rustdv-sim 0.1.0

//! Tiny deterministic RNG (SplitMix64). Zero-dependency substitute for the
//! `rand` crate; seeded by the runner from RUSTDV_RANDOM_SEED (port of
//! cocotb's RANDOM_SEED handling, design-doc D2.4).

#[derive(Clone, Debug)]
pub struct Rng {
    state: u64,
}

impl Rng {
    pub fn new(seed: u64) -> Rng {
        Rng { state: seed }
    }

    pub fn next_u64(&mut self) -> u64 {
        self.state = self.state.wrapping_add(0x9E37_79B9_7F4A_7C15);
        let mut z = self.state;
        z = (z ^ (z >> 30)).wrapping_mul(0xBF58_476D_1CE4_E5B9);
        z = (z ^ (z >> 27)).wrapping_mul(0x94D0_49BB_1331_11EB);
        z ^ (z >> 31)
    }

    /// Uniform in [0, n). n must be > 0.
    pub fn below(&mut self, n: u64) -> u64 {
        self.next_u64() % n
    }

    pub fn u8(&mut self) -> u8 {
        (self.next_u64() & 0xFF) as u8
    }

    pub fn bool(&mut self) -> bool {
        self.next_u64() & 1 == 1
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn deterministic() {
        let mut a = Rng::new(42);
        let mut b = Rng::new(42);
        for _ in 0..100 {
            assert_eq!(a.next_u64(), b.next_u64());
        }
    }

    #[test]
    fn below_in_range() {
        let mut r = Rng::new(7);
        for _ in 0..1000 {
            assert!(r.below(5) < 5);
        }
    }
}