tokio/util/
rand.rs

1cfg_rt! {
2    mod rt;
3    pub(crate) use rt::RngSeedGenerator;
4
5    cfg_unstable! {
6        mod rt_unstable;
7    }
8}
9
10/// A seed for random number generation.
11///
12/// In order to make certain functions within a runtime deterministic, a seed
13/// can be specified at the time of creation.
14#[allow(unreachable_pub)]
15#[derive(Clone, Debug)]
16pub struct RngSeed {
17    s: u32,
18    r: u32,
19}
20
21/// Fast random number generate.
22///
23/// Implement `xorshift64+`: 2 32-bit `xorshift` sequences added together.
24/// Shift triplet `[17,7,16]` was calculated as indicated in Marsaglia's
25/// `Xorshift` paper: <https://www.jstatsoft.org/article/view/v008i14/xorshift.pdf>
26/// This generator passes the SmallCrush suite, part of TestU01 framework:
27/// <http://simul.iro.umontreal.ca/testu01/tu01.html>
28#[derive(Clone, Copy, Debug)]
29pub(crate) struct FastRand {
30    one: u32,
31    two: u32,
32}
33
34impl RngSeed {
35    /// Creates a random seed using loom internally.
36    pub(crate) fn new() -> Self {
37        Self::from_u64(crate::loom::rand::seed())
38    }
39
40    fn from_u64(seed: u64) -> Self {
41        let one = (seed >> 32) as u32;
42        let two = seed as u32;
43
44        Self::from_pair(one, two)
45    }
46
47    fn from_pair(s: u32, r: u32) -> Self {
48        if s == 0 && r == 0 {
49            Self { s: 0, r: 1 }
50        } else {
51            Self { s, r }
52        }
53    }
54}
55
56impl FastRand {
57    /// Initialize a new fast random number generator using the default source of entropy.
58    pub(crate) fn new() -> FastRand {
59        FastRand::from_seed(RngSeed::new())
60    }
61
62    /// Initializes a new, thread-local, fast random number generator.
63    pub(crate) fn from_seed(seed: RngSeed) -> FastRand {
64        FastRand {
65            one: seed.s,
66            two: seed.r,
67        }
68    }
69
70    #[cfg(any(feature = "macros", feature = "sync", feature = "rt"))]
71    pub(crate) fn fastrand_n(&mut self, n: u32) -> u32 {
72        // This is similar to fastrand() % n, but faster.
73        // See https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
74        let mul = (self.fastrand() as u64).wrapping_mul(n as u64);
75        (mul >> 32) as u32
76    }
77
78    fn fastrand(&mut self) -> u32 {
79        let mut s1 = self.one;
80        let s0 = self.two;
81
82        s1 ^= s1 << 17;
83        s1 = s1 ^ s0 ^ s1 >> 7 ^ s0 >> 16;
84
85        self.one = s0;
86        self.two = s1;
87
88        s0.wrapping_add(s1)
89    }
90}
91
92#[cfg(test)]
93mod tests {
94    use super::*;
95
96    #[test]
97    fn non_zero_seed_from_u64() {
98        let seed = RngSeed::from_u64(0);
99        assert_eq!(seed.s, 0);
100        assert_eq!(seed.r, 1);
101    }
102
103    #[test]
104    fn non_zero_seed_from_pair() {
105        let seed = RngSeed::from_pair(0, 0);
106        assert_eq!(seed.s, 0);
107        assert_eq!(seed.r, 1);
108    }
109}
tokio/util/rand.rs

tokio/util/
rand.rs
