symplex 0.13.0

//! The deterministic generator behind [`Distribution::sample`](super::Distribution::sample)
//! (SplitMix64): seeded and reproducible, so Monte-Carlo sanity checks of
//! exact results are repeatable.

/// A deterministic pseudo-random generator (SplitMix64).  Not
/// cryptographic; seeded, reproducible, and good enough for Monte-Carlo
/// sanity checks of exact results.
#[derive(Clone, Debug)]
pub struct Rng(u64);

impl Rng {
    /// A generator with the given seed.
    pub fn new(seed: u64) -> Self {
        Rng(seed)
    }

    /// The next 64 random bits.
    pub fn next_u64(&mut self) -> u64 {
        self.0 = self.0.wrapping_add(0x9E37_79B9_7F4A_7C15);
        let mut z = self.0;
        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, 1)` with 53 random bits.
    pub fn next_f64(&mut self) -> f64 {
        (self.next_u64() >> 11) as f64 / (1u64 << 53) as f64
    }

    /// Uniform in `0..n` (`n > 0`; `0` for `n == 0`).
    pub fn below(&mut self, n: usize) -> usize {
        if n == 0 {
            return 0;
        }
        (self.next_u64() % n as u64) as usize
    }
}