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    use super::{Interest, Ready, ReadyEvent, Tick};
    use crate::loom::sync::atomic::AtomicUsize;
    use crate::loom::sync::Mutex;
    use crate::util::bit;
    use crate::util::slab::Entry;

    use std::sync::atomic::Ordering::{AcqRel, Acquire, Release};
    use std::task::{Context, Poll, Waker};

    use super::Direction;

    cfg_io_readiness! {
        use crate::util::linked_list::{self, LinkedList};

        use std::cell::UnsafeCell;
        use std::future::Future;
        use std::marker::PhantomPinned;
        use std::pin::Pin;
        use std::ptr::NonNull;
    }

    /// Stored in the I/O driver resource slab.
    #[derive(Debug)]
    pub(crate) struct ScheduledIo {
        /// Packs the resource's readiness with the resource's generation.
        readiness: AtomicUsize,

        waiters: Mutex<Waiters>,
    }

    cfg_io_readiness! {
        type WaitList = LinkedList<Waiter, <Waiter as linked_list::Link>::Target>;
    }

    #[derive(Debug, Default)]
    struct Waiters {
        #[cfg(feature = "net")]
        /// List of all current waiters
        list: WaitList,

        /// Waker used for AsyncRead
        reader: Option<Waker>,

        /// Waker used for AsyncWrite
        writer: Option<Waker>,

        /// True if this ScheduledIo has been killed due to IO driver shutdown
        is_shutdown: bool,
    }

    cfg_io_readiness! {
        #[derive(Debug)]
        struct Waiter {
            pointers: linked_list::Pointers<Waiter>,

            /// The waker for this task
            waker: Option<Waker>,

            /// The interest this waiter is waiting on
            interest: Interest,

            is_ready: bool,

            /// Should never be `!Unpin`
            _p: PhantomPinned,
        }

        /// Future returned by `readiness()`
        struct Readiness<'a> {
            scheduled_io: &'a ScheduledIo,

            state: State,

            /// Entry in the waiter `LinkedList`.
            waiter: UnsafeCell<Waiter>,
        }

        enum State {
            Init,
            Waiting,
            Done,
        }
    }

    // The `ScheduledIo::readiness` (`AtomicUsize`) is packed full of goodness.
    //
    // | reserved | generation |  driver tick | readinesss |
    // |----------+------------+--------------+------------|
    // |   1 bit  |   7 bits   +    8 bits    +   16 bits  |

    const READINESS: bit::Pack = bit::Pack::least_significant(16);

    const TICK: bit::Pack = READINESS.then(8);

    const GENERATION: bit::Pack = TICK.then(7);

    #[test]
    fn test_generations_assert_same() {
        assert_eq!(super::GENERATION, GENERATION);
    }

    // ===== impl ScheduledIo =====

    impl Entry for ScheduledIo {
        fn reset(&self) {
            let state = self.readiness.load(Acquire);

            let generation = GENERATION.unpack(state);
            let next = GENERATION.pack_lossy(generation + 1, 0);

            self.readiness.store(next, Release);
        }
    }

    impl Default for ScheduledIo {
        fn default() -> ScheduledIo {
            ScheduledIo {
                readiness: AtomicUsize::new(0),
                waiters: Mutex::new(Default::default()),
            }
        }
    }

    impl ScheduledIo {
        pub(crate) fn generation(&self) -> usize {
            GENERATION.unpack(self.readiness.load(Acquire))
        }

        /// Invoked when the IO driver is shut down; forces this ScheduledIo into a
        /// permanently ready state.
        pub(super) fn shutdown(&self) {
            self.wake0(Ready::ALL, true)
        }

        /// Sets the readiness on this `ScheduledIo` by invoking the given closure on
        /// the current value, returning the previous readiness value.
        ///
        /// # Arguments
        /// - `token`: the token for this `ScheduledIo`.
        /// - `tick`: whether setting the tick or trying to clear readiness for a
        ///    specific tick.
        /// - `f`: a closure returning a new readiness value given the previous
        ///   readiness.
        ///
        /// # Returns
        ///
        /// If the given token's generation no longer matches the `ScheduledIo`'s
        /// generation, then the corresponding IO resource has been removed and
        /// replaced with a new resource. In that case, this method returns `Err`.
        /// Otherwise, this returns the previous readiness.
        pub(super) fn set_readiness(
            &self,
            token: Option<usize>,
            tick: Tick,
            f: impl Fn(Ready) -> Ready,
        ) -> Result<(), ()> {
            let mut current = self.readiness.load(Acquire);

            loop {
                let current_generation = GENERATION.unpack(current);

                if let Some(token) = token {
                    // Check that the generation for this access is still the
                    // current one.
                    if GENERATION.unpack(token) != current_generation {
                        return Err(());
                    }
                }

                // Mask out the tick/generation bits so that the modifying
                // function doesn't see them.
                let current_readiness = Ready::from_usize(current);
                let new = f(current_readiness);

                let packed = match tick {
                    Tick::Set(t) => TICK.pack(t as usize, new.as_usize()),
                    Tick::Clear(t) => {
                        if TICK.unpack(current) as u8 != t {
                            // Trying to clear readiness with an old event!
                            return Err(());
                        }

                        TICK.pack(t as usize, new.as_usize())
                    }
                };

                let next = GENERATION.pack(current_generation, packed);

                match self
                    .readiness
                    .compare_exchange(current, next, AcqRel, Acquire)
                {
                    Ok(_) => return Ok(()),
                    // we lost the race, retry!
                    Err(actual) => current = actual,
                }
            }
        }

        /// Notifies all pending waiters that have registered interest in `ready`.
        ///
        /// There may be many waiters to notify. Waking the pending task **must** be
        /// done from outside of the lock otherwise there is a potential for a
        /// deadlock.
        ///
        /// A stack array of wakers is created and filled with wakers to notify, the
        /// lock is released, and the wakers are notified. Because there may be more
        /// than 32 wakers to notify, if the stack array fills up, the lock is
        /// released, the array is cleared, and the iteration continues.
        pub(super) fn wake(&self, ready: Ready) {
            self.wake0(ready, false);
        }

        fn wake0(&self, ready: Ready, shutdown: bool) {
            const NUM_WAKERS: usize = 32;

            let mut wakers: [Option<Waker>; NUM_WAKERS] = Default::default();
            let mut curr = 0;

            let mut waiters = self.waiters.lock();

            waiters.is_shutdown |= shutdown;

            // check for AsyncRead slot
            if ready.is_readable() {
                if let Some(waker) = waiters.reader.take() {
                    wakers[curr] = Some(waker);
                    curr += 1;
                }
            }

            // check for AsyncWrite slot
            if ready.is_writable() {
                if let Some(waker) = waiters.writer.take() {
                    wakers[curr] = Some(waker);
                    curr += 1;
                }
            }

            #[cfg(feature = "net")]
            'outer: loop {
                let mut iter = waiters.list.drain_filter(|w| ready.satisfies(w.interest));

                while curr < NUM_WAKERS {
                    match iter.next() {
                        Some(waiter) => {
                            let waiter = unsafe { &mut *waiter.as_ptr() };

                            if let Some(waker) = waiter.waker.take() {
                                waiter.is_ready = true;
                                wakers[curr] = Some(waker);
                                curr += 1;
                            }
                        }
                        None => {
                            break 'outer;
                        }
                    }
                }

                drop(waiters);

                for waker in wakers.iter_mut().take(curr) {
                    waker.take().unwrap().wake();
                }

                curr = 0;

                // Acquire the lock again.
                waiters = self.waiters.lock();
            }

            // Release the lock before notifying
            drop(waiters);

            for waker in wakers.iter_mut().take(curr) {
                waker.take().unwrap().wake();
            }
        }

        pub(super) fn ready_event(&self, interest: Interest) -> ReadyEvent {
            let curr = self.readiness.load(Acquire);

            ReadyEvent {
                tick: TICK.unpack(curr) as u8,
                ready: interest.mask() & Ready::from_usize(READINESS.unpack(curr)),
            }
        }

        /// Poll version of checking readiness for a certain direction.
        ///
        /// These are to support `AsyncRead` and `AsyncWrite` polling methods,
        /// which cannot use the `async fn` version. This uses reserved reader
        /// and writer slots.
        pub(super) fn poll_readiness(
            &self,
            cx: &mut Context<'_>,
            direction: Direction,
        ) -> Poll<ReadyEvent> {
            let curr = self.readiness.load(Acquire);

            let ready = direction.mask() & Ready::from_usize(READINESS.unpack(curr));

            if ready.is_empty() {
                // Update the task info
                let mut waiters = self.waiters.lock();
                let slot = match direction {
                    Direction::Read => &mut waiters.reader,
                    Direction::Write => &mut waiters.writer,
                };

                // Avoid cloning the waker if one is already stored that matches the
                // current task.
                match slot {
                    Some(existing) => {
                        if !existing.will_wake(cx.waker()) {
                            *existing = cx.waker().clone();
                        }
                    }
                    None => {
                        *slot = Some(cx.waker().clone());
                    }
                }

                // Try again, in case the readiness was changed while we were
                // taking the waiters lock
                let curr = self.readiness.load(Acquire);
                let ready = direction.mask() & Ready::from_usize(READINESS.unpack(curr));
                if waiters.is_shutdown {
                    Poll::Ready(ReadyEvent {
                        tick: TICK.unpack(curr) as u8,
                        ready: direction.mask(),
                    })
                } else if ready.is_empty() {
                    Poll::Pending
                } else {
                    Poll::Ready(ReadyEvent {
                        tick: TICK.unpack(curr) as u8,
                        ready,
                    })
                }
            } else {
                Poll::Ready(ReadyEvent {
                    tick: TICK.unpack(curr) as u8,
                    ready,
                })
            }
        }

        pub(crate) fn clear_readiness(&self, event: ReadyEvent) {
            // This consumes the current readiness state **except** for closed
            // states. Closed states are excluded because they are final states.
            let mask_no_closed = event.ready - Ready::READ_CLOSED - Ready::WRITE_CLOSED;

            // result isn't important
            let _ = self.set_readiness(None, Tick::Clear(event.tick), |curr| curr - mask_no_closed);
        }

        pub(crate) fn clear_wakers(&self) {
            let mut waiters = self.waiters.lock();
            waiters.reader.take();
            waiters.writer.take();
        }
    }

    impl Drop for ScheduledIo {
        fn drop(&mut self) {
            self.wake(Ready::ALL);
        }
    }

    unsafe impl Send for ScheduledIo {}
    unsafe impl Sync for ScheduledIo {}

    cfg_io_readiness! {
        impl ScheduledIo {
            /// An async version of `poll_readiness` which uses a linked list of wakers
            pub(crate) async fn readiness(&self, interest: Interest) -> ReadyEvent {
                self.readiness_fut(interest).await
            }

            // This is in a separate function so that the borrow checker doesn't think
            // we are borrowing the `UnsafeCell` possibly over await boundaries.
            //
            // Go figure.
            fn readiness_fut(&self, interest: Interest) -> Readiness<'_> {
                Readiness {
                    scheduled_io: self,
                    state: State::Init,
                    waiter: UnsafeCell::new(Waiter {
                        pointers: linked_list::Pointers::new(),
                        waker: None,
                        is_ready: false,
                        interest,
                        _p: PhantomPinned,
                    }),
                }
            }
        }

        unsafe impl linked_list::Link for Waiter {
            type Handle = NonNull<Waiter>;
            type Target = Waiter;

            fn as_raw(handle: &NonNull<Waiter>) -> NonNull<Waiter> {
                *handle
            }

            unsafe fn from_raw(ptr: NonNull<Waiter>) -> NonNull<Waiter> {
                ptr
            }

            unsafe fn pointers(mut target: NonNull<Waiter>) -> NonNull<linked_list::Pointers<Waiter>> {
                NonNull::from(&mut target.as_mut().pointers)
            }
        }

        // ===== impl Readiness =====

        impl Future for Readiness<'_> {
            type Output = ReadyEvent;

            fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
                use std::sync::atomic::Ordering::SeqCst;

                let (scheduled_io, state, waiter) = unsafe {
                    let me = self.get_unchecked_mut();
                    (&me.scheduled_io, &mut me.state, &me.waiter)
                };

                loop {
                    match *state {
                        State::Init => {
                            // Optimistically check existing readiness
                            let curr = scheduled_io.readiness.load(SeqCst);
                            let ready = Ready::from_usize(READINESS.unpack(curr));

                            // Safety: `waiter.interest` never changes
                            let interest = unsafe { (*waiter.get()).interest };
                            let ready = ready.intersection(interest);

                            if !ready.is_empty() {
                                // Currently ready!
                                let tick = TICK.unpack(curr) as u8;
                                *state = State::Done;
                                return Poll::Ready(ReadyEvent { ready, tick });
                            }

                            // Wasn't ready, take the lock (and check again while locked).
                            let mut waiters = scheduled_io.waiters.lock();

                            let curr = scheduled_io.readiness.load(SeqCst);
                            let mut ready = Ready::from_usize(READINESS.unpack(curr));

                            if waiters.is_shutdown {
                                ready = Ready::ALL;
                            }

                            let ready = ready.intersection(interest);

                            if !ready.is_empty() {
                                // Currently ready!
                                let tick = TICK.unpack(curr) as u8;
                                *state = State::Done;
                                return Poll::Ready(ReadyEvent { ready, tick });
                            }

                            // Not ready even after locked, insert into list...

                            // Safety: called while locked
                            unsafe {
                                (*waiter.get()).waker = Some(cx.waker().clone());
                            }

                            // Insert the waiter into the linked list
                            //
                            // safety: pointers from `UnsafeCell` are never null.
                            waiters
                                .list
                                .push_front(unsafe { NonNull::new_unchecked(waiter.get()) });
                            *state = State::Waiting;
                        }
                        State::Waiting => {
                            // Currently in the "Waiting" state, implying the caller has
                            // a waiter stored in the waiter list (guarded by
                            // `notify.waiters`). In order to access the waker fields,
                            // we must hold the lock.

                            let waiters = scheduled_io.waiters.lock();

                            // Safety: called while locked
                            let w = unsafe { &mut *waiter.get() };

                            if w.is_ready {
                                // Our waker has been notified.
                                *state = State::Done;
                            } else {
                                // Update the waker, if necessary.
                                if !w.waker.as_ref().unwrap().will_wake(cx.waker()) {
                                    w.waker = Some(cx.waker().clone());
                                }

                                return Poll::Pending;
                            }

                            // Explicit drop of the lock to indicate the scope that the
                            // lock is held. Because holding the lock is required to
                            // ensure safe access to fields not held within the lock, it
                            // is helpful to visualize the scope of the critical
                            // section.
                            drop(waiters);
                        }
                        State::Done => {
                            let tick = TICK.unpack(scheduled_io.readiness.load(Acquire)) as u8;

                            // Safety: State::Done means it is no longer shared
                            let w = unsafe { &mut *waiter.get() };

                            return Poll::Ready(ReadyEvent {
                                tick,
                                ready: Ready::from_interest(w.interest),
                            });
                        }
                    }
                }
            }
        }

        impl Drop for Readiness<'_> {
            fn drop(&mut self) {
                let mut waiters = self.scheduled_io.waiters.lock();

                // Safety: `waiter` is only ever stored in `waiters`
                unsafe {
                    waiters
                        .list
                        .remove(NonNull::new_unchecked(self.waiter.get()))
                };
            }
        }

        unsafe impl Send for Readiness<'_> {}
        unsafe impl Sync for Readiness<'_> {}
    }