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    use super::Task;

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

    /// A synchronization primitive for task notification.
    ///
    /// `AtomicTask` will coordinate concurrent notifications with the consumer
    /// potentially "updating" the underlying task to notify. 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 `notify` after producing the computation (this
    /// differs from the usual `thread::park` pattern). It is also permitted for
    /// `notify` to be called **before** `register`. This results in a no-op.
    ///
    /// A single `AtomicTask` may be reused for any number of calls to `register` or
    /// `notify`.
    ///
    /// `AtomicTask` does not provide any memory ordering guarantees, as such the
    /// user should use caution and use other synchronization primitives to guard
    /// the result of the underlying computation.
    pub struct AtomicTask {
        state: AtomicUsize,
        task: UnsafeCell<Option<Task>>,
    }

    // `AtomicTask` is a multi-consumer, single-producer transfer cell. The cell
    // stores a `Task` value produced by calls to `register` and many threads can
    // race to take the task (to notify it) by calling `notify.
    //
    // If a new `Task` instance is produced by calling `register` before an existing
    // one is consumed, then the existing one is overwritten.
    //
    // While `AtomicTask` is single-producer, the implementation ensures memory
    // safety. In the event of concurrent calls to `register`, there will be a
    // single winner whose task will get stored in the cell. The losers will not
    // have their tasks notified. As such, callers should ensure to add
    // synchronization to calls to `register`.
    //
    // The implementation uses a single `AtomicUsize` value to coordinate access to
    // the `Task` cell. There are two bits that are operated on independently. These
    // are represented by `REGISTERING` and `NOTIFYING`.
    //
    // The `REGISTERING` bit is set when a producer enters the critical section. The
    // `NOTIFYING` 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 task cell by transitioning the
    // state from `WAITING` to `REGISTERING` or `NOTIFYING`, depending on the
    // operation the thread wishes to perform. When this transition is made, it is
    // guaranteed that no other thread will access the task 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 task cell.
    //
    // If the lock is obtained, then the thread sets the task cell to the task
    // 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 `notify` will observe the task.
    //
    // If the transition fails, then there was a concurrent call to `notify` that
    // was unable to access the task cell (due to the registering thread holding the
    // lock). To handle this, the registering thread removes the task it just set
    // from the cell and calls `notify` on it. This call to notify represents the
    // attempt to notify by the other thread (that set the `NOTIFYING` bit). The
    // state is then transitioned from `REGISTERING | NOTIFYING` back to `WAITING`.
    // This transition must succeed because, at this point, the state cannot be
    // transitioned by another thread.
    //
    // # Notifying
    //
    // On a call to `notify`, an attempt to transition the state from `WAITING` to
    // `NOTIFYING` is made. On success, the caller obtains a lock on the task cell.
    //
    // If the lock is obtained, then the thread takes ownership of the current value
    // in teh task cell, and calls `notify` 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 `NOTIFYING` 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
    // `NOTIFYING` critical section. Either way, no action must be taken.
    //
    // If the current thread is the only concurrent call to `notify` and another
    // thread is in the `register` critical section, when the other thread **exits**
    // the `register` critical section, it will observe the `NOTIFYING` bit and
    // handle the notify itself.
    //
    // If another thread is in the `notify` critical section, then it will handle
    // notifying the task.
    //
    // # A potential race (is safely handled).
    //
    // Imagine the following situation:
    //
    // * Thread A obtains the `notify` lock and notifies a task.
    //
    // * Before thread A releases the `notify` lock, the notified task is scheduled.
    //
    // * Thread B attempts to notify the task. In theory this should result in the
    //   task being notified, but it cannot because thread A still holds the notify
    //   lock.
    //
    // This case is handled by requiring users of `AtomicTask` to call `register`
    // **before** attempting to observe the application state change that resulted
    // in the task being notified. The notifiers also change the application state
    // before calling notify.
    //
    // Because of this, the task will do one of two things.
    //
    // 1) Observe the application state change that Thread B is notifying on. In
    //    this case, it is OK for Thread B's notification to be lost.
    //
    // 2) Call register before attempting to observe the application state. Since
    //    Thread A still holds the `notify` lock, the call to `register` will result
    //    in the task notifying itself and get scheduled again.

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

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

    /// The task currently registered with the `AtomicTask` cell is being notified.
    const NOTIFYING: usize = 0b10;

    impl AtomicTask {
        /// Create an `AtomicTask` initialized with the given `Task`
        pub fn new() -> AtomicTask {
            // Make sure that task is Sync
            trait AssertSync: Sync {}
            impl AssertSync for Task {}

            AtomicTask {
                state: AtomicUsize::new(WAITING),
                task: UnsafeCell::new(None),
            }
        }

        /// Registers the current task to be notified on calls to `notify`.
        ///
        /// This is the same as calling `register_task` with `task::current()`.
        pub fn register(&self) {
            self.register_task(super::current());
        }

        /// Registers the provided task to be notified on calls to `notify`.
        ///
        /// The new task will take place of any previous tasks that were registered
        /// by previous calls to `register`. Any calls to `notify` that happen after
        /// a call to `register` (as defined by the memory ordering rules), will
        /// notify the `register` caller's task.
        ///
        /// It is safe to call `register` with multiple other threads concurrently
        /// calling `notify`. 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.
        pub fn register_task(&self, task: Task) {
            match self.state.compare_and_swap(WAITING, REGISTERING, Acquire) {
                WAITING => {
                    unsafe {
                        // Locked acquired, update the waker cell
                        *self.task.get() = Some(task.clone());

                        // Release the lock. If the state transitioned to include
                        // the `NOTIFYING` bit, this means that a notify has been
                        // called concurrently, so we have to remove the task and
                        // notify it.`
                        //
                        // Start by assuming that the state is `REGISTERING` as this
                        // is what we jut set it to.
                        let mut curr = REGISTERING;

                        // If a task has to be notified, it will be set here.
                        let mut notify: Option<Task> = None;

                        loop {
                            let res = self.state.compare_exchange(
                                curr, WAITING, AcqRel, Acquire);

                            match res {
                                Ok(_) => {
                                    // The atomic exchange was successful, now
                                    // notify the task (if set) and return.
                                    if let Some(task) = notify {
                                        task.notify();
                                    }

                                    return;
                                }
                                Err(actual) => {
                                    // This branch can only be reached if a
                                    // concurrent thread called `notify`. In this
                                    // case, `actual` **must** be `REGISTERING |
                                    // `NOTIFYING`.
                                    debug_assert_eq!(actual, REGISTERING | NOTIFYING);

                                    // Take the task to notify once the atomic operation has
                                    // completed.
                                    notify = (*self.task.get()).take();

                                    // Update `curr` for the next iteration of the
                                    // loop
                                    curr = actual;
                                }
                            }
                        }
                    }
                }
                NOTIFYING => {
                    // Currently in the process of notifying the task, i.e.,
                    // `notify` is currently being called on the old task handle.
                    // So, we call notify on the new task handle
                    task.notify();
                }
                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.
                    //
                    // We just want to maintain memory safety. It is ok to drop the
                    // call to `register`.
                    debug_assert!(
                        state == REGISTERING ||
                        state == REGISTERING | NOTIFYING);
                }
            }
        }

        /// Notifies the task that last called `register`.
        ///
        /// If `register` has not been called yet, then this does nothing.
        pub fn notify(&self) {
            // 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 `AtomicTask` is associated with.
            match self.state.fetch_or(NOTIFYING, AcqRel) {
                WAITING => {
                    // The notifying lock has been acquired.
                    let task = unsafe { (*self.task.get()).take() };

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

                    if let Some(task) = task {
                        task.notify();
                    }
                }
                state => {
                    // There is a concurrent thread currently updating the
                    // associated task.
                    //
                    // Nothing more to do as the `NOTIFYING` bit has been set. It
                    // doesn't matter if there are concurrent registering threads or
                    // not.
                    //
                    debug_assert!(
                        state == REGISTERING ||
                        state == REGISTERING | NOTIFYING ||
                        state == NOTIFYING);
                }
            }
        }
    }

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

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

    unsafe impl Send for AtomicTask {}
    unsafe impl Sync for AtomicTask {}

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