//! Repeatable pseudo random numbers (SplitMix64).
//!
//! The core never uses any other source of randomness. The same seed always
//! gives the same sequence, on every machine.

/// Pseudo random number generator with 64 bits of state.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct Rng {
    state: u64,
}

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

    /// Returns the next value of the sequence.
    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)
    }

    /// Returns a value in `0..bound`.
    ///
    /// # Panics
    /// Panics if `bound` is zero.
    pub fn below(&mut self, bound: u32) -> u32 {
        assert!(bound > 0, "bound must be greater than zero");
        ((u128::from(self.next_u64()) * u128::from(bound)) >> 64) as u32
    }
}