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    #![cfg_attr(not(feature = "full"), allow(dead_code))]

    use crate::loom::sync::atomic::AtomicUsize;
    use crate::loom::sync::{Arc, Condvar, Mutex};
    use crate::park::{Park, Unpark};

    use std::sync::atomic::Ordering::SeqCst;
    use std::time::Duration;

    #[derive(Debug)]
    pub(crate) struct ParkThread {
        inner: Arc<Inner>,
    }

    pub(crate) type ParkError = ();

    /// Unblocks a thread that was blocked by `ParkThread`.
    #[derive(Clone, Debug)]
    pub(crate) struct UnparkThread {
        inner: Arc<Inner>,
    }

    #[derive(Debug)]
    struct Inner {
        state: AtomicUsize,
        mutex: Mutex<()>,
        condvar: Condvar,
    }

    const EMPTY: usize = 0;
    const PARKED: usize = 1;
    const NOTIFIED: usize = 2;

    thread_local! {
        static CURRENT_PARKER: ParkThread = ParkThread::new();
    }

    // ==== impl ParkThread ====

    impl ParkThread {
        pub(crate) fn new() -> Self {
            Self {
                inner: Arc::new(Inner {
                    state: AtomicUsize::new(EMPTY),
                    mutex: Mutex::new(()),
                    condvar: Condvar::new(),
                }),
            }
        }
    }

    impl Park for ParkThread {
        type Unpark = UnparkThread;
        type Error = ParkError;

        fn unpark(&self) -> Self::Unpark {
            let inner = self.inner.clone();
            UnparkThread { inner }
        }

        fn park(&mut self) -> Result<(), Self::Error> {
            self.inner.park();
            Ok(())
        }

        fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> {
            self.inner.park_timeout(duration);
            Ok(())
        }

        fn shutdown(&mut self) {
            self.inner.shutdown();
        }
    }

    // ==== impl Inner ====

    impl Inner {
        /// Park the current thread for at most `dur`.
        fn park(&self) {
            // If we were previously notified then we consume this notification and
            // return quickly.
            if self
                .state
                .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
                .is_ok()
            {
                return;
            }

            // Otherwise we need to coordinate going to sleep
            let mut m = self.mutex.lock();

            match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
                Ok(_) => {}
                Err(NOTIFIED) => {
                    // We must read here, even though we know it will be `NOTIFIED`.
                    // This is because `unpark` may have been called again since we read
                    // `NOTIFIED` in the `compare_exchange` above. We must perform an
                    // acquire operation that synchronizes with that `unpark` to observe
                    // any writes it made before the call to unpark. To do that we must
                    // read from the write it made to `state`.
                    let old = self.state.swap(EMPTY, SeqCst);
                    debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly");

                    return;
                }
                Err(actual) => panic!("inconsistent park state; actual = {}", actual),
            }

            loop {
                m = self.condvar.wait(m).unwrap();

                if self
                    .state
                    .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
                    .is_ok()
                {
                    // got a notification
                    return;
                }

                // spurious wakeup, go back to sleep
            }
        }

        fn park_timeout(&self, dur: Duration) {
            // Like `park` above we have a fast path for an already-notified thread,
            // and afterwards we start coordinating for a sleep. Return quickly.
            if self
                .state
                .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
                .is_ok()
            {
                return;
            }

            if dur == Duration::from_millis(0) {
                return;
            }

            let m = self.mutex.lock();

            match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
                Ok(_) => {}
                Err(NOTIFIED) => {
                    // We must read again here, see `park`.
                    let old = self.state.swap(EMPTY, SeqCst);
                    debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly");

                    return;
                }
                Err(actual) => panic!("inconsistent park_timeout state; actual = {}", actual),
            }

            // Wait with a timeout, and if we spuriously wake up or otherwise wake up
            // from a notification, we just want to unconditionally set the state back to
            // empty, either consuming a notification or un-flagging ourselves as
            // parked.
            let (_m, _result) = self.condvar.wait_timeout(m, dur).unwrap();

            match self.state.swap(EMPTY, SeqCst) {
                NOTIFIED => {} // got a notification, hurray!
                PARKED => {}   // no notification, alas
                n => panic!("inconsistent park_timeout state: {}", n),
            }
        }

        fn unpark(&self) {
            // To ensure the unparked thread will observe any writes we made before
            // this call, we must perform a release operation that `park` can
            // synchronize with. To do that we must write `NOTIFIED` even if `state`
            // is already `NOTIFIED`. That is why this must be a swap rather than a
            // compare-and-swap that returns if it reads `NOTIFIED` on failure.
            match self.state.swap(NOTIFIED, SeqCst) {
                EMPTY => return,    // no one was waiting
                NOTIFIED => return, // already unparked
                PARKED => {}        // gotta go wake someone up
                _ => panic!("inconsistent state in unpark"),
            }

            // There is a period between when the parked thread sets `state` to
            // `PARKED` (or last checked `state` in the case of a spurious wake
            // up) and when it actually waits on `cvar`. If we were to notify
            // during this period it would be ignored and then when the parked
            // thread went to sleep it would never wake up. Fortunately, it has
            // `lock` locked at this stage so we can acquire `lock` to wait until
            // it is ready to receive the notification.
            //
            // Releasing `lock` before the call to `notify_one` means that when the
            // parked thread wakes it doesn't get woken only to have to wait for us
            // to release `lock`.
            drop(self.mutex.lock());

            self.condvar.notify_one()
        }

        fn shutdown(&self) {
            self.condvar.notify_all();
        }
    }

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

    // ===== impl UnparkThread =====

    impl Unpark for UnparkThread {
        fn unpark(&self) {
            self.inner.unpark();
        }
    }

    use std::future::Future;
    use std::marker::PhantomData;
    use std::mem;
    use std::rc::Rc;
    use std::task::{RawWaker, RawWakerVTable, Waker};

    /// Blocks the current thread using a condition variable.
    #[derive(Debug)]
    pub(crate) struct CachedParkThread {
        _anchor: PhantomData<Rc<()>>,
    }

    impl CachedParkThread {
        /// Create a new `ParkThread` handle for the current thread.
        ///
        /// This type cannot be moved to other threads, so it should be created on
        /// the thread that the caller intends to park.
        pub(crate) fn new() -> CachedParkThread {
            CachedParkThread {
                _anchor: PhantomData,
            }
        }

        pub(crate) fn get_unpark(&self) -> Result<UnparkThread, ParkError> {
            self.with_current(|park_thread| park_thread.unpark())
        }

        /// Get a reference to the `ParkThread` handle for this thread.
        fn with_current<F, R>(&self, f: F) -> Result<R, ParkError>
        where
            F: FnOnce(&ParkThread) -> R,
        {
            CURRENT_PARKER.try_with(|inner| f(inner)).map_err(|_| ())
        }

        pub(crate) fn block_on<F: Future>(&mut self, f: F) -> Result<F::Output, ParkError> {
            use std::task::Context;
            use std::task::Poll::Ready;

            // `get_unpark()` should not return a Result
            let waker = self.get_unpark()?.into_waker();
            let mut cx = Context::from_waker(&waker);

            pin!(f);

            loop {
                if let Ready(v) = crate::coop::budget(|| f.as_mut().poll(&mut cx)) {
                    return Ok(v);
                }

                self.park()?;
            }
        }
    }

    impl Park for CachedParkThread {
        type Unpark = UnparkThread;
        type Error = ParkError;

        fn unpark(&self) -> Self::Unpark {
            self.get_unpark().unwrap()
        }

        fn park(&mut self) -> Result<(), Self::Error> {
            self.with_current(|park_thread| park_thread.inner.park())?;
            Ok(())
        }

        fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> {
            self.with_current(|park_thread| park_thread.inner.park_timeout(duration))?;
            Ok(())
        }

        fn shutdown(&mut self) {
            let _ = self.with_current(|park_thread| park_thread.inner.shutdown());
        }
    }

    impl UnparkThread {
        pub(crate) fn into_waker(self) -> Waker {
            unsafe {
                let raw = unparker_to_raw_waker(self.inner);
                Waker::from_raw(raw)
            }
        }
    }

    impl Inner {
        #[allow(clippy::wrong_self_convention)]
        fn into_raw(this: Arc<Inner>) -> *const () {
            Arc::into_raw(this) as *const ()
        }

        unsafe fn from_raw(ptr: *const ()) -> Arc<Inner> {
            Arc::from_raw(ptr as *const Inner)
        }
    }

    unsafe fn unparker_to_raw_waker(unparker: Arc<Inner>) -> RawWaker {
        RawWaker::new(
            Inner::into_raw(unparker),
            &RawWakerVTable::new(clone, wake, wake_by_ref, drop_waker),
        )
    }

    unsafe fn clone(raw: *const ()) -> RawWaker {
        let unparker = Inner::from_raw(raw);

        // Increment the ref count
        mem::forget(unparker.clone());

        unparker_to_raw_waker(unparker)
    }

    unsafe fn drop_waker(raw: *const ()) {
        let _ = Inner::from_raw(raw);
    }

    unsafe fn wake(raw: *const ()) {
        let unparker = Inner::from_raw(raw);
        unparker.unpark();
    }

    unsafe fn wake_by_ref(raw: *const ()) {
        let unparker = Inner::from_raw(raw);
        unparker.unpark();

        // We don't actually own a reference to the unparker
        mem::forget(unparker);
    }