1// Copyright 2018 Developers of the Rand project.
2//
3// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
4// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
5// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
6// option. This file may not be copied, modified, or distributed
7// except according to those terms.
8
9//! The xorshift random number generator.
10
11#![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk.png",
12 html_favicon_url = "https://www.rust-lang.org/favicon.ico",
13 html_root_url = "https://docs.rs/rand_xorshift/0.3.0")]
14
15#![deny(missing_docs)]
16#![deny(missing_debug_implementations)]
17
18#![no_std]
19
20use core::num::Wrapping as w;
21use core::fmt;
22use rand_core::{RngCore, SeedableRng, Error, impls, le};
23#[cfg(feature="serde1")] use serde::{Serialize, Deserialize};
24
25/// An Xorshift random number generator.
26///
27/// The Xorshift[^1] algorithm is not suitable for cryptographic purposes
28/// but is very fast. If you do not know for sure that it fits your
29/// requirements, use a more secure one such as `StdRng` or `OsRng`.
30///
31/// [^1]: Marsaglia, George (July 2003).
32/// ["Xorshift RNGs"](https://www.jstatsoft.org/v08/i14/paper).
33/// *Journal of Statistical Software*. Vol. 8 (Issue 14).
34#[derive(Clone, PartialEq, Eq)]
35#[cfg_attr(feature="serde1", derive(Serialize,Deserialize))]
36pub struct XorShiftRng {
37 x: w<u32>,
38 y: w<u32>,
39 z: w<u32>,
40 w: w<u32>,
41}
42
43// Custom Debug implementation that does not expose the internal state
44impl fmt::Debug for XorShiftRng {
45 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
46 write!(f, "XorShiftRng {{}}")
47 }
48}
49
50impl RngCore for XorShiftRng {
51 #[inline]
52 fn next_u32(&mut self) -> u32 {
53 let x = self.x;
54 let t = x ^ (x << 11);
55 self.x = self.y;
56 self.y = self.z;
57 self.z = self.w;
58 let w_ = self.w;
59 self.w = w_ ^ (w_ >> 19) ^ (t ^ (t >> 8));
60 self.w.0
61 }
62
63 #[inline]
64 fn next_u64(&mut self) -> u64 {
65 impls::next_u64_via_u32(self)
66 }
67
68 #[inline]
69 fn fill_bytes(&mut self, dest: &mut [u8]) {
70 impls::fill_bytes_via_next(self, dest)
71 }
72
73 fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
74 self.fill_bytes(dest);
75 Ok(())
76 }
77}
78
79impl SeedableRng for XorShiftRng {
80 type Seed = [u8; 16];
81
82 fn from_seed(seed: Self::Seed) -> Self {
83 let mut seed_u32 = [0u32; 4];
84 le::read_u32_into(&seed, &mut seed_u32);
85
86 // Xorshift cannot be seeded with 0 and we cannot return an Error, but
87 // also do not wish to panic (because a random seed can legitimately be
88 // 0); our only option is therefore to use a preset value.
89 if seed_u32.iter().all(|&x| x == 0) {
90 seed_u32 = [0xBAD_5EED, 0xBAD_5EED, 0xBAD_5EED, 0xBAD_5EED];
91 }
92
93 XorShiftRng {
94 x: w(seed_u32[0]),
95 y: w(seed_u32[1]),
96 z: w(seed_u32[2]),
97 w: w(seed_u32[3]),
98 }
99 }
100
101 fn from_rng<R: RngCore>(mut rng: R) -> Result<Self, Error> {
102 let mut b = [0u8; 16];
103 loop {
104 rng.try_fill_bytes(&mut b[..])?;
105 if !b.iter().all(|&x| x == 0) {
106 break;
107 }
108 }
109
110 Ok(XorShiftRng {
111 x: w(u32::from_le_bytes([b[0], b[1], b[2], b[3]])),
112 y: w(u32::from_le_bytes([b[4], b[5], b[6], b[7]])),
113 z: w(u32::from_le_bytes([b[8], b[9], b[10], b[11]])),
114 w: w(u32::from_le_bytes([b[12], b[13], b[14], b[15]])),
115 })
116 }
117}
118