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// Copyright 2018 Developers of the Rand project.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//! The xorshift random number generator.
#![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk.png",
html_favicon_url = "https://www.rust-lang.org/favicon.ico",
html_root_url = "https://rust-random.github.io/rand/")]
#![deny(missing_docs)]
#![deny(missing_debug_implementations)]
#![no_std]
use core::num::Wrapping as w;
use core::fmt;
use rand_core::{RngCore, SeedableRng, Error, impls, le};
#[cfg(feature="serde1")] use serde::{Serialize, Deserialize};
/// An Xorshift random number generator.
///
/// The Xorshift[^1] algorithm is not suitable for cryptographic purposes
/// but is very fast. If you do not know for sure that it fits your
/// requirements, use a more secure one such as `StdRng` or `OsRng`.
///
/// [^1]: Marsaglia, George (July 2003).
/// ["Xorshift RNGs"](https://www.jstatsoft.org/v08/i14/paper).
/// *Journal of Statistical Software*. Vol. 8 (Issue 14).
#[derive(Clone)]
#[cfg_attr(feature="serde1", derive(Serialize,Deserialize))]
pub struct XorShiftRng {
x: w<u32>,
y: w<u32>,
z: w<u32>,
w: w<u32>,
}
// Custom Debug implementation that does not expose the internal state
impl fmt::Debug for XorShiftRng {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "XorShiftRng {{}}")
}
}
impl RngCore for XorShiftRng {
#[inline]
fn next_u32(&mut self) -> u32 {
let x = self.x;
let t = x ^ (x << 11);
self.x = self.y;
self.y = self.z;
self.z = self.w;
let w_ = self.w;
self.w = w_ ^ (w_ >> 19) ^ (t ^ (t >> 8));
self.w.0
}
#[inline]
fn next_u64(&mut self) -> u64 {
impls::next_u64_via_u32(self)
}
#[inline]
fn fill_bytes(&mut self, dest: &mut [u8]) {
impls::fill_bytes_via_next(self, dest)
}
fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
self.fill_bytes(dest);
Ok(())
}
}
impl SeedableRng for XorShiftRng {
type Seed = [u8; 16];
fn from_seed(seed: Self::Seed) -> Self {
let mut seed_u32 = [0u32; 4];
le::read_u32_into(&seed, &mut seed_u32);
// Xorshift cannot be seeded with 0 and we cannot return an Error, but
// also do not wish to panic (because a random seed can legitimately be
// 0); our only option is therefore to use a preset value.
if seed_u32.iter().all(|&x| x == 0) {
seed_u32 = [0xBAD_5EED, 0xBAD_5EED, 0xBAD_5EED, 0xBAD_5EED];
}
XorShiftRng {
x: w(seed_u32[0]),
y: w(seed_u32[1]),
z: w(seed_u32[2]),
w: w(seed_u32[3]),
}
}
fn from_rng<R: RngCore>(mut rng: R) -> Result<Self, Error> {
let mut b = [0u8; 16];
loop {
rng.try_fill_bytes(&mut b[..])?;
if !b.iter().all(|&x| x == 0) {
break;
}
}
Ok(XorShiftRng {
x: w(u32::from_le_bytes([b[0], b[1], b[2], b[3]])),
y: w(u32::from_le_bytes([b[4], b[5], b[6], b[7]])),
z: w(u32::from_le_bytes([b[8], b[9], b[10], b[11]])),
w: w(u32::from_le_bytes([b[12], b[13], b[14], b[15]])),
})
}
}
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