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    use std::fmt;
    use std::marker::PhantomData;
    use std::sync::{Arc, Condvar, Mutex};
    use std::sync::atomic::AtomicUsize;
    use std::sync::atomic::Ordering::SeqCst;
    use std::time::Duration;

    /// A thread parking primitive.
    ///
    /// Conceptually, each `Parker` has an associated token which is initially not present:
    ///
    /// * The [`park`] method blocks the current thread unless or until the token is available, at
    ///   which point it automatically consumes the token. It may also return *spuriously*, without
    ///   consuming the token.
    ///
    /// * The [`park_timeout`] method works the same as [`park`], but blocks for a specified maximum
    ///   time.
    ///
    /// * The [`unpark`] method atomically makes the token available if it wasn't already. Because the
    ///   token is initially absent, [`unpark`] followed by [`park`] will result in the second call
    ///   returning immediately.
    ///
    /// In other words, each `Parker` acts a bit like a spinlock that can be locked and unlocked using
    /// [`park`] and [`unpark`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    /// use std::time::Duration;
    /// use crossbeam_utils::sync::Parker;
    ///
    /// let mut p = Parker::new();
    /// let u = p.unparker().clone();
    ///
    /// // Make the token available.
    /// u.unpark();
    /// // Wakes up immediately and consumes the token.
    /// p.park();
    ///
    /// thread::spawn(move || {
    ///     thread::sleep(Duration::from_millis(500));
    ///     u.unpark();
    /// });
    ///
    /// // Wakes up when `u.unpark()` provides the token, but may also wake up
    /// // spuriously before that without consuming the token.
    /// p.park();
    /// ```
    ///
    /// [`park`]: struct.Parker.html#method.park
    /// [`park_timeout`]: struct.Parker.html#method.park_timeout
    /// [`unpark`]: struct.Unparker.html#method.unpark
    pub struct Parker {
        unparker: Unparker,
        _marker: PhantomData<*const ()>,
    }

    unsafe impl Send for Parker {}

    impl Parker {
        /// Creates a new `Parker`.
        ///
        /// # Examples
        ///
        /// ```
        /// use crossbeam_utils::sync::Parker;
        ///
        /// let p = Parker::new();
        /// ```
        ///
        pub fn new() -> Parker {
            Parker {
                unparker: Unparker {
                    inner: Arc::new(Inner {
                        state: AtomicUsize::new(EMPTY),
                        lock: Mutex::new(()),
                        cvar: Condvar::new(),
                    }),
                },
                _marker: PhantomData,
            }
        }

        /// Blocks the current thread until the token is made available.
        ///
        /// A call to `park` may wake up spuriously without consuming the token, and callers should be
        /// prepared for this possibility.
        ///
        /// # Examples
        ///
        /// ```
        /// use crossbeam_utils::sync::Parker;
        ///
        /// let mut p = Parker::new();
        /// let u = p.unparker().clone();
        ///
        /// // Make the token available.
        /// u.unpark();
        ///
        /// // Wakes up immediately and consumes the token.
        /// p.park();
        /// ```
        pub fn park(&self) {
            self.unparker.inner.park(None);
        }

        /// Blocks the current thread until the token is made available, but only for a limited time.
        ///
        /// A call to `park_timeout` may wake up spuriously without consuming the token, and callers
        /// should be prepared for this possibility.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::time::Duration;
        /// use crossbeam_utils::sync::Parker;
        ///
        /// let mut p = Parker::new();
        ///
        /// // Waits for the token to become available, but will not wait longer than 500 ms.
        /// p.park_timeout(Duration::from_millis(500));
        /// ```
        pub fn park_timeout(&self, timeout: Duration) {
            self.unparker.inner.park(Some(timeout));
        }

        /// Returns a reference to an associated [`Unparker`].
        ///
        /// The returned [`Unparker`] doesn't have to be used by reference - it can also be cloned.
        ///
        /// # Examples
        ///
        /// ```
        /// use crossbeam_utils::sync::Parker;
        ///
        /// let mut p = Parker::new();
        /// let u = p.unparker().clone();
        ///
        /// // Make the token available.
        /// u.unpark();
        /// // Wakes up immediately and consumes the token.
        /// p.park();
        /// ```
        ///
        /// [`park`]: struct.Parker.html#method.park
        /// [`park_timeout`]: struct.Parker.html#method.park_timeout
        ///
        /// [`Unparker`]: struct.Unparker.html
        pub fn unparker(&self) -> &Unparker {
            &self.unparker
        }
    }

    impl fmt::Debug for Parker {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.pad("Parker { .. }")
        }
    }

    /// Unparks a thread parked by the associated [`Parker`].
    ///
    /// [`Parker`]: struct.Parker.html
    pub struct Unparker {
        inner: Arc<Inner>,
    }

    unsafe impl Send for Unparker {}
    unsafe impl Sync for Unparker {}

    impl Unparker {
        /// Atomically makes the token available if it is not already.
        ///
        /// This method will wake up the thread blocked on [`park`] or [`park_timeout`], if there is
        /// any.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::thread;
        /// use std::time::Duration;
        /// use crossbeam_utils::sync::Parker;
        ///
        /// let mut p = Parker::new();
        /// let u = p.unparker().clone();
        ///
        /// thread::spawn(move || {
        ///     thread::sleep(Duration::from_millis(500));
        ///     u.unpark();
        /// });
        ///
        /// // Wakes up when `u.unpark()` provides the token, but may also wake up
        /// // spuriously before that without consuming the token.
        /// p.park();
        /// ```
        ///
        /// [`park`]: struct.Parker.html#method.park
        /// [`park_timeout`]: struct.Parker.html#method.park_timeout
        pub fn unpark(&self) {
            self.inner.unpark()
        }
    }

    impl fmt::Debug for Unparker {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.pad("Unparker { .. }")
        }
    }

    impl Clone for Unparker {
        fn clone(&self) -> Unparker {
            Unparker {
                inner: self.inner.clone(),
            }
        }
    }

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

    struct Inner {
        state: AtomicUsize,
        lock: Mutex<()>,
        cvar: Condvar,
    }

    impl Inner {
        fn park(&self, timeout: Option<Duration>) {
            // 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;
            }

            // If the timeout is zero, then there is no need to actually block.
            if let Some(ref dur) = timeout {
                if *dur == Duration::from_millis(0) {
                    return;
                }
            }

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

            match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
                Ok(_) => {}
                // Consume this notification to avoid spurious wakeups in the next park.
                Err(NOTIFIED) => {
                    // We must read `state` 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);
                    assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
                    return;
                }
                Err(n) => panic!("inconsistent park_timeout state: {}", n),
            }

            match timeout {
                None => {
                    loop {
                        // Block the current thread on the conditional variable.
                        m = self.cvar.wait(m).unwrap();

                        match self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) {
                            Ok(_) => return, // got a notification
                            Err(_) => {} // spurious wakeup, go back to sleep
                        }
                    }
                }
                Some(timeout) => {
                    // Wait with a timeout, and if we spuriously wake up or otherwise wake up from a
                    // notification we just want to unconditionally set `state` back to `EMPTY`, either
                    // consuming a notification or un-flagging ourselves as parked.
                    let (_m, _result) = self.cvar.wait_timeout(m, timeout).unwrap();

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

        pub 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 wakeup) 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.lock.lock().unwrap());
            self.cvar.notify_one();
        }
    }

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