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use std::{
cell::Cell,
collections::hash_map::DefaultHasher,
hash::Hasher,
num::Wrapping,
sync::atomic::{AtomicUsize, Ordering},
};
// Based on [Fisher–Yates shuffle].
//
// [Fisher–Yates shuffle]: https://en.wikipedia.org/wiki/Fisher–Yates_shuffle
#[doc(hidden)]
pub fn shuffle<T>(slice: &mut [T]) {
for i in (1..slice.len()).rev() {
slice.swap(i, gen_index(i + 1));
}
}
/// Return a value from `0..n`.
fn gen_index(n: usize) -> usize {
(random() % n as u64) as usize
}
/// Pseudorandom number generator based on [xorshift*].
///
/// [xorshift*]: https://en.wikipedia.org/wiki/Xorshift#xorshift*
fn random() -> u64 {
thread_local! {
static RNG: Cell<Wrapping<u64>> = Cell::new(Wrapping(prng_seed()));
}
fn prng_seed() -> u64 {
static COUNTER: AtomicUsize = AtomicUsize::new(0);
// Any non-zero seed will do
let mut seed = 0;
while seed == 0 {
let mut hasher = DefaultHasher::new();
hasher.write_usize(COUNTER.fetch_add(1, Ordering::Relaxed));
seed = hasher.finish();
}
seed
}
RNG.with(|rng| {
let mut x = rng.get();
debug_assert_ne!(x.0, 0);
x ^= x >> 12;
x ^= x << 25;
x ^= x >> 27;
rng.set(x);
x.0.wrapping_mul(0x2545_f491_4f6c_dd1d)
})
}
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