rand_pcg/
pcg64.rs

1// Copyright 2018 Developers of the Rand project.
2// Copyright 2017 Paul Dicker.
3// Copyright 2014-2017 Melissa O'Neill and PCG Project contributors
4//
5// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
8// option. This file may not be copied, modified, or distributed
9// except according to those terms.
10
11//! PCG random number generators
12
13use core::fmt;
14use core::mem::transmute;
15use rand_core::{RngCore, SeedableRng, Error, le, impls};
16
17// This is the default multiplier used by PCG for 64-bit state.
18const MULTIPLIER: u64 = 6364136223846793005;
19
20/// A PCG random number generator (XSH RR 64/32 (LCG) variant).
21///
22/// Permuted Congruential Generator with 64-bit state, internal Linear
23/// Congruential Generator, and 32-bit output via "xorshift high (bits),
24/// random rotation" output function.
25///
26/// This is a 64-bit LCG with explicitly chosen stream with the PCG-XSH-RR
27/// output function. This combination is the standard `pcg32`.
28///
29/// Despite the name, this implementation uses 16 bytes (128 bit) space
30/// comprising 64 bits of state and 64 bits stream selector. These are both set
31/// by `SeedableRng`, using a 128-bit seed.
32#[derive(Clone)]
33#[cfg_attr(feature="serde1", derive(Serialize,Deserialize))]
34pub struct Lcg64Xsh32 {
35    state: u64,
36    increment: u64,
37}
38
39/// `Lcg64Xsh32` is also officially known as `pcg32`.
40pub type Pcg32 = Lcg64Xsh32;
41
42impl Lcg64Xsh32 {
43    /// Construct an instance compatible with PCG seed and stream.
44    ///
45    /// Note that PCG specifies default values for both parameters:
46    ///
47    /// - `state = 0xcafef00dd15ea5e5`
48    /// - `stream = 721347520444481703`
49    pub fn new(state: u64, stream: u64) -> Self {
50        // The increment must be odd, hence we discard one bit:
51        let increment = (stream << 1) | 1;
52        Lcg64Xsh32::from_state_incr(state, increment)
53    }
54
55    #[inline]
56    fn from_state_incr(state: u64, increment: u64) -> Self {
57        let mut pcg = Lcg64Xsh32 { state, increment };
58        // Move away from inital value:
59        pcg.state = pcg.state.wrapping_add(pcg.increment);
60        pcg.step();
61        pcg
62    }
63
64    #[inline]
65    fn step(&mut self) {
66        // prepare the LCG for the next round
67        self.state = self.state
68            .wrapping_mul(MULTIPLIER)
69            .wrapping_add(self.increment);
70    }
71}
72
73// Custom Debug implementation that does not expose the internal state
74impl fmt::Debug for Lcg64Xsh32 {
75    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
76        write!(f, "Lcg64Xsh32 {{}}")
77    }
78}
79
80/// We use a single 127-bit seed to initialise the state and select a stream.
81/// One `seed` bit (lowest bit of `seed[8]`) is ignored.
82impl SeedableRng for Lcg64Xsh32 {
83    type Seed = [u8; 16];
84
85    fn from_seed(seed: Self::Seed) -> Self {
86        let mut seed_u64 = [0u64; 2];
87        le::read_u64_into(&seed, &mut seed_u64);
88
89        // The increment must be odd, hence we discard one bit:
90        Lcg64Xsh32::from_state_incr(seed_u64[0], seed_u64[1] | 1)
91    }
92}
93
94impl RngCore for Lcg64Xsh32 {
95    #[inline]
96    fn next_u32(&mut self) -> u32 {
97        let state = self.state;
98        self.step();
99
100        // Output function XSH RR: xorshift high (bits), followed by a random rotate
101        // Constants are for 64-bit state, 32-bit output
102        const ROTATE: u32 = 59; // 64 - 5
103        const XSHIFT: u32 = 18; // (5 + 32) / 2
104        const SPARE: u32 = 27;  // 64 - 32 - 5
105
106        let rot = (state >> ROTATE) as u32;
107        let xsh = (((state >> XSHIFT) ^ state) >> SPARE) as u32;
108        xsh.rotate_right(rot)
109    }
110
111    #[inline]
112    fn next_u64(&mut self) -> u64 {
113        impls::next_u64_via_u32(self)
114    }
115
116    #[inline]
117    fn fill_bytes(&mut self, dest: &mut [u8]) {
118        // specialisation of impls::fill_bytes_via_next; approx 40% faster
119        let mut left = dest;
120        while left.len() >= 4 {
121            let (l, r) = {left}.split_at_mut(4);
122            left = r;
123            let chunk: [u8; 4] = unsafe {
124                transmute(self.next_u32().to_le())
125            };
126            l.copy_from_slice(&chunk);
127        }
128        let n = left.len();
129        if n > 0 {
130            let chunk: [u8; 4] = unsafe {
131                transmute(self.next_u32().to_le())
132            };
133            left.copy_from_slice(&chunk[..n]);
134        }
135    }
136
137    #[inline]
138    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
139        Ok(self.fill_bytes(dest))
140    }
141}
rand_pcg/pcg64.rs

rand_pcg/
pcg64.rs
