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 rand_core::{impls, le, RngCore, SeedableRng};
15#[cfg(feature = "serde")]
16use serde::{Deserialize, Serialize};
17
18// This is the default multiplier used by PCG for 64-bit state.
19const MULTIPLIER: u64 = 6364136223846793005;
20
21/// A PCG random number generator (XSH RR 64/32 (LCG) variant).
22///
23/// Permuted Congruential Generator with 64-bit state, internal Linear
24/// Congruential Generator, and 32-bit output via "xorshift high (bits),
25/// random rotation" output function.
26///
27/// This is a 64-bit LCG with explicitly chosen stream with the PCG-XSH-RR
28/// output function. This combination is the standard `pcg32`.
29///
30/// Despite the name, this implementation uses 16 bytes (128 bit) space
31/// comprising 64 bits of state and 64 bits stream selector. These are both set
32/// by `SeedableRng`, using a 128-bit seed.
33///
34/// Note that two generators with different stream parameter may be closely
35/// correlated.
36#[derive(Clone, PartialEq, Eq)]
37#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
38pub struct Lcg64Xsh32 {
39    state: u64,
40    increment: u64,
41}
42
43/// [`Lcg64Xsh32`] is also officially known as `pcg32`.
44pub type Pcg32 = Lcg64Xsh32;
45
46impl Lcg64Xsh32 {
47    /// Multi-step advance functions (jump-ahead, jump-back)
48    ///
49    /// The method used here is based on Brown, "Random Number Generation
50    /// with Arbitrary Stride,", Transactions of the American Nuclear
51    /// Society (Nov. 1994).  The algorithm is very similar to fast
52    /// exponentiation.
53    ///
54    /// Even though delta is an unsigned integer, we can pass a
55    /// signed integer to go backwards, it just goes "the long way round".
56    ///
57    /// Using this function is equivalent to calling `next_32()` `delta`
58    /// number of times.
59    #[inline]
60    pub fn advance(&mut self, delta: u64) {
61        let mut acc_mult: u64 = 1;
62        let mut acc_plus: u64 = 0;
63        let mut cur_mult = MULTIPLIER;
64        let mut cur_plus = self.increment;
65        let mut mdelta = delta;
66
67        while mdelta > 0 {
68            if (mdelta & 1) != 0 {
69                acc_mult = acc_mult.wrapping_mul(cur_mult);
70                acc_plus = acc_plus.wrapping_mul(cur_mult).wrapping_add(cur_plus);
71            }
72            cur_plus = cur_mult.wrapping_add(1).wrapping_mul(cur_plus);
73            cur_mult = cur_mult.wrapping_mul(cur_mult);
74            mdelta /= 2;
75        }
76        self.state = acc_mult.wrapping_mul(self.state).wrapping_add(acc_plus);
77    }
78
79    /// Construct an instance compatible with PCG seed and stream.
80    ///
81    /// Note that the highest bit of the `stream` parameter is discarded
82    /// to simplify upholding internal invariants.
83    ///
84    /// Note that two generators with different stream parameters may be closely
85    /// correlated.
86    ///
87    /// PCG specifies the following default values for both parameters:
88    ///
89    /// - `state = 0xcafef00dd15ea5e5`
90    /// - `stream = 0xa02bdbf7bb3c0a7`
91    // Note: stream is 1442695040888963407u64 >> 1
92    pub fn new(state: u64, stream: u64) -> Self {
93        // The increment must be odd, hence we discard one bit:
94        let increment = (stream << 1) | 1;
95        Lcg64Xsh32::from_state_incr(state, increment)
96    }
97
98    #[inline]
99    fn from_state_incr(state: u64, increment: u64) -> Self {
100        let mut pcg = Lcg64Xsh32 { state, increment };
101        // Move away from initial value:
102        pcg.state = pcg.state.wrapping_add(pcg.increment);
103        pcg.step();
104        pcg
105    }
106
107    #[inline]
108    fn step(&mut self) {
109        // prepare the LCG for the next round
110        self.state = self
111            .state
112            .wrapping_mul(MULTIPLIER)
113            .wrapping_add(self.increment);
114    }
115}
116
117// Custom Debug implementation that does not expose the internal state
118impl fmt::Debug for Lcg64Xsh32 {
119    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
120        write!(f, "Lcg64Xsh32 {{}}")
121    }
122}
123
124impl SeedableRng for Lcg64Xsh32 {
125    type Seed = [u8; 16];
126
127    /// We use a single 127-bit seed to initialise the state and select a stream.
128    /// One `seed` bit (lowest bit of `seed[8]`) is ignored.
129    fn from_seed(seed: Self::Seed) -> Self {
130        let mut seed_u64 = [0u64; 2];
131        le::read_u64_into(&seed, &mut seed_u64);
132
133        // The increment must be odd, hence we discard one bit:
134        Lcg64Xsh32::from_state_incr(seed_u64[0], seed_u64[1] | 1)
135    }
136}
137
138impl RngCore for Lcg64Xsh32 {
139    #[inline]
140    fn next_u32(&mut self) -> u32 {
141        let state = self.state;
142        self.step();
143
144        // Output function XSH RR: xorshift high (bits), followed by a random rotate
145        // Constants are for 64-bit state, 32-bit output
146        const ROTATE: u32 = 59; // 64 - 5
147        const XSHIFT: u32 = 18; // (5 + 32) / 2
148        const SPARE: u32 = 27; // 64 - 32 - 5
149
150        let rot = (state >> ROTATE) as u32;
151        let xsh = (((state >> XSHIFT) ^ state) >> SPARE) as u32;
152        xsh.rotate_right(rot)
153    }
154
155    #[inline]
156    fn next_u64(&mut self) -> u64 {
157        impls::next_u64_via_u32(self)
158    }
159
160    #[inline]
161    fn fill_bytes(&mut self, dest: &mut [u8]) {
162        impls::fill_bytes_via_next(self, dest)
163    }
164}
rand_pcg/pcg64.rs

rand_pcg/
pcg64.rs
