139 lines
4.5 KiB
Rust
139 lines
4.5 KiB
Rust
// Copyright 2018 Developers of the Rand project.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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use rand_core::impls::{fill_bytes_via_next, next_u64_via_u32};
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use rand_core::le::read_u32_into;
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use rand_core::{RngCore, SeedableRng};
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#[cfg(feature = "serde")]
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use serde::{Deserialize, Serialize};
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/// A xoshiro128++ random number generator.
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///
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/// The xoshiro128++ algorithm is not suitable for cryptographic purposes, but
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/// is very fast and has excellent statistical properties.
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///
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/// The algorithm used here is translated from [the `xoshiro128plusplus.c`
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/// reference source code](http://xoshiro.di.unimi.it/xoshiro128plusplus.c) by
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/// David Blackman and Sebastiano Vigna.
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#[derive(Debug, Clone, PartialEq, Eq)]
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#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
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pub struct Xoshiro128PlusPlus {
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s: [u32; 4],
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}
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impl SeedableRng for Xoshiro128PlusPlus {
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type Seed = [u8; 16];
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/// Create a new `Xoshiro128PlusPlus`. If `seed` is entirely 0, it will be
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/// mapped to a different seed.
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#[inline]
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fn from_seed(seed: [u8; 16]) -> Xoshiro128PlusPlus {
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let mut state = [0; 4];
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read_u32_into(&seed, &mut state);
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// Check for zero on aligned integers for better code generation.
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// Furtermore, seed_from_u64(0) will expand to a constant when optimized.
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if state.iter().all(|&x| x == 0) {
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return Self::seed_from_u64(0);
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}
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Xoshiro128PlusPlus { s: state }
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}
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/// Create a new `Xoshiro128PlusPlus` from a `u64` seed.
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///
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/// This uses the SplitMix64 generator internally.
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#[inline]
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fn seed_from_u64(mut state: u64) -> Self {
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const PHI: u64 = 0x9e3779b97f4a7c15;
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let mut s = [0; 4];
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for i in s.chunks_exact_mut(2) {
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state = state.wrapping_add(PHI);
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let mut z = state;
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z = (z ^ (z >> 30)).wrapping_mul(0xbf58476d1ce4e5b9);
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z = (z ^ (z >> 27)).wrapping_mul(0x94d049bb133111eb);
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z = z ^ (z >> 31);
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i[0] = z as u32;
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i[1] = (z >> 32) as u32;
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}
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// By using a non-zero PHI we are guaranteed to generate a non-zero state
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// Thus preventing a recursion between from_seed and seed_from_u64.
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debug_assert_ne!(s, [0; 4]);
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Xoshiro128PlusPlus { s }
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}
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}
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impl RngCore for Xoshiro128PlusPlus {
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#[inline]
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fn next_u32(&mut self) -> u32 {
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let res = self.s[0]
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.wrapping_add(self.s[3])
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.rotate_left(7)
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.wrapping_add(self.s[0]);
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let t = self.s[1] << 9;
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self.s[2] ^= self.s[0];
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self.s[3] ^= self.s[1];
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self.s[1] ^= self.s[2];
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self.s[0] ^= self.s[3];
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self.s[2] ^= t;
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self.s[3] = self.s[3].rotate_left(11);
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res
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}
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#[inline]
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fn next_u64(&mut self) -> u64 {
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next_u64_via_u32(self)
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}
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#[inline]
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fn fill_bytes(&mut self, dst: &mut [u8]) {
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fill_bytes_via_next(self, dst)
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}
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}
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#[cfg(test)]
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mod tests {
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use super::Xoshiro128PlusPlus;
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use rand_core::{RngCore, SeedableRng};
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#[test]
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fn reference() {
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let mut rng =
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Xoshiro128PlusPlus::from_seed([1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0]);
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// These values were produced with the reference implementation:
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// http://xoshiro.di.unimi.it/xoshiro128plusplus.c
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let expected = [
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641, 1573767, 3222811527, 3517856514, 836907274, 4247214768, 3867114732, 1355841295,
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495546011, 621204420,
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];
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for &e in &expected {
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assert_eq!(rng.next_u32(), e);
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}
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}
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#[test]
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fn stable_seed_from_u64_and_from_seed() {
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// We don't guarantee value-stability for SmallRng but this
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// could influence keeping stability whenever possible (e.g. after optimizations).
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let mut rng = Xoshiro128PlusPlus::seed_from_u64(0);
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// from_seed([0; 16]) should produce the same state as seed_from_u64(0).
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let mut rng_from_seed_0 = Xoshiro128PlusPlus::from_seed([0; 16]);
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let expected = [
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1179900579, 1938959192, 3089844957, 3657088315, 1015453891, 479942911, 3433842246,
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669252886, 3985671746, 2737205563,
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];
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for &e in &expected {
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assert_eq!(rng.next_u32(), e);
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assert_eq!(rng_from_seed_0.next_u32(), e);
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}
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}
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}
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