gnitz-zset 0.1.3

The Z-set kernel of the gnitz database: schema, columnar batches, cursors and operators
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//! The test-only deterministic PRNG, compiled here and as part of
//! `gnitz-zset-testkit`.
//!
//! Tiny xorshift64* — no `rand` crate dependency, deterministic across hosts. One
//! copy for every property / differential test in the crate (k-way merge, range
//! join, …) instead of a per-module re-implementation.

pub struct Rng(u64);

impl Rng {
    pub fn new(seed: u64) -> Self {
        Self(seed | 1)
    }

    pub fn next_u64(&mut self) -> u64 {
        let mut x = self.0;
        x ^= x >> 12;
        x ^= x << 25;
        x ^= x >> 27;
        self.0 = x;
        x.wrapping_mul(0x2545F4914F6CDD1D)
    }

    pub fn gen_u128(&mut self) -> u128 {
        ((self.next_u64() as u128) << 64) | (self.next_u64() as u128)
    }

    pub fn gen_range(&mut self, max: u64) -> u64 {
        self.next_u64() % max
    }

    /// A uniformly drawn element of `items`.
    pub fn pick<T: Clone>(&mut self, items: &[T]) -> T {
        items[self.gen_range(items.len() as u64) as usize].clone()
    }

    /// Fisher–Yates, in place.
    pub fn shuffle<T>(&mut self, items: &mut [T]) {
        for i in (1..items.len()).rev() {
            items.swap(i, self.gen_range(i as u64 + 1) as usize);
        }
    }
}