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    use core::cell::UnsafeCell;
    use core::fmt;
    use core::sync::atomic::AtomicUsize;
    use core::sync::atomic::Ordering::{Acquire, Release, AcqRel};
    use core::task::Waker;

    /// A synchronization primitive for task wakeup.
    ///
    /// Sometimes the task interested in a given event will change over time.
    /// An `AtomicWaker` can coordinate concurrent notifications with the consumer
    /// potentially "updating" the underlying task to wake up. This is useful in
    /// scenarios where a computation completes in another thread and wants to
    /// notify the consumer, but the consumer is in the process of being migrated to
    /// a new logical task.
    ///
    /// Consumers should call `register` before checking the result of a computation
    /// and producers should call `wake` after producing the computation (this
    /// differs from the usual `thread::park` pattern). It is also permitted for
    /// `wake` to be called **before** `register`. This results in a no-op.
    ///
    /// A single `AtomicWaker` may be reused for any number of calls to `register` or
    /// `wake`.
    ///
    /// # Memory ordering
    ///
    /// Calling `register` "acquires" all memory "released" by calls to `wake`
    /// before the call to `register`.  Later calls to `wake` will wake the
    /// registered waker (on contention this wake might be triggered in `register`).
    ///
    /// For concurrent calls to `register` (should be avoided) the ordering is only
    /// guaranteed for the winning call.
    ///
    /// # Examples
    ///
    /// Here is a simple example providing a `Flag` that can be signalled manually
    /// when it is ready.
    ///
    /// ```
    /// use futures::future::Future;
    /// use futures::task::{Context, Poll, AtomicWaker};
    /// use std::sync::Arc;
    /// use std::sync::atomic::AtomicBool;
    /// use std::sync::atomic::Ordering::Relaxed;
    /// use std::pin::Pin;
    ///
    /// struct Inner {
    ///     waker: AtomicWaker,
    ///     set: AtomicBool,
    /// }
    ///
    /// #[derive(Clone)]
    /// struct Flag(Arc<Inner>);
    ///
    /// impl Flag {
    ///     pub fn new() -> Self {
    ///         Self(Arc::new(Inner {
    ///             waker: AtomicWaker::new(),
    ///             set: AtomicBool::new(false),
    ///         }))
    ///     }
    ///
    ///     pub fn signal(&self) {
    ///         self.0.set.store(true, Relaxed);
    ///         self.0.waker.wake();
    ///     }
    /// }
    ///
    /// impl Future for Flag {
    ///     type Output = ();
    ///
    ///     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
    ///         // quick check to avoid registration if already done.
    ///         if self.0.set.load(Relaxed) {
    ///             return Poll::Ready(());
    ///         }
    ///
    ///         self.0.waker.register(cx.waker());
    ///
    ///         // Need to check condition **after** `register` to avoid a race
    ///         // condition that would result in lost notifications.
    ///         if self.0.set.load(Relaxed) {
    ///             Poll::Ready(())
    ///         } else {
    ///             Poll::Pending
    ///         }
    ///     }
    /// }
    /// ```
    pub struct AtomicWaker {
        state: AtomicUsize,
        waker: UnsafeCell<Option<Waker>>,
    }

    // `AtomicWaker` is a multi-consumer, single-producer transfer cell. The cell
    // stores a `Waker` value produced by calls to `register` and many threads can
    // race to take the waker (to wake it) by calling `wake`.
    //
    // If a new `Waker` instance is produced by calling `register` before an
    // existing one is consumed, then the existing one is overwritten.
    //
    // While `AtomicWaker` is single-producer, the implementation ensures memory
    // safety. In the event of concurrent calls to `register`, there will be a
    // single winner whose waker will get stored in the cell. The losers will not
    // have their tasks woken. As such, callers should ensure to add synchronization
    // to calls to `register`.
    //
    // The implementation uses a single `AtomicUsize` value to coordinate access to
    // the `Waker` cell. There are two bits that are operated on independently.
    // These are represented by `REGISTERING` and `WAKING`.
    //
    // The `REGISTERING` bit is set when a producer enters the critical section. The
    // `WAKING` bit is set when a consumer enters the critical section. Neither bit
    // being set is represented by `WAITING`.
    //
    // A thread obtains an exclusive lock on the waker cell by transitioning the
    // state from `WAITING` to `REGISTERING` or `WAKING`, depending on the operation
    // the thread wishes to perform. When this transition is made, it is guaranteed
    // that no other thread will access the waker cell.
    //
    // # Registering
    //
    // On a call to `register`, an attempt to transition the state from WAITING to
    // REGISTERING is made. On success, the caller obtains a lock on the waker cell.
    //
    // If the lock is obtained, then the thread sets the waker cell to the waker
    // provided as an argument. Then it attempts to transition the state back from
    // `REGISTERING` -> `WAITING`.
    //
    // If this transition is successful, then the registering process is complete
    // and the next call to `wake` will observe the waker.
    //
    // If the transition fails, then there was a concurrent call to `wake` that was
    // unable to access the waker cell (due to the registering thread holding the
    // lock). To handle this, the registering thread removes the waker it just set
    // from the cell and calls `wake` on it. This call to wake represents the
    // attempt to wake by the other thread (that set the `WAKING` bit). The state is
    // then transitioned from `REGISTERING | WAKING` back to `WAITING`.  This
    // transition must succeed because, at this point, the state cannot be
    // transitioned by another thread.
    //
    // # Waking
    //
    // On a call to `wake`, an attempt to transition the state from `WAITING` to
    // `WAKING` is made. On success, the caller obtains a lock on the waker cell.
    //
    // If the lock is obtained, then the thread takes ownership of the current value
    // in the waker cell, and calls `wake` on it. The state is then transitioned
    // back to `WAITING`. This transition must succeed as, at this point, the state
    // cannot be transitioned by another thread.
    //
    // If the thread is unable to obtain the lock, the `WAKING` bit is still.  This
    // is because it has either been set by the current thread but the previous
    // value included the `REGISTERING` bit **or** a concurrent thread is in the
    // `WAKING` critical section. Either way, no action must be taken.
    //
    // If the current thread is the only concurrent call to `wake` and another
    // thread is in the `register` critical section, when the other thread **exits**
    // the `register` critical section, it will observe the `WAKING` bit and handle
    // the wake itself.
    //
    // If another thread is in the `wake` critical section, then it will handle
    // waking the task.
    //
    // # A potential race (is safely handled).
    //
    // Imagine the following situation:
    //
    // * Thread A obtains the `wake` lock and wakes a task.
    //
    // * Before thread A releases the `wake` lock, the woken task is scheduled.
    //
    // * Thread B attempts to wake the task. In theory this should result in the
    //   task being woken, but it cannot because thread A still holds the wake lock.
    //
    // This case is handled by requiring users of `AtomicWaker` to call `register`
    // **before** attempting to observe the application state change that resulted
    // in the task being awoken. The wakers also change the application state before
    // calling wake.
    //
    // Because of this, the waker will do one of two things.
    //
    // 1) Observe the application state change that Thread B is woken for. In this
    //    case, it is OK for Thread B's wake to be lost.
    //
    // 2) Call register before attempting to observe the application state. Since
    //    Thread A still holds the `wake` lock, the call to `register` will result
    //    in the task waking itself and get scheduled again.

    /// Idle state
    const WAITING: usize = 0;

    /// A new waker value is being registered with the `AtomicWaker` cell.
    const REGISTERING: usize = 0b01;

    /// The waker currently registered with the `AtomicWaker` cell is being woken.
    const WAKING: usize = 0b10;

    impl AtomicWaker {
        /// Create an `AtomicWaker`.
        pub const fn new() -> Self {
            // Make sure that task is Sync
            trait AssertSync: Sync {}
            impl AssertSync for Waker {}

            Self {
                state: AtomicUsize::new(WAITING),
                waker: UnsafeCell::new(None),
            }
        }

        /// Registers the waker to be notified on calls to `wake`.
        ///
        /// The new task will take place of any previous tasks that were registered
        /// by previous calls to `register`. Any calls to `wake` that happen after
        /// a call to `register` (as defined by the memory ordering rules), will
        /// notify the `register` caller's task and deregister the waker from future
        /// notifications. Because of this, callers should ensure `register` gets
        /// invoked with a new `Waker` **each** time they require a wakeup.
        ///
        /// It is safe to call `register` with multiple other threads concurrently
        /// calling `wake`. This will result in the `register` caller's current
        /// task being notified once.
        ///
        /// This function is safe to call concurrently, but this is generally a bad
        /// idea. Concurrent calls to `register` will attempt to register different
        /// tasks to be notified. One of the callers will win and have its task set,
        /// but there is no guarantee as to which caller will succeed.
        ///
        /// # Examples
        ///
        /// Here is how `register` is used when implementing a flag.
        ///
        /// ```
        /// use futures::future::Future;
        /// use futures::task::{Context, Poll, AtomicWaker};
        /// use std::sync::atomic::AtomicBool;
        /// use std::sync::atomic::Ordering::Relaxed;
        /// use std::pin::Pin;
        ///
        /// struct Flag {
        ///     waker: AtomicWaker,
        ///     set: AtomicBool,
        /// }
        ///
        /// impl Future for Flag {
        ///     type Output = ();
        ///
        ///     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
        ///         // Register **before** checking `set` to avoid a race condition
        ///         // that would result in lost notifications.
        ///         self.waker.register(cx.waker());
        ///
        ///         if self.set.load(Relaxed) {
        ///             Poll::Ready(())
        ///         } else {
        ///             Poll::Pending
        ///         }
        ///     }
        /// }
        /// ```
        pub fn register(&self, waker: &Waker) {
            match self
                .state
                .compare_exchange(WAITING, REGISTERING, Acquire, Acquire)
                .unwrap_or_else(|x| x)
            {
                WAITING => {
                    unsafe {
                        // Locked acquired, update the waker cell
                        *self.waker.get() = Some(waker.clone());

                        // Release the lock. If the state transitioned to include
                        // the `WAKING` bit, this means that at least one wake has
                        // been called concurrently.
                        //
                        // Start by assuming that the state is `REGISTERING` as this
                        // is what we just set it to. If this holds, we know that no
                        // other writes were performed in the meantime, so there is
                        // nothing to acquire, only release. In case of concurrent
                        // wakers, we need to acquire their releases, so success needs
                        // to do both.
                        let res = self.state.compare_exchange(
                            REGISTERING, WAITING, AcqRel, Acquire);

                        match res {
                            Ok(_) => {
                                // memory ordering: acquired self.state during CAS
                                // - if previous wakes went through it syncs with
                                //   their final release (`fetch_and`)
                                // - if there was no previous wake the next wake
                                //   will wake us, no sync needed.
                            }
                            Err(actual) => {
                                // This branch can only be reached if at least one
                                // concurrent thread called `wake`. In this
                                // case, `actual` **must** be `REGISTERING |
                                // `WAKING`.
                                debug_assert_eq!(actual, REGISTERING | WAKING);

                                // Take the waker to wake once the atomic operation has
                                // completed.
                                let waker = (*self.waker.get()).take().unwrap();

                                // We need to return to WAITING state (clear our lock and
                                // concurrent WAKING flag). This needs to acquire all
                                // WAKING fetch_or releases and it needs to release our
                                // update to self.waker, so we need a `swap` operation.
                                self.state.swap(WAITING, AcqRel);

                                // memory ordering: we acquired the state for all
                                // concurrent wakes, but future wakes might still
                                // need to wake us in case we can't make progress
                                // from the pending wakes.
                                //
                                // So we simply schedule to come back later (we could
                                // also simply leave the registration in place above).
                                waker.wake();
                            }
                        }
                    }
                }
                WAKING => {
                    // Currently in the process of waking the task, i.e.,
                    // `wake` is currently being called on the old task handle.
                    //
                    // memory ordering: we acquired the state for all
                    // concurrent wakes, but future wakes might still
                    // need to wake us in case we can't make progress
                    // from the pending wakes.
                    //
                    // So we simply schedule to come back later (we
                    // could also spin here trying to acquire the lock
                    // to register).
                    waker.wake_by_ref();
                }
                state => {
                    // In this case, a concurrent thread is holding the
                    // "registering" lock. This probably indicates a bug in the
                    // caller's code as racing to call `register` doesn't make much
                    // sense.
                    //
                    // memory ordering: don't care. a concurrent register() is going
                    // to succeed and provide proper memory ordering.
                    //
                    // We just want to maintain memory safety. It is ok to drop the
                    // call to `register`.
                    debug_assert!(
                        state == REGISTERING ||
                        state == REGISTERING | WAKING);
                }
            }
        }

        /// Calls `wake` on the last `Waker` passed to `register`.
        ///
        /// If `register` has not been called yet, then this does nothing.
        pub fn wake(&self) {
            if let Some(waker) = self.take() {
                waker.wake();
            }
        }

        /// Returns the last `Waker` passed to `register`, so that the user can wake it.
        ///
        ///
        /// Sometimes, just waking the AtomicWaker is not fine grained enough. This allows the user
        /// to take the waker and then wake it separately, rather than performing both steps in one
        /// atomic action.
        ///
        /// If a waker has not been registered, this returns `None`.
        pub fn take(&self) -> Option<Waker> {
            // AcqRel ordering is used in order to acquire the value of the `task`
            // cell as well as to establish a `release` ordering with whatever
            // memory the `AtomicWaker` is associated with.
            match self.state.fetch_or(WAKING, AcqRel) {
                WAITING => {
                    // The waking lock has been acquired.
                    let waker = unsafe { (*self.waker.get()).take() };

                    // Release the lock
                    self.state.fetch_and(!WAKING, Release);

                    waker
                }
                state => {
                    // There is a concurrent thread currently updating the
                    // associated task.
                    //
                    // Nothing more to do as the `WAKING` bit has been set. It
                    // doesn't matter if there are concurrent registering threads or
                    // not.
                    //
                    debug_assert!(
                        state == REGISTERING ||
                        state == REGISTERING | WAKING ||
                        state == WAKING);
                    None
                }
            }
        }
    }

    impl Default for AtomicWaker {
        fn default() -> Self {
            Self::new()
        }
    }

    impl fmt::Debug for AtomicWaker {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "AtomicWaker")
        }
    }

    unsafe impl Send for AtomicWaker {}
    unsafe impl Sync for AtomicWaker {}