1use std::{
2 cell::Cell,
3 collections::hash_map::DefaultHasher,
4 hash::Hasher,
5 num::Wrapping,
6 sync::atomic::{AtomicUsize, Ordering},
7};
8
9// Based on [Fisher–Yates shuffle].
10//
11// [Fisher–Yates shuffle]: https://en.wikipedia.org/wiki/Fisher–Yates_shuffle
12#[doc(hidden)]
13pub fn shuffle<T>(slice: &mut [T]) {
14 for i: usize in (1..slice.len()).rev() {
15 slice.swap(a:i, b:gen_index(i + 1));
16 }
17}
18
19/// Return a value from `0..n`.
20fn gen_index(n: usize) -> usize {
21 (random() % n as u64) as usize
22}
23
24/// Pseudorandom number generator based on [xorshift*].
25///
26/// [xorshift*]: https://en.wikipedia.org/wiki/Xorshift#xorshift*
27fn random() -> u64 {
28 std::thread_local! {
29 static RNG: Cell<Wrapping<u64>> = Cell::new(Wrapping(prng_seed()));
30 }
31
32 fn prng_seed() -> u64 {
33 static COUNTER: AtomicUsize = AtomicUsize::new(0);
34
35 // Any non-zero seed will do
36 let mut seed = 0;
37 while seed == 0 {
38 let mut hasher = DefaultHasher::new();
39 hasher.write_usize(COUNTER.fetch_add(1, Ordering::Relaxed));
40 seed = hasher.finish();
41 }
42 seed
43 }
44
45 RNG.with(|rng| {
46 let mut x = rng.get();
47 debug_assert_ne!(x.0, 0);
48 x ^= x >> 12;
49 x ^= x << 25;
50 x ^= x >> 27;
51 rng.set(x);
52 x.0.wrapping_mul(0x2545_f491_4f6c_dd1d)
53 })
54}
55