pcg64.rs

use core::num::Wrapping;

pub(crate) struct Pcg64 {
    state: Wrapping<u128>,
    increment: Wrapping<u128>,
}

impl Pcg64 {
    const MULTIPLIER: Wrapping<u128> = Wrapping(6_364_136_223_846_793_005);

    pub fn new(seed: u64, mut increment: u64) -> Self {
        if increment % 2 != 0 {
            increment += 1;
        }
        let (seed, increment) = (Wrapping(seed as u128), Wrapping(increment as u128));

        let mut gen = Self {
            state: seed + increment,
            increment,
        };
        gen.next_u64();

        gen
    }

    pub fn next_u64(&mut self) -> u64 {
        let x = self.state;
        let count = (x >> 122).0 as u32;

        self.state = x * Self::MULTIPLIER + self.increment;
        let x = (x ^ (x >> 64)).0 as u64;

        x.rotate_right(count)
    }
}