96 lines
2.4 KiB
Rust
96 lines
2.4 KiB
Rust
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cfg_rt! {
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mod rt;
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pub(crate) use rt::RngSeedGenerator;
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cfg_unstable! {
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mod rt_unstable;
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}
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}
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/// A seed for random number generation.
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///
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/// In order to make certain functions within a runtime deterministic, a seed
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/// can be specified at the time of creation.
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#[allow(unreachable_pub)]
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#[derive(Clone, Debug)]
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pub struct RngSeed {
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s: u32,
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r: u32,
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}
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/// Fast random number generate.
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///
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/// Implement `xorshift64+`: 2 32-bit `xorshift` sequences added together.
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/// Shift triplet `[17,7,16]` was calculated as indicated in Marsaglia's
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/// `Xorshift` paper: <https://www.jstatsoft.org/article/view/v008i14/xorshift.pdf>
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/// This generator passes the SmallCrush suite, part of TestU01 framework:
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/// <http://simul.iro.umontreal.ca/testu01/tu01.html>
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#[derive(Clone, Copy, Debug)]
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pub(crate) struct FastRand {
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one: u32,
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two: u32,
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}
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impl RngSeed {
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/// Creates a random seed using loom internally.
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pub(crate) fn new() -> Self {
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Self::from_u64(crate::loom::rand::seed())
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}
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fn from_u64(seed: u64) -> Self {
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let one = (seed >> 32) as u32;
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let mut two = seed as u32;
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if two == 0 {
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// This value cannot be zero
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two = 1;
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}
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Self::from_pair(one, two)
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}
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fn from_pair(s: u32, r: u32) -> Self {
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Self { s, r }
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}
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}
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impl FastRand {
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/// Initialize a new fast random number generator using the default source of entropy.
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pub(crate) fn new() -> FastRand {
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FastRand::from_seed(RngSeed::new())
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}
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/// Initializes a new, thread-local, fast random number generator.
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pub(crate) fn from_seed(seed: RngSeed) -> FastRand {
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FastRand {
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one: seed.s,
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two: seed.r,
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}
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}
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#[cfg(any(
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feature = "macros",
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feature = "rt-multi-thread",
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all(feature = "sync", feature = "rt")
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))]
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pub(crate) fn fastrand_n(&mut self, n: u32) -> u32 {
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// This is similar to fastrand() % n, but faster.
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// See https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
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let mul = (self.fastrand() as u64).wrapping_mul(n as u64);
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(mul >> 32) as u32
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}
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fn fastrand(&mut self) -> u32 {
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let mut s1 = self.one;
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let s0 = self.two;
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s1 ^= s1 << 17;
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s1 = s1 ^ s0 ^ s1 >> 7 ^ s0 >> 16;
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self.one = s0;
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self.two = s1;
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s0.wrapping_add(s1)
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}
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}
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