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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
// This is the default multiplier used by PCG for 64-bit state.
const MULTIPLIER: u128 = 0x2360_ED05_1FC6_5DA4_4385_DF64_9FCC_F645;
use core::fmt;
use core::mem::transmute;
use rand_core::{RngCore, SeedableRng, Error, le};
/// A PCG random number generator (XSL 128/64 (MCG) variant).
///
/// Permuted Congruential Generator with 128-bit state, internal Multiplicative
/// Congruential Generator, and 64-bit output via "xorshift low (bits),
/// random rotation" output function.
///
/// This is a 128-bit MCG with the PCG-XSL-RR output function.
/// Note that compared to the standard `pcg64` (128-bit LCG with PCG-XSL-RR
/// output function), this RNG is faster, also has a long cycle, and still has
/// good performance on statistical tests.
///
/// Note: this RNG is only available using Rust 1.26 or later.
#[derive(Clone)]
#[cfg_attr(feature="serde1", derive(Serialize,Deserialize))]
pub struct Mcg128Xsl64 {
state: u128,
}
/// A friendly name for `Mcg128Xsl64`.
pub type Pcg64Mcg = Mcg128Xsl64;
impl Mcg128Xsl64 {
/// Construct an instance compatible with PCG seed.
///
/// Note that PCG specifies a default value for the parameter:
///
/// - `state = 0xcafef00dd15ea5e5`
pub fn new(state: u128) -> Self {
// Force low bit to 1, as in C version (C++ uses `state | 3` instead).
Mcg128Xsl64 { state: state | 1 }
}
}
// Custom Debug implementation that does not expose the internal state
impl fmt::Debug for Mcg128Xsl64 {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "Mcg128Xsl64 {{}}")
}
}
/// We use a single 126-bit seed to initialise the state and select a stream.
/// Two `seed` bits (lowest order of last byte) are ignored.
impl SeedableRng for Mcg128Xsl64 {
type Seed = [u8; 16];
fn from_seed(seed: Self::Seed) -> Self {
// Read as if a little-endian u128 value:
let mut seed_u64 = [0u64; 2];
le::read_u64_into(&seed, &mut seed_u64);
let state = (seed_u64[0] as u128) |
(seed_u64[1] as u128) << 64;
Mcg128Xsl64::new(state)
}
}
impl RngCore for Mcg128Xsl64 {
#[inline]
fn next_u32(&mut self) -> u32 {
self.next_u64() as u32
}
#[inline]
fn next_u64(&mut self) -> u64 {
// prepare the LCG for the next round
let state = self.state.wrapping_mul(MULTIPLIER);
self.state = state;
// Output function XSL RR ("xorshift low (bits), random rotation")
// Constants are for 128-bit state, 64-bit output
const XSHIFT: u32 = 64; // (128 - 64 + 64) / 2
const ROTATE: u32 = 122; // 128 - 6
let rot = (state >> ROTATE) as u32;
let xsl = ((state >> XSHIFT) as u64) ^ (state as u64);
xsl.rotate_right(rot)
}
#[inline]
fn fill_bytes(&mut self, dest: &mut [u8]) {
// specialisation of impls::fill_bytes_via_next; approx 3x faster
let mut left = dest;
while left.len() >= 8 {
let (l, r) = {left}.split_at_mut(8);
left = r;
let chunk: [u8; 8] = unsafe {
transmute(self.next_u64().to_le())
};
l.copy_from_slice(&chunk);
}
let n = left.len();
if n > 0 {
let chunk: [u8; 8] = unsafe {
transmute(self.next_u64().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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