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use rand_core::impls::fill_bytes_via_next;
use rand_core::le::read_u64_into;
use rand_core::{Error, RngCore, SeedableRng};
/// A xoshiro256** random number generator.
///
/// The xoshiro256** algorithm is not suitable for cryptographic purposes, but
/// is very fast and has excellent statistical properties.
///
/// The algorithm used here is translated from [the `xoshiro256starstar.c`
/// reference source code](http://xoshiro.di.unimi.it/xoshiro256starstar.c) by
/// David Blackman and Sebastiano Vigna.
#[cfg_attr(not(target_os = "cuda"), derive(Copy, cust_core::DeviceCopy))]
#[derive(Debug, Clone, PartialEq, Eq)]
#[repr(C)]
pub struct Xoshiro256StarStar {
s: [u64; 4],
}
impl Xoshiro256StarStar {
/// Jump forward, equivalently to 2^128 calls to `next_u64()`.
///
/// This can be used to generate 2^128 non-overlapping subsequences for
/// parallel computations.
///
/// ```
/// use rand_xoshiro::rand_core::SeedableRng;
/// use rand_xoshiro::Xoshiro256StarStar;
///
/// let rng1 = Xoshiro256StarStar::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,
[
0x180ec6d33cfd0aba,
0xd5a61266f0c9392c,
0xa9582618e03fc9aa,
0x39abdc4529b1661c
]
);
}
/// Jump forward, equivalently to 2^192 calls to `next_u64()`.
///
/// This can be used to generate 2^64 starting points, from each of which
/// `jump()` will generate 2^64 non-overlapping subsequences for parallel
/// distributed computations.
pub fn long_jump(&mut self) {
impl_jump!(
u64,
self,
[
0x76e15d3efefdcbbf,
0xc5004e441c522fb3,
0x77710069854ee241,
0x39109bb02acbe635
]
);
}
/// Initializes multiple RNG states such that each state corresponds to a subsequence
/// separated by `2**128` steps from eachother in the main sequence. This ensures that as long as
/// no state requests more than `2**128` random numbers, the states are guaranteed to be fully independent.
#[cfg(not(target_os = "cuda"))]
pub fn initialize_states(seed: u64, num_states: usize) -> Vec<Self> {
impl_initialize_states!(seed, num_states)
}
}
impl SeedableRng for Xoshiro256StarStar {
type Seed = [u8; 32];
/// Create a new `Xoshiro256StarStar`. If `seed` is entirely 0, it will be
/// mapped to a different seed.
#[inline]
fn from_seed(seed: [u8; 32]) -> Xoshiro256StarStar {
deal_with_zero_seed!(seed, Self);
let mut state = [0; 4];
read_u64_into(&seed, &mut state);
Xoshiro256StarStar { s: state }
}
/// Seed a `Xoshiro256StarStar` from a `u64` using `SplitMix64`.
fn seed_from_u64(seed: u64) -> Xoshiro256StarStar {
from_splitmix!(seed)
}
}
impl RngCore for Xoshiro256StarStar {
#[inline]
fn next_u32(&mut self) -> u32 {
// The lowest bits have some linear dependencies, so we use the
// upper bits instead.
(self.next_u64() >> 32) as u32
}
#[inline]
fn next_u64(&mut self) -> u64 {
let result_starstar = starstar_u64!(self.s[1]);
impl_xoshiro_u64!(self);
result_starstar
}
#[inline]
fn fill_bytes(&mut self, dest: &mut [u8]) {
fill_bytes_via_next(self, dest);
}
#[inline]
fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
self.fill_bytes(dest);
Ok(())
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn reference() {
let mut rng = Xoshiro256StarStar::from_seed([
1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0,
0, 0, 0,
]);
// These values were produced with the reference implementation:
// http://xoshiro.di.unimi.it/xoshiro128starstar.c
let expected = [
11520,
0,
1509978240,
1215971899390074240,
1216172134540287360,
607988272756665600,
16172922978634559625,
8476171486693032832,
10595114339597558777,
2904607092377533576,
];
for &e in &expected {
assert_eq!(rng.next_u64(), e);
}
}
}