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