rand_xoshiro 0.8.0

// Copyright 2018 Developers of the Rand project.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use core::convert::Infallible;
use rand_core::{Rng, SeedableRng, TryRng, utils};
#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};

/// A xoroshiro128+ random number generator.
///
/// The xoroshiro128+ algorithm is not suitable for cryptographic purposes, but
/// is very fast and has good statistical properties, besides a low linear
/// complexity in the lowest bits.
///
/// The algorithm used here is translated from [the `xoroshiro128plus.c`
/// reference source code](http://xoshiro.di.unimi.it/xoroshiro128plus.c) by
/// David Blackman and Sebastiano Vigna.
#[allow(missing_copy_implementations)]
#[derive(Debug, Clone, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
pub struct Xoroshiro128Plus {
    s0: u64,
    s1: u64,
}

impl Xoroshiro128Plus {
    /// Jump forward, equivalently to 2^64 calls to `next_u64()`.
    ///
    /// This can be used to generate 2^64 non-overlapping subsequences for
    /// parallel computations.
    ///
    /// ```
    /// use rand_xoshiro::rand_core::SeedableRng;
    /// use rand_xoshiro::Xoroshiro128Plus;
    ///
    /// let rng1 = Xoroshiro128Plus::seed_from_u64(0);
    /// let mut rng2 = rng1.clone();
    /// rng2.jump();
    /// let mut rng3 = rng2.clone();
    /// rng3.jump();
    /// ```
    pub fn jump(&mut self) {
        impl_jump!(u64, self, [0xdf900294d8f554a5, 0x170865df4b3201fc]);
    }

    /// Jump forward, equivalently to 2^96 calls to `next_u64()`.
    ///
    /// This can be used to generate 2^32 starting points, from each of which
    /// `jump()` will generate 2^32 non-overlapping subsequences for parallel
    /// distributed computations.
    pub fn long_jump(&mut self) {
        impl_jump!(u64, self, [0xd2a98b26625eee7b, 0xdddf9b1090aa7ac1]);
    }
}

impl TryRng for Xoroshiro128Plus {
    type Error = Infallible;

    #[inline]
    fn try_next_u32(&mut self) -> Result<u32, Self::Error> {
        // The two lowest bits have some linear dependencies, so we use the
        // upper bits instead.
        Ok((self.next_u64() >> 32) as u32)
    }

    #[inline]
    fn try_next_u64(&mut self) -> Result<u64, Self::Error> {
        let r = self.s0.wrapping_add(self.s1);
        impl_xoroshiro_u64!(self);
        Ok(r)
    }

    #[inline]
    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Self::Error> {
        utils::fill_bytes_via_next_word(dest, || self.try_next_u64())
    }
}
impl SeedableRng for Xoroshiro128Plus {
    type Seed = [u8; 16];

    /// Create a new `Xoroshiro128Plus`.  If `seed` is entirely 0, it will be
    /// mapped to a different seed.
    fn from_seed(seed: [u8; 16]) -> Xoroshiro128Plus {
        deal_with_zero_seed!(seed, Self, 16);
        let s: [_; 2] = utils::read_words(&seed);

        Xoroshiro128Plus { s0: s[0], s1: s[1] }
    }

    /// Seed a `Xoroshiro128Plus` from a `u64` using `SplitMix64`.
    fn seed_from_u64(seed: u64) -> Xoroshiro128Plus {
        from_splitmix!(seed)
    }
}

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

    #[test]
    fn reference() {
        let mut rng = Xoroshiro128Plus::from_seed([1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0]);
        // These values were produced with the reference implementation:
        // http://xoshiro.di.unimi.it/xoshiro128starstar.c
        let expected = [
            3,
            412333834243,
            2360170716294286339,
            9295852285959843169,
            2797080929874688578,
            6019711933173041966,
            3076529664176959358,
            3521761819100106140,
            7493067640054542992,
            920801338098114767,
        ];
        for &e in &expected {
            assert_eq!(rng.next_u64(), e);
        }
    }
}