use core::cell::Cell;

/// A simple implementation of the XorShift64* pseudorandom number generator (PRNG).
/// Based on the algorithm described by George Marsaglia.
pub struct XorShift64Star {
    state: Cell<u64>,
}

impl XorShift64Star {
    /// Initializes the prng with a seed. Provided seed must be nonzero.
    pub fn new(seed: u64) -> Self {
        XorShift64Star {
            state: Cell::new(seed),
        }
    }

    /// Returns a pseudorandom number.
    pub fn next(&self) -> u64 {
        let mut x = self.state.get();
        debug_assert_ne!(x, 0);
        x ^= x >> 12;
        x ^= x << 25;
        x ^= x >> 27;
        self.state.set(x);
        x.wrapping_mul(0x2545_f491_4f6c_dd1d)
    }

    /// Return a pseudorandom number from `0..n`.
    pub fn next_usize(&self, n: usize) -> usize {
        (self.next() % n as u64) as usize
    }
}