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    // Copyright 2016 Amanieu d'Antras
    //
    // Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
    // http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
    // http://opensource.org/licenses/MIT>, at your option. This file may not be
    // copied, modified, or distributed except according to those terms.

    use crate::thread_parker;
    use std::sync::atomic::spin_loop_hint;

    // Wastes some CPU time for the given number of iterations,
    // using a hint to indicate to the CPU that we are spinning.
    #[inline]
    fn cpu_relax(iterations: u32) {
        for _ in 0..iterations {
            spin_loop_hint()
        }
    }

    /// A counter used to perform exponential backoff in spin loops.
    #[derive(Default)]
    pub struct SpinWait {
        counter: u32,
    }

    impl SpinWait {
        /// Creates a new `SpinWait`.
        #[inline]
        pub fn new() -> Self {
            Self::default()
        }

        /// Resets a `SpinWait` to its initial state.
        #[inline]
        pub fn reset(&mut self) {
            self.counter = 0;
        }

        /// Spins until the sleep threshold has been reached.
        ///
        /// This function returns whether the sleep threshold has been reached, at
        /// which point further spinning has diminishing returns and the thread
        /// should be parked instead.
        ///
        /// The spin strategy will initially use a CPU-bound loop but will fall back
        /// to yielding the CPU to the OS after a few iterations.
        #[inline]
        pub fn spin(&mut self) -> bool {
            if self.counter >= 10 {
                return false;
            }
            self.counter += 1;
            if self.counter <= 3 {
                cpu_relax(1 << self.counter);
            } else {
                thread_parker::thread_yield();
            }
            true
        }

        /// Spins without yielding the thread to the OS.
        ///
        /// Instead, the backoff is simply capped at a maximum value. This can be
        /// used to improve throughput in `compare_exchange` loops that have high
        /// contention.
        #[inline]
        pub fn spin_no_yield(&mut self) {
            self.counter += 1;
            if self.counter > 10 {
                self.counter = 10;
            }
            cpu_relax(1 << self.counter);
        }
    }