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use std::cell::Cell;
/// 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
/// Xorshift paper: <https://www.jstatsoft.org/article/view/v008i14/xorshift.pdf>
/// This generator passes the SmallCrush suite, part of TestU01 framework:
/// <http://simul.iro.umontreal.ca/testu01/tu01.html>
#[derive(Debug)]
pub(crate) struct FastRand {
one: Cell<u32>,
two: Cell<u32>,
}
impl FastRand {
/// Initializes a new, thread-local, fast random number generator.
pub(crate) fn new(seed: u64) -> FastRand {
let one = (seed >> 32) as u32;
let mut two = seed as u32;
if two == 0 {
// This value cannot be zero
two = 1;
}
FastRand {
one: Cell::new(one),
two: Cell::new(two),
}
}
pub(crate) fn fastrand_n(&self, n: u32) -> u32 {
// This is similar to fastrand() % n, but faster.
// See https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
let mul = (self.fastrand() as u64).wrapping_mul(n as u64);
(mul >> 32) as u32
}
fn fastrand(&self) -> u32 {
let mut s1 = self.one.get();
let s0 = self.two.get();
s1 ^= s1 << 17;
s1 = s1 ^ s0 ^ s1 >> 7 ^ s0 >> 16;
self.one.set(s0);
self.two.set(s1);
s0.wrapping_add(s1)
}
}
// Used by the select macro and `StreamMap`
#[cfg(any(feature = "macros"))]
#[doc(hidden)]
#[cfg_attr(not(feature = "macros"), allow(unreachable_pub))]
pub fn thread_rng_n(n: u32) -> u32 {
thread_local! {
static THREAD_RNG: FastRand = FastRand::new(crate::loom::rand::seed());
}
THREAD_RNG.with(|rng| rng.fastrand_n(n))
}
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