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