vendor/crossbeam-channel/src/utils.rs

   1 //! Miscellaneous utilities.
   2
   3 use std::cell::{Cell, UnsafeCell};
   4 use std::num::Wrapping;
   5 use std::ops::{Deref, DerefMut};
   6 use std::sync::atomic::{AtomicBool, Ordering};
   7 use std::thread;
   8 use std::time::{Duration, Instant};
   9
  10 use crossbeam_utils::Backoff;
  11
  12 /// Randomly shuffles a slice.
  13 pub fn shuffle<T>(v: &mut [T]) {
  14     let len = v.len();
  15     if len <= 1 {
  16         return;
  17     }
  18
  19     thread_local! {
  20         static RNG: Cell<Wrapping<u32>> = Cell::new(Wrapping(1406868647));
  21     }
  22
  23     let _ = RNG.try_with(|rng| {
  24         for i in 1..len {
  25             // This is the 32-bit variant of Xorshift.
  26             //
  27             // Source: https://en.wikipedia.org/wiki/Xorshift
  28             let mut x = rng.get();
  29             x ^= x << 13;
  30             x ^= x >> 17;
  31             x ^= x << 5;
  32             rng.set(x);
  33
  34             let x = x.0;
  35             let n = i + 1;
  36
  37             // This is a fast alternative to `let j = x % n`.
  38             //
  39             // Author: Daniel Lemire
  40             // Source: https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
  41             let j = ((x as u64).wrapping_mul(n as u64) >> 32) as u32 as usize;
  42
  43             v.swap(i, j);
  44         }
  45     });
  46 }
  47
  48 /// Sleeps until the deadline, or forever if the deadline isn't specified.
  49 pub fn sleep_until(deadline: Option<Instant>) {
  50     loop {
  51         match deadline {
  52             None => thread::sleep(Duration::from_secs(1000)),
  53             Some(d) => {
  54                 let now = Instant::now();
  55                 if now >= d {
  56                     break;
  57                 }
  58                 thread::sleep(d - now);
  59             }
  60         }
  61     }
  62 }
  63
  64 /// A simple spinlock.
  65 pub struct Spinlock<T> {
  66     flag: AtomicBool,
  67     value: UnsafeCell<T>,
  68 }
  69
  70 impl<T> Spinlock<T> {
  71     /// Returns a new spinlock initialized with `value`.
  72     pub fn new(value: T) -> Spinlock<T> {
  73         Spinlock {
  74             flag: AtomicBool::new(false),
  75             value: UnsafeCell::new(value),
  76         }
  77     }
  78
  79     /// Locks the spinlock.
  80     pub fn lock(&self) -> SpinlockGuard<'_, T> {
  81         let backoff = Backoff::new();
  82         while self.flag.swap(true, Ordering::Acquire) {
  83             backoff.snooze();
  84         }
  85         SpinlockGuard { parent: self }
  86     }
  87 }
  88
  89 /// A guard holding a spinlock locked.
  90 pub struct SpinlockGuard<'a, T: 'a> {
  91     parent: &'a Spinlock<T>,
  92 }
  93
  94 impl<'a, T> Drop for SpinlockGuard<'a, T> {
  95     fn drop(&mut self) {
  96         self.parent.flag.store(false, Ordering::Release);
  97     }
  98 }
  99
 100 impl<'a, T> Deref for SpinlockGuard<'a, T> {
 101     type Target = T;
 102
 103     fn deref(&self) -> &T {
 104         unsafe { &*self.parent.value.get() }
 105     }
 106 }
 107
 108 impl<'a, T> DerefMut for SpinlockGuard<'a, T> {
 109     fn deref_mut(&mut self) -> &mut T {
 110         unsafe { &mut *self.parent.value.get() }
 111     }
 112 }