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// Copyright 2018 Developers of the Rand project.
// Copyright 2017 Paul Dicker.
// Copyright 2014-2017 Melissa O'Neill and PCG Project contributors
//
// 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.
//! PCG random number generators
use core::fmt;
use core::mem::transmute;
use rand_core::{RngCore, SeedableRng, Error, le, impls};
// This is the default multiplier used by PCG for 64-bit state.
const MULTIPLIER: u64 = 6364136223846793005;
/// A PCG random number generator (XSH RR 64/32 (LCG) variant).
///
/// Permuted Congruential Generator with 64-bit state, internal Linear
/// Congruential Generator, and 32-bit output via "xorshift high (bits),
/// random rotation" output function.
///
/// This is a 64-bit LCG with explicitly chosen stream with the PCG-XSH-RR
/// output function. This combination is the standard `pcg32`.
///
/// Despite the name, this implementation uses 16 bytes (128 bit) space
/// comprising 64 bits of state and 64 bits stream selector. These are both set
/// by `SeedableRng`, using a 128-bit seed.
#[derive(Clone)]
#[cfg_attr(feature="serde1", derive(Serialize,Deserialize))]
pub struct Lcg64Xsh32 {
state: u64,
increment: u64,
}
/// `Lcg64Xsh32` is also officially known as `pcg32`.
pub type Pcg32 = Lcg64Xsh32;
impl Lcg64Xsh32 {
/// Construct an instance compatible with PCG seed and stream.
///
/// Note that PCG specifies default values for both parameters:
///
/// - `state = 0xcafef00dd15ea5e5`
/// - `stream = 721347520444481703`
pub fn new(state: u64, stream: u64) -> Self {
// The increment must be odd, hence we discard one bit:
let increment = (stream << 1) | 1;
Lcg64Xsh32::from_state_incr(state, increment)
}
#[inline]
fn from_state_incr(state: u64, increment: u64) -> Self {
let mut pcg = Lcg64Xsh32 { state, increment };
// Move away from inital value:
pcg.state = pcg.state.wrapping_add(pcg.increment);
pcg.step();
pcg
}
#[inline]
fn step(&mut self) {
// prepare the LCG for the next round
self.state = self.state
.wrapping_mul(MULTIPLIER)
.wrapping_add(self.increment);
}
}
// Custom Debug implementation that does not expose the internal state
impl fmt::Debug for Lcg64Xsh32 {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "Lcg64Xsh32 {{}}")
}
}
/// We use a single 127-bit seed to initialise the state and select a stream.
/// One `seed` bit (lowest bit of `seed[8]`) is ignored.
impl SeedableRng for Lcg64Xsh32 {
type Seed = [u8; 16];
fn from_seed(seed: Self::Seed) -> Self {
let mut seed_u64 = [0u64; 2];
le::read_u64_into(&seed, &mut seed_u64);
// The increment must be odd, hence we discard one bit:
Lcg64Xsh32::from_state_incr(seed_u64[0], seed_u64[1] | 1)
}
}
impl RngCore for Lcg64Xsh32 {
#[inline]
fn next_u32(&mut self) -> u32 {
let state = self.state;
self.step();
// Output function XSH RR: xorshift high (bits), followed by a random rotate
// Constants are for 64-bit state, 32-bit output
const ROTATE: u32 = 59; // 64 - 5
const XSHIFT: u32 = 18; // (5 + 32) / 2
const SPARE: u32 = 27; // 64 - 32 - 5
let rot = (state >> ROTATE) as u32;
let xsh = (((state >> XSHIFT) ^ state) >> SPARE) as u32;
xsh.rotate_right(rot)
}
#[inline]
fn next_u64(&mut self) -> u64 {
impls::next_u64_via_u32(self)
}
#[inline]
fn fill_bytes(&mut self, dest: &mut [u8]) {
// specialisation of impls::fill_bytes_via_next; approx 40% faster
let mut left = dest;
while left.len() >= 4 {
let (l, r) = {left}.split_at_mut(4);
left = r;
let chunk: [u8; 4] = unsafe {
transmute(self.next_u32().to_le())
};
l.copy_from_slice(&chunk);
}
let n = left.len();
if n > 0 {
let chunk: [u8; 4] = unsafe {
transmute(self.next_u32().to_le())
};
left.copy_from_slice(&chunk[..n]);
}
}
#[inline]
fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
Ok(self.fill_bytes(dest))
}
}
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