let me = &mut me as *mut *const ArcNode<T> as *mut Arc<Node<T>>;
            ptr::drop_in_place(me);
        }
    }

    unsafe fn hide_lt<T>(p: *mut ArcNode<T>) -> *mut UnsafeNotify {
        mem::transmute(p as *mut UnsafeNotify)
    }

    impl<T> Node<T> {
        fn notify(me: &Arc<Node<T>>) {
            let inner = match me.queue.upgrade() {
                Some(inner) => inner,
                None => return,
            };

            // It's our job to notify the node that it's ready to get polled,
            // meaning that we need to enqueue it into the readiness queue. To
            // do this we flag that we're ready to be queued, and if successful
            // we then do the literal queueing operation, ensuring that we're
            // only queued once.
            //
            // Once the node is inserted we be sure to notify the parent task,
            // as it'll want to come along and pick up our node now.
            //
            // Note that we don't change the reference count of the node here,
            // we're just enqueueing the raw pointer. The `FuturesUnordered`
            // implementation guarantees that if we set the `queued` flag true that
            // there's a reference count held by the main `FuturesUnordered` queue
            // still.
            let prev = me.queued.swap(true, SeqCst);
            if !prev {
                inner.enqueue(&**me);
                inner.parent.notify();
            }
        }
    }

    impl<T> Drop for Node<T> {
        fn drop(&mut self) {
            // Currently a `Node<T>` is sent across all threads for any lifetime,
            // regardless of `T`. This means that for memory safety we can't
            // actually touch `T` at any time except when we have a reference to the
            // `FuturesUnordered` itself.
            //
            // Consequently it *should* be the case that we always drop futures from
            // the `FuturesUnordered` instance, but this is a bomb in place to catch
            // any bugs in that logic.
            unsafe {
                if (*self.future.get()).is_some() {
                    abort("future still here when dropping");
                }
            }
        }
    }

    fn arc2ptr<T>(ptr: Arc<T>) -> *const T {
        let addr = &*ptr as *const T;
        mem::forget(ptr);
        return addr
    }

    unsafe fn ptr2arc<T>(ptr: *const T) -> Arc<T> {
        let anchor = mem::transmute::<usize, Arc<T>>(0x10);
        let addr = &*anchor as *const T;
        mem::forget(anchor);
        let offset = addr as isize - 0x10;
        mem::transmute::<isize, Arc<T>>(ptr as isize - offset)
    }

    fn abort(s: &str) -> ! {
        struct DoublePanic;

        impl Drop for DoublePanic {
            fn drop(&mut self) {
                panic!("panicking twice to abort the program");
            }
        }

        let _bomb = DoublePanic;
        panic!("{}", s);
    }

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