rand_xoshiro/
xoshiro256plus.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
// 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.

#[cfg(feature="serde1")] use serde::{Serialize, Deserialize};
use rand_core::impls::fill_bytes_via_next;
use rand_core::le::read_u64_into;
use rand_core::{SeedableRng, RngCore, Error};

/// A xoshiro256+ random number generator.
///
/// The xoshiro256+ algorithm is not suitable for cryptographic purposes, but
/// is very fast and has good statistical properties, besides a low linear
/// complexity in the lowest bits.
///
/// The algorithm used here is translated from [the `xoshiro256plus.c`
/// reference source code](http://xoshiro.di.unimi.it/xoshiro256plus.c) by
/// David Blackman and Sebastiano Vigna.
#[derive(Debug, Clone, PartialEq, Eq)]
#[cfg_attr(feature="serde1", derive(Serialize, Deserialize))]
pub struct Xoshiro256Plus {
    s: [u64; 4],
}

impl Xoshiro256Plus {
    /// Jump forward, equivalently to 2^128 calls to `next_u64()`.
    ///
    /// This can be used to generate 2^128 non-overlapping subsequences for
    /// parallel computations.
    ///
    /// ```
    /// use rand_xoshiro::rand_core::SeedableRng;
    /// use rand_xoshiro::Xoshiro256Plus;
    ///
    /// let rng1 = Xoshiro256Plus::seed_from_u64(0);
    /// let mut rng2 = rng1.clone();
    /// rng2.jump();
    /// let mut rng3 = rng2.clone();
    /// rng3.jump();
    /// ```
    pub fn jump(&mut self) {
        impl_jump!(u64, self, [
            0x180ec6d33cfd0aba, 0xd5a61266f0c9392c,
            0xa9582618e03fc9aa, 0x39abdc4529b1661c
        ]);
    }

    /// Jump forward, equivalently to 2^192 calls to `next_u64()`.
    ///
    /// This can be used to generate 2^64 starting points, from each of which
    /// `jump()` will generate 2^64 non-overlapping subsequences for parallel
    /// distributed computations.
    pub fn long_jump(&mut self) {
        impl_jump!(u64, self, [
            0x76e15d3efefdcbbf, 0xc5004e441c522fb3,
            0x77710069854ee241, 0x39109bb02acbe635
        ]);
    }
}

impl SeedableRng for Xoshiro256Plus {
    type Seed = [u8; 32];

    /// Create a new `Xoshiro256Plus`.  If `seed` is entirely 0, it will be
    /// mapped to a different seed.
    #[inline]
    fn from_seed(seed: [u8; 32]) -> Xoshiro256Plus {
        deal_with_zero_seed!(seed, Self);
        let mut state = [0; 4];
        read_u64_into(&seed, &mut state);
        Xoshiro256Plus { s: state }
    }

    /// Seed a `Xoshiro256Plus` from a `u64` using `SplitMix64`.
    fn seed_from_u64(seed: u64) -> Xoshiro256Plus {
        from_splitmix!(seed)
    }
}

impl RngCore for Xoshiro256Plus {
    #[inline]
    fn next_u32(&mut self) -> u32 {
        // The lowest bits have some linear dependencies, so we use the
        // upper bits instead.
        (self.next_u64() >> 32) as u32
    }

    #[inline]
    fn next_u64(&mut self) -> u64 {
        let result_plus = self.s[0].wrapping_add(self.s[3]);
        impl_xoshiro_u64!(self);
        result_plus
    }

    #[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(())
    }
}

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

    #[test]
    fn reference() {
        let mut rng = Xoshiro256Plus::from_seed(
            [1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0,
             3, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0]);
        // These values were produced with the reference implementation:
        // http://xoshiro.di.unimi.it/xoshiro256plus.c
        let expected = [
            5, 211106232532999, 211106635186183, 9223759065350669058,
            9250833439874351877, 13862484359527728515, 2346507365006083650,
            1168864526675804870, 34095955243042024, 3466914240207415127,
        ];
        for &e in &expected {
            assert_eq!(rng.next_u64(), e);
        }
    }
}