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    use core::cell::Cell;
    use core::fmt;
    use core::sync::atomic;

    const SPIN_LIMIT: u32 = 6;
    const YIELD_LIMIT: u32 = 10;

    /// Performs exponential backoff in spin loops.
    ///
    /// Backing off in spin loops reduces contention and improves overall performance.
    ///
    /// This primitive can execute *YIELD* and *PAUSE* instructions, yield the current thread to the OS
    /// scheduler, and tell when is a good time to block the thread using a different synchronization
    /// mechanism. Each step of the back off procedure takes roughly twice as long as the previous
    /// step.
    ///
    /// # Examples
    ///
    /// Backing off in a lock-free loop:
    ///
    /// ```
    /// use crossbeam_utils::Backoff;
    /// use std::sync::atomic::AtomicUsize;
    /// use std::sync::atomic::Ordering::SeqCst;
    ///
    /// fn fetch_mul(a: &AtomicUsize, b: usize) -> usize {
    ///     let backoff = Backoff::new();
    ///     loop {
    ///         let val = a.load(SeqCst);
    ///         if a.compare_and_swap(val, val.wrapping_mul(b), SeqCst) == val {
    ///             return val;
    ///         }
    ///         backoff.spin();
    ///     }
    /// }
    /// ```
    ///
    /// Waiting for an [`AtomicBool`] to become `true`:
    ///
    /// ```
    /// use crossbeam_utils::Backoff;
    /// use std::sync::atomic::AtomicBool;
    /// use std::sync::atomic::Ordering::SeqCst;
    ///
    /// fn spin_wait(ready: &AtomicBool) {
    ///     let backoff = Backoff::new();
    ///     while !ready.load(SeqCst) {
    ///         backoff.snooze();
    ///     }
    /// }
    /// ```
    ///
    /// Waiting for an [`AtomicBool`] to become `true` and parking the thread after a long wait.
    /// Note that whoever sets the atomic variable to `true` must notify the parked thread by calling
    /// [`unpark()`]:
    ///
    /// ```
    /// use crossbeam_utils::Backoff;
    /// use std::sync::atomic::AtomicBool;
    /// use std::sync::atomic::Ordering::SeqCst;
    /// use std::thread;
    ///
    /// fn blocking_wait(ready: &AtomicBool) {
    ///     let backoff = Backoff::new();
    ///     while !ready.load(SeqCst) {
    ///         if backoff.is_completed() {
    ///             thread::park();
    ///         } else {
    ///             backoff.snooze();
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// [`is_completed`]: struct.Backoff.html#method.is_completed
    /// [`std::thread::park()`]: https://doc.rust-lang.org/std/thread/fn.park.html
    /// [`Condvar`]: https://doc.rust-lang.org/std/sync/struct.Condvar.html
    /// [`AtomicBool`]: https://doc.rust-lang.org/std/sync/atomic/struct.AtomicBool.html
    /// [`unpark()`]: https://doc.rust-lang.org/std/thread/struct.Thread.html#method.unpark
    pub struct Backoff {
        step: Cell<u32>,
    }

    impl Backoff {
        /// Creates a new `Backoff`.
        ///
        /// # Examples
        ///
        /// ```
        /// use crossbeam_utils::Backoff;
        ///
        /// let backoff = Backoff::new();
        /// ```
        #[inline]
        pub fn new() -> Self {
            Backoff {
                step: Cell::new(0),
            }
        }

        /// Resets the `Backoff`.
        ///
        /// # Examples
        ///
        /// ```
        /// use crossbeam_utils::Backoff;
        ///
        /// let backoff = Backoff::new();
        /// backoff.reset();
        /// ```
        #[inline]
        pub fn reset(&self) {
            self.step.set(0);
        }

        /// Backs off in a lock-free loop.
        ///
        /// This method should be used when we need to retry an operation because another thread made
        /// progress.
        ///
        /// The processor may yield using the *YIELD* or *PAUSE* instruction.
        ///
        /// # Examples
        ///
        /// Backing off in a lock-free loop:
        ///
        /// ```
        /// use crossbeam_utils::Backoff;
        /// use std::sync::atomic::AtomicUsize;
        /// use std::sync::atomic::Ordering::SeqCst;
        ///
        /// fn fetch_mul(a: &AtomicUsize, b: usize) -> usize {
        ///     let backoff = Backoff::new();
        ///     loop {
        ///         let val = a.load(SeqCst);
        ///         if a.compare_and_swap(val, val.wrapping_mul(b), SeqCst) == val {
        ///             return val;
        ///         }
        ///         backoff.spin();
        ///     }
        /// }
        ///
        /// let a = AtomicUsize::new(7);
        /// assert_eq!(fetch_mul(&a, 8), 7);
        /// assert_eq!(a.load(SeqCst), 56);
        /// ```
        #[inline]
        pub fn spin(&self) {
            for _ in 0..1 << self.step.get().min(SPIN_LIMIT) {
                atomic::spin_loop_hint();
            }

            if self.step.get() <= SPIN_LIMIT {
                self.step.set(self.step.get() + 1);
            }
        }

        /// Backs off in a blocking loop.
        ///
        /// This method should be used when we need to wait for another thread to make progress.
        ///
        /// The processor may yield using the *YIELD* or *PAUSE* instruction and the current thread
        /// may yield by giving up a timeslice to the OS scheduler.
        ///
        /// In `#[no_std]` environments, this method is equivalent to [`spin`].
        ///
        /// If possible, use [`is_completed`] to check when it is advised to stop using backoff and
        /// block the current thread using a different synchronization mechanism instead.
        ///
        /// [`spin`]: struct.Backoff.html#method.spin
        /// [`is_completed`]: struct.Backoff.html#method.is_completed
        ///
        /// # Examples
        ///
        /// Waiting for an [`AtomicBool`] to become `true`:
        ///
        /// ```
        /// use crossbeam_utils::Backoff;
        /// use std::sync::Arc;
        /// use std::sync::atomic::AtomicBool;
        /// use std::sync::atomic::Ordering::SeqCst;
        /// use std::thread;
        /// use std::time::Duration;
        ///
        /// fn spin_wait(ready: &AtomicBool) {
        ///     let backoff = Backoff::new();
        ///     while !ready.load(SeqCst) {
        ///         backoff.snooze();
        ///     }
        /// }
        ///
        /// let ready = Arc::new(AtomicBool::new(false));
        /// let ready2 = ready.clone();
        ///
        /// thread::spawn(move || {
        ///     thread::sleep(Duration::from_millis(100));
        ///     ready2.store(true, SeqCst);
        /// });
        ///
        /// assert_eq!(ready.load(SeqCst), false);
        /// spin_wait(&ready);
        /// assert_eq!(ready.load(SeqCst), true);
        /// ```
        ///
        /// [`AtomicBool`]: https://doc.rust-lang.org/std/sync/atomic/struct.AtomicBool.html
        #[inline]
        pub fn snooze(&self) {
            if self.step.get() <= SPIN_LIMIT {
                for _ in 0..1 << self.step.get() {
                    atomic::spin_loop_hint();
                }
            } else {
                #[cfg(not(feature = "std"))]
                for _ in 0..1 << self.step.get() {
                    atomic::spin_loop_hint();
                }

                #[cfg(feature = "std")]
                ::std::thread::yield_now();
            }

            if self.step.get() <= YIELD_LIMIT {
                self.step.set(self.step.get() + 1);
            }
        }

        /// Returns `true` if exponential backoff has completed and blocking the thread is advised.
        ///
        /// # Examples
        ///
        /// Waiting for an [`AtomicBool`] to become `true` and parking the thread after a long wait:
        ///
        /// ```
        /// use crossbeam_utils::Backoff;
        /// use std::sync::Arc;
        /// use std::sync::atomic::AtomicBool;
        /// use std::sync::atomic::Ordering::SeqCst;
        /// use std::thread;
        /// use std::time::Duration;
        ///
        /// fn blocking_wait(ready: &AtomicBool) {
        ///     let backoff = Backoff::new();
        ///     while !ready.load(SeqCst) {
        ///         if backoff.is_completed() {
        ///             thread::park();
        ///         } else {
        ///             backoff.snooze();
        ///         }
        ///     }
        /// }
        ///
        /// let ready = Arc::new(AtomicBool::new(false));
        /// let ready2 = ready.clone();
        /// let waiter = thread::current();
        ///
        /// thread::spawn(move || {
        ///     thread::sleep(Duration::from_millis(100));
        ///     ready2.store(true, SeqCst);
        ///     waiter.unpark();
        /// });
        ///
        /// assert_eq!(ready.load(SeqCst), false);
        /// blocking_wait(&ready);
        /// assert_eq!(ready.load(SeqCst), true);
        /// ```
        ///
        /// [`AtomicBool`]: https://doc.rust-lang.org/std/sync/atomic/struct.AtomicBool.html
        #[inline]
        pub fn is_completed(&self) -> bool {
            self.step.get() > YIELD_LIMIT
        }

        #[inline]
        #[doc(hidden)]
        #[deprecated(note = "use `is_completed` instead")]
        pub fn is_complete(&self) -> bool {
            self.is_completed()
        }
    }

    impl fmt::Debug for Backoff {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Backoff")
                .field("step", &self.step)
                .field("is_completed", &self.is_completed())
                .finish()
        }
    }

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

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