crossbeam_channel/
utils.rs

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1//! Miscellaneous utilities.
2
3use std::cell::{Cell, UnsafeCell};
4use std::num::Wrapping;
5use std::ops::{Deref, DerefMut};
6use std::sync::atomic::{AtomicBool, Ordering};
7use std::thread;
8use std::time::{Duration, Instant};
9
10use crossbeam_utils::Backoff;
11
12/// Randomly shuffles a slice.
13pub(crate) 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(1_406_868_647));
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.
49pub(crate) 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.
65pub(crate) struct Spinlock<T> {
66    flag: AtomicBool,
67    value: UnsafeCell<T>,
68}
69
70impl<T> Spinlock<T> {
71    /// Returns a new spinlock initialized with `value`.
72    pub(crate) 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(crate) 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.
90pub(crate) struct SpinlockGuard<'a, T> {
91    parent: &'a Spinlock<T>,
92}
93
94impl<T> Drop for SpinlockGuard<'_, T> {
95    fn drop(&mut self) {
96        self.parent.flag.store(false, Ordering::Release);
97    }
98}
99
100impl<T> Deref for SpinlockGuard<'_, T> {
101    type Target = T;
102
103    fn deref(&self) -> &T {
104        unsafe { &*self.parent.value.get() }
105    }
106}
107
108impl<T> DerefMut for SpinlockGuard<'_, T> {
109    fn deref_mut(&mut self) -> &mut T {
110        unsafe { &mut *self.parent.value.get() }
111    }
112}
crossbeam_channel/utils.rs

crossbeam_channel/
utils.rs
