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// Copyright 2018 Developers of the Rand project.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// option. This file may not be copied, modified, or distributed
// except according to those terms.
#[cfg(feature="serde1")] use serde::{Serialize, Deserialize};
use rand_core::impls::fill_bytes_via_next;
use rand_core::le::read_u64_into;
use rand_core::{SeedableRng, RngCore, Error};
/// A xoshiro256++ random number generator.
///
/// The xoshiro256++ algorithm is not suitable for cryptographic purposes, but
/// is very fast and has excellent statistical properties.
///
/// The algorithm used here is translated from [the `xoshiro256plusplus.c`
/// David Blackman and Sebastiano Vigna.
#[derive(Debug, Clone, PartialEq, Eq)]
#[cfg_attr(feature="serde1", derive(Serialize, Deserialize))]
pub struct Xoshiro256PlusPlus {
s: [u64; 4],
}
impl SeedableRng for Xoshiro256PlusPlus {
type Seed = [u8; 32];
/// Create a new `Xoshiro256PlusPlus`. If `seed` is entirely 0, it will be
/// mapped to a different seed.
#[inline]
fn from_seed(seed: [u8; 32]) -> Xoshiro256PlusPlus {
if seed.iter().all(|&x| x == 0) {
return Self::seed_from_u64(0);
}
let mut state = [0; 4];
read_u64_into(&seed, &mut state);
Xoshiro256PlusPlus { s: state }
}
/// Create a new `Xoshiro256PlusPlus` from a `u64` seed.
///
/// This uses the SplitMix64 generator internally.
fn seed_from_u64(mut state: u64) -> Self {
const PHI: u64 = 0x9e3779b97f4a7c15;
let mut seed = Self::Seed::default();
for chunk in seed.as_mut().chunks_mut(8) {
state = state.wrapping_add(PHI);
let mut z = state;
z = (z ^ (z >> 30)).wrapping_mul(0xbf58476d1ce4e5b9);
z = (z ^ (z >> 27)).wrapping_mul(0x94d049bb133111eb);
z = z ^ (z >> 31);
chunk.copy_from_slice(&z.to_le_bytes());
}
Self::from_seed(seed)
}
}
impl RngCore for Xoshiro256PlusPlus {
#[inline]
fn next_u32(&mut self) -> u32 {
// The lowest bits have some linear dependencies, so we use the
// upper bits instead.
(self.next_u64() >> 32) as u32
}
#[inline]
fn next_u64(&mut self) -> u64 {
let result_plusplus = self.s[0]
.wrapping_add(self.s[3])
.rotate_left(23)
.wrapping_add(self.s[0]);
let t = self.s[1] << 17;
self.s[2] ^= self.s[0];
self.s[3] ^= self.s[1];
self.s[1] ^= self.s[2];
self.s[0] ^= self.s[3];
self.s[2] ^= t;
self.s[3] = self.s[3].rotate_left(45);
result_plusplus
}
#[inline]
fn fill_bytes(&mut self, dest: &mut [u8]) {
fill_bytes_via_next(self, dest);
}
#[inline]
fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
self.fill_bytes(dest);
Ok(())
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn reference() {
let mut rng = Xoshiro256PlusPlus::from_seed(
[1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0,
3, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0]);
// These values were produced with the reference implementation:
let expected = [
41943041, 58720359, 3588806011781223, 3591011842654386,
9228616714210784205, 9973669472204895162, 14011001112246962877,
12406186145184390807, 15849039046786891736, 10450023813501588000,
];
for &e in &expected {
assert_eq!(rng.next_u64(), e);
}
}
}