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
9use rand_core::impls::fill_bytes_via_next;
10use rand_core::le::read_u64_into;
11use rand_core::{RngCore, SeedableRng};
12#[cfg(feature = "serde")]
13use serde::{Deserialize, Serialize};
14
15/// A xoroshiro128+ random number generator.
16///
17/// The xoroshiro128+ algorithm is not suitable for cryptographic purposes, but
18/// is very fast and has good statistical properties, besides a low linear
19/// complexity in the lowest bits.
20///
21/// The algorithm used here is translated from [the `xoroshiro128plus.c`
22/// reference source code](http://xoshiro.di.unimi.it/xoroshiro128plus.c) by
23/// David Blackman and Sebastiano Vigna.
24#[allow(missing_copy_implementations)]
25#[derive(Debug, Clone, PartialEq, Eq)]
26#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
27pub struct Xoroshiro128Plus {
28 s0: u64,
29 s1: u64,
30}
31
32impl Xoroshiro128Plus {
33 /// Jump forward, equivalently to 2^64 calls to `next_u64()`.
34 ///
35 /// This can be used to generate 2^64 non-overlapping subsequences for
36 /// parallel computations.
37 ///
38 /// ```
39 /// use rand_xoshiro::rand_core::SeedableRng;
40 /// use rand_xoshiro::Xoroshiro128Plus;
41 ///
42 /// let rng1 = Xoroshiro128Plus::seed_from_u64(0);
43 /// let mut rng2 = rng1.clone();
44 /// rng2.jump();
45 /// let mut rng3 = rng2.clone();
46 /// rng3.jump();
47 /// ```
48 pub fn jump(&mut self) {
49 impl_jump!(u64, self, [0xdf900294d8f554a5, 0x170865df4b3201fc]);
50 }
51
52 /// Jump forward, equivalently to 2^96 calls to `next_u64()`.
53 ///
54 /// This can be used to generate 2^32 starting points, from each of which
55 /// `jump()` will generate 2^32 non-overlapping subsequences for parallel
56 /// distributed computations.
57 pub fn long_jump(&mut self) {
58 impl_jump!(u64, self, [0xd2a98b26625eee7b, 0xdddf9b1090aa7ac1]);
59 }
60}
61
62impl RngCore for Xoroshiro128Plus {
63 #[inline]
64 fn next_u32(&mut self) -> u32 {
65 // The two lowest bits have some linear dependencies, so we use the
66 // upper bits instead.
67 (self.next_u64() >> 32) as u32
68 }
69
70 #[inline]
71 fn next_u64(&mut self) -> u64 {
72 let r: u64 = self.s0.wrapping_add(self.s1);
73 impl_xoroshiro_u64!(self);
74 r
75 }
76
77 #[inline]
78 fn fill_bytes(&mut self, dest: &mut [u8]) {
79 fill_bytes_via_next(self, dest);
80 }
81}
82impl SeedableRng for Xoroshiro128Plus {
83 type Seed = [u8; 16];
84
85 /// Create a new `Xoroshiro128Plus`. If `seed` is entirely 0, it will be
86 /// mapped to a different seed.
87 fn from_seed(seed: [u8; 16]) -> Xoroshiro128Plus {
88 deal_with_zero_seed!(seed, Self, 16);
89 let mut s: [u64; 2] = [0; 2];
90 read_u64_into(&seed, &mut s);
91
92 Xoroshiro128Plus { s0: s[0], s1: s[1] }
93 }
94
95 /// Seed a `Xoroshiro128Plus` from a `u64` using `SplitMix64`.
96 fn seed_from_u64(seed: u64) -> Xoroshiro128Plus {
97 from_splitmix!(seed)
98 }
99}
100
101#[cfg(test)]
102mod tests {
103 use super::*;
104
105 #[test]
106 fn reference() {
107 let mut rng = Xoroshiro128Plus::from_seed([1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0]);
108 // These values were produced with the reference implementation:
109 // http://xoshiro.di.unimi.it/xoshiro128starstar.c
110 let expected = [
111 3,
112 412333834243,
113 2360170716294286339,
114 9295852285959843169,
115 2797080929874688578,
116 6019711933173041966,
117 3076529664176959358,
118 3521761819100106140,
119 7493067640054542992,
120 920801338098114767,
121 ];
122 for &e in &expected {
123 assert_eq!(rng.next_u64(), e);
124 }
125 }
126}
127