1// Copyright 2018-2023 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//! # Example
12//!
13//! To initialize a generator, use the [`SeedableRng`][rand_core::SeedableRng] trait:
14//!
15//! ```
16//! use rand_core::{SeedableRng, RngCore};
17//! use rand_xorshift::XorShiftRng;
18//!
19//! let mut rng = XorShiftRng::seed_from_u64(0);
20//! let x = rng.next_u32();
21//! ```
22
23#![doc(
24 html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk.png",
25 html_favicon_url = "https://www.rust-lang.org/favicon.ico",
26 html_root_url = "https://docs.rs/rand_xorshift/0.4.0"
27)]
28#![forbid(unsafe_code)]
29#![deny(missing_docs)]
30#![deny(missing_debug_implementations)]
31#![no_std]
32
33use core::fmt;
34use core::num::Wrapping as w;
35use rand_core::{impls, le, RngCore, SeedableRng, TryRngCore};
36#[cfg(feature = "serde")]
37use serde::{Deserialize, Serialize};
38
39/// An Xorshift random number generator.
40///
41/// The Xorshift[^1] algorithm is not suitable for cryptographic purposes
42/// but is very fast. If you do not know for sure that it fits your
43/// requirements, use a more secure one such as `StdRng` or `OsRng`.
44///
45/// When seeded with zero (i.e. `XorShiftRng::from_seed(0)` is called), this implementation
46/// actually uses `0xBAD_5EED_0BAD_5EED_0BAD_5EED_0BAD_5EED` for the seed. This arbitrary value is
47/// used because the underlying algorithm can't escape from an all-zero state, and the function is
48/// infallible so it can't signal this by returning an error.
49///
50/// [^1]: Marsaglia, George (July 2003).
51/// ["Xorshift RNGs"](https://www.jstatsoft.org/v08/i14/paper).
52/// *Journal of Statistical Software*. Vol. 8 (Issue 14).
53#[derive(Clone, PartialEq, Eq)]
54#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
55pub struct XorShiftRng {
56 x: w<u32>,
57 y: w<u32>,
58 z: w<u32>,
59 w: w<u32>,
60}
61
62// Custom Debug implementation that does not expose the internal state
63impl fmt::Debug for XorShiftRng {
64 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
65 write!(f, "XorShiftRng {{}}")
66 }
67}
68
69impl RngCore for XorShiftRng {
70 #[inline]
71 fn next_u32(&mut self) -> u32 {
72 // These shifts are taken from the example in the Summary section of
73 // the paper 'Xorshift RNGs'. (On the bottom of page 5.)
74 let x = self.x;
75 let t = x ^ (x << 11);
76 self.x = self.y;
77 self.y = self.z;
78 self.z = self.w;
79 let w_ = self.w;
80 self.w = w_ ^ (w_ >> 19) ^ (t ^ (t >> 8));
81 self.w.0
82 }
83
84 #[inline]
85 fn next_u64(&mut self) -> u64 {
86 impls::next_u64_via_u32(self)
87 }
88
89 #[inline]
90 fn fill_bytes(&mut self, dest: &mut [u8]) {
91 impls::fill_bytes_via_next(self, dest)
92 }
93}
94
95impl SeedableRng for XorShiftRng {
96 type Seed = [u8; 16];
97
98 fn from_seed(seed: Self::Seed) -> Self {
99 let mut seed_u32 = [0u32; 4];
100 le::read_u32_into(&seed, &mut seed_u32);
101
102 // Xorshift cannot be seeded with 0 and we cannot return an Error, but
103 // also do not wish to panic (because a random seed can legitimately be
104 // 0); our only option is therefore to use a preset value.
105 if seed_u32 == [0; 4] {
106 seed_u32 = [0xBAD_5EED, 0xBAD_5EED, 0xBAD_5EED, 0xBAD_5EED];
107 }
108
109 XorShiftRng {
110 x: w(seed_u32[0]),
111 y: w(seed_u32[1]),
112 z: w(seed_u32[2]),
113 w: w(seed_u32[3]),
114 }
115 }
116
117 fn from_rng(rng: &mut impl RngCore) -> Self {
118 let mut b = [0u8; 16];
119 loop {
120 rng.fill_bytes(b.as_mut());
121 if b != [0; 16] {
122 break;
123 }
124 }
125
126 XorShiftRng {
127 x: w(u32::from_le_bytes([b[0], b[1], b[2], b[3]])),
128 y: w(u32::from_le_bytes([b[4], b[5], b[6], b[7]])),
129 z: w(u32::from_le_bytes([b[8], b[9], b[10], b[11]])),
130 w: w(u32::from_le_bytes([b[12], b[13], b[14], b[15]])),
131 }
132 }
133
134 fn try_from_rng<R: TryRngCore>(rng: &mut R) -> Result<Self, R::Error> {
135 let mut b = [0u8; 16];
136 loop {
137 rng.try_fill_bytes(b.as_mut())?;
138 if b != [0; 16] {
139 break;
140 }
141 }
142
143 Ok(XorShiftRng {
144 x: w(u32::from_le_bytes([b[0], b[1], b[2], b[3]])),
145 y: w(u32::from_le_bytes([b[4], b[5], b[6], b[7]])),
146 z: w(u32::from_le_bytes([b[8], b[9], b[10], b[11]])),
147 w: w(u32::from_le_bytes([b[12], b[13], b[14], b[15]])),
148 })
149 }
150}
151