rand_pcg/
pcg128.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
13// This is the default multiplier used by PCG for 64-bit state.
14const MULTIPLIER: u128 = 0x2360_ED05_1FC6_5DA4_4385_DF64_9FCC_F645;
15
16use core::fmt;
17use core::mem::transmute;
18use rand_core::{RngCore, SeedableRng, Error, le};
19
20/// A PCG random number generator (XSL 128/64 (MCG) variant).
21///
22/// Permuted Congruential Generator with 128-bit state, internal Multiplicative
23/// Congruential Generator, and 64-bit output via "xorshift low (bits),
24/// random rotation" output function.
25///
26/// This is a 128-bit MCG with the PCG-XSL-RR output function.
27/// Note that compared to the standard `pcg64` (128-bit LCG with PCG-XSL-RR
28/// output function), this RNG is faster, also has a long cycle, and still has
29/// good performance on statistical tests.
30///
31/// Note: this RNG is only available using Rust 1.26 or later.
32#[derive(Clone)]
33#[cfg_attr(feature="serde1", derive(Serialize,Deserialize))]
34pub struct Mcg128Xsl64 {
35    state: u128,
36}
37
38/// A friendly name for `Mcg128Xsl64`.
39pub type Pcg64Mcg = Mcg128Xsl64;
40
41impl Mcg128Xsl64 {
42    /// Construct an instance compatible with PCG seed.
43    ///
44    /// Note that PCG specifies a default value for the parameter:
45    ///
46    /// - `state = 0xcafef00dd15ea5e5`
47    pub fn new(state: u128) -> Self {
48        // Force low bit to 1, as in C version (C++ uses `state | 3` instead).
49        Mcg128Xsl64 { state: state | 1 }
50    }
51}
52
53// Custom Debug implementation that does not expose the internal state
54impl fmt::Debug for Mcg128Xsl64 {
55    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
56        write!(f, "Mcg128Xsl64 {{}}")
57    }
58}
59
60/// We use a single 126-bit seed to initialise the state and select a stream.
61/// Two `seed` bits (lowest order of last byte) are ignored.
62impl SeedableRng for Mcg128Xsl64 {
63    type Seed = [u8; 16];
64
65    fn from_seed(seed: Self::Seed) -> Self {
66        // Read as if a little-endian u128 value:
67        let mut seed_u64 = [0u64; 2];
68        le::read_u64_into(&seed, &mut seed_u64);
69        let state = (seed_u64[0] as u128) |
70                    (seed_u64[1] as u128) << 64;
71        Mcg128Xsl64::new(state)
72    }
73}
74
75impl RngCore for Mcg128Xsl64 {
76    #[inline]
77    fn next_u32(&mut self) -> u32 {
78        self.next_u64() as u32
79    }
80
81    #[inline]
82    fn next_u64(&mut self) -> u64 {
83        // prepare the LCG for the next round
84        let state = self.state.wrapping_mul(MULTIPLIER);
85        self.state = state;
86
87        // Output function XSL RR ("xorshift low (bits), random rotation")
88        // Constants are for 128-bit state, 64-bit output
89        const XSHIFT: u32 = 64;     // (128 - 64 + 64) / 2
90        const ROTATE: u32 = 122;    // 128 - 6
91
92        let rot = (state >> ROTATE) as u32;
93        let xsl = ((state >> XSHIFT) as u64) ^ (state as u64);
94        xsl.rotate_right(rot)
95    }
96
97    #[inline]
98    fn fill_bytes(&mut self, dest: &mut [u8]) {
99        // specialisation of impls::fill_bytes_via_next; approx 3x faster
100        let mut left = dest;
101        while left.len() >= 8 {
102            let (l, r) = {left}.split_at_mut(8);
103            left = r;
104            let chunk: [u8; 8] = unsafe {
105                transmute(self.next_u64().to_le())
106            };
107            l.copy_from_slice(&chunk);
108        }
109        let n = left.len();
110        if n > 0 {
111            let chunk: [u8; 8] = unsafe {
112                transmute(self.next_u64().to_le())
113            };
114            left.copy_from_slice(&chunk[..n]);
115        }
116    }
117
118    #[inline]
119    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
120        Ok(self.fill_bytes(dest))
121    }
122}
rand_pcg/pcg128.rs

rand_pcg/
pcg128.rs
