rand_xoshiro/
splitmix64.rs

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
// 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.

use byteorder::{ByteOrder, LittleEndian};
use rand_core::le::read_u64_into;
use rand_core::impls::fill_bytes_via_next;
use rand_core::{RngCore, SeedableRng, Error};

/// A splitmix64 random number generator.
///
/// The splitmix algorithm is not suitable for cryptographic purposes, but is
/// very fast and has a 64 bit state.
///
/// The algorithm used here is translated from [the `splitmix64.c`
/// reference source code](http://xoshiro.di.unimi.it/splitmix64.c) by
/// Sebastiano Vigna. For `next_u32`, a more efficient mixing function taken
/// from [`dsiutils`](http://dsiutils.di.unimi.it/) is used.
#[allow(missing_copy_implementations)]
#[derive(Debug, Clone)]
pub struct SplitMix64 {
    x: u64,
}

const PHI: u64 = 0x9e3779b97f4a7c15;

impl RngCore for SplitMix64 {
    #[inline]
    fn next_u32(&mut self) -> u32 {
        self.x = self.x.wrapping_add(PHI);
        let mut z = self.x;
        // David Stafford's
        // (http://zimbry.blogspot.com/2011/09/better-bit-mixing-improving-on.html)
        // "Mix4" variant of the 64-bit finalizer in Austin Appleby's
        // MurmurHash3 algorithm.
        z = (z ^ (z >> 33)).wrapping_mul(0x62A9D9ED799705F5);
        z = (z ^ (z >> 28)).wrapping_mul(0xCB24D0A5C88C35B3);
        (z >> 32) as u32
    }

    #[inline]
    fn next_u64(&mut self) -> u64 {
        self.x = self.x.wrapping_add(PHI);
        let mut z = self.x;
        z = (z ^ (z >> 30)).wrapping_mul(0xbf58476d1ce4e5b9);
        z = (z ^ (z >> 27)).wrapping_mul(0x94d049bb133111eb);
        z ^ (z >> 31)
    }

    #[inline]
    fn fill_bytes(&mut self, dest: &mut [u8]) {
        fill_bytes_via_next(self, dest);
    }

    #[inline]
    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
        self.fill_bytes(dest);
        Ok(())
    }
}

impl SeedableRng for SplitMix64 {
    type Seed = [u8; 8];

    /// Create a new `SplitMix64`.
    fn from_seed(seed: [u8; 8]) -> SplitMix64 {
        let mut state = [0; 1];
        read_u64_into(&seed, &mut state);
        SplitMix64 {
            x: state[0],
        }
    }

    /// Seed a `SplitMix64` from a `u64`.
    fn seed_from_u64(seed: u64) -> SplitMix64 {
        let mut x = [0; 8];
        LittleEndian::write_u64(&mut x, seed);
        SplitMix64::from_seed(x)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn reference() {
        let mut rng = SplitMix64::seed_from_u64(1477776061723855037);
        // These values were produced with the reference implementation:
        // http://xoshiro.di.unimi.it/splitmix64.c
        let expected : [u64 ; 50]= [
            1985237415132408290, 2979275885539914483, 13511426838097143398,
            8488337342461049707, 15141737807933549159, 17093170987380407015,
            16389528042912955399, 13177319091862933652, 10841969400225389492,
            17094824097954834098, 3336622647361835228, 9678412372263018368,
            11111587619974030187, 7882215801036322410, 5709234165213761869,
            7799681907651786826, 4616320717312661886, 4251077652075509767,
            7836757050122171900, 5054003328188417616, 12919285918354108358,
            16477564761813870717, 5124667218451240549, 18099554314556827626,
            7603784838804469118, 6358551455431362471, 3037176434532249502,
            3217550417701719149, 9958699920490216947, 5965803675992506258,
            12000828378049868312, 12720568162811471118, 245696019213873792,
            8351371993958923852, 14378754021282935786, 5655432093647472106,
            5508031680350692005, 8515198786865082103, 6287793597487164412,
            14963046237722101617, 3630795823534910476, 8422285279403485710,
            10554287778700714153, 10871906555720704584, 8659066966120258468,
            9420238805069527062, 10338115333623340156, 13514802760105037173,
            14635952304031724449, 15419692541594102413,
        ];
        for &e in expected.iter() {
            assert_eq!(rng.next_u64(), e);
        }
    }

    #[test]
    fn next_u32() {
        let mut rng = SplitMix64::seed_from_u64(10);
        // These values were produced with the reference implementation:
        // http://dsiutils.di.unimi.it/dsiutils-2.5.1-src.tar.gz
        let expected : [u32 ; 100]= [
            3930361779, 4016923089, 4113052479, 925926767, 1755287528,
            802865554, 954171070, 3724185978, 173676273, 1414488795, 12664133,
            1784889697, 1303817078, 261610523, 941280008, 2571813643,
            2954453492, 378291111, 2546873158, 3923319175, 645257028,
            3881821278, 2681538690, 3037029984, 1999958137, 1853970361,
            2989951788, 2126166628, 839962987, 3989679659, 3656977858,
            684284364, 1673258011, 170979192, 3037622326, 1600748179,
            1780764218, 1141430714, 4139736875, 3336905707, 2262051600,
            3830850262, 2430765325, 1073032139, 1668888979, 2716938970,
            4102420032, 40305196, 386350562, 2754480591, 622869439, 2129598760,
            2306038241, 4218338739, 412298926, 3453855056, 3061469690,
            4284292697, 994843708, 1591016681, 414726151, 1238182607, 18073498,
            1237631493, 351884714, 2347486264, 2488990876, 802846256, 645670443,
            957607012, 3126589776, 1966356370, 3036485766, 868696717,
            2808613630, 2070968151, 1025536863, 1743949425, 466212687,
            2994327271, 209776458, 1246125124, 3344380309, 2203947859,
            968313105, 2805485302, 197484837, 3472483632, 3931823935,
            3288490351, 4165666529, 3671080416, 689542830, 1272555356,
            1039141475, 3984640460, 4142959054, 2252788890, 2459379590,
            991872507,
        ];
        for &e in expected.iter() {
            assert_eq!(rng.next_u32(), e);
        }
    }
}