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Make the PCG implementation const compatible

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This commit is contained in:
Robbert van der Helm 2022-03-08 20:51:01 +01:00
parent 817ff72265
commit 4cca7af930
1 changed files with 15 additions and 10 deletions
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25
plugins/crisp/src/pcg.rs
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@ -33,15 +33,23 @@ impl Pcg32iState {
/// Initialize the PRNG, aka `*_srandom()`.
///
/// <https://github.com/imneme/pcg-c/blob/83252d9c23df9c82ecb42210afed61a7b42402d7/include/pcg_variants.h#L757-L765>
pub fn new(state: u32, sequence: u32) -> Self {
pub const fn new(state: u32, sequence: u32) -> Self {
let mut rng = Self {
state: 0,
inc: (sequence << 1) | 1,
};
rng.step();
// https://github.com/imneme/pcg-c/blob/83252d9c23df9c82ecb42210afed61a7b42402d7/include/pcg_variants.h#L540-L543,
// inlined so we can make this a const function
rng.state = rng
.state
.wrapping_mul(PCG_DEFAULT_MULTIPLIER_32)
.wrapping_add(rng.inc);
rng.state += state;
rng.step();
rng.state = rng
.state
.wrapping_mul(PCG_DEFAULT_MULTIPLIER_32)
.wrapping_add(rng.inc);
rng
}
@ -52,7 +60,10 @@ impl Pcg32iState {
#[inline]
pub fn next_u32(&mut self) -> u32 {
let old_state = self.state;
self.step();
self.state = self
.state
.wrapping_mul(PCG_DEFAULT_MULTIPLIER_32)
.wrapping_add(self.inc);
let word = ((old_state >> ((old_state >> 28) + 4)) ^ old_state) * 277803737;
(word >> 22) ^ word
@ -70,10 +81,4 @@ impl Pcg32iState {
let exponent_bits: u32 = ((f32::MAX_EXP - 1) as u32) << (f32::MANTISSA_DIGITS - 1);
f32::from_bits(fraction | exponent_bits) - (1.0 - f32::EPSILON / 2.0)
}
/// <https://github.com/imneme/pcg-c/blob/83252d9c23df9c82ecb42210afed61a7b42402d7/include/pcg_variants.h#L540-L543>
#[inline]
fn step(&mut self) {
self.state = self.state * PCG_DEFAULT_MULTIPLIER_32 + self.inc;
}
}