pcg128.rs

// 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 128-bit state.
const MULTIPLIER: u128 = 0x2360_ED05_1FC6_5DA4_4385_DF64_9FCC_F645;

use core::fmt;
use rand_core::{le, Error, RngCore, SeedableRng};
#[cfg(feature = "serde1")] use serde::{Deserialize, Serialize};

/// A PCG random number generator (XSL RR 128/64 (LCG) variant).
///
/// Permuted Congruential Generator with 128-bit state, internal Linear
/// Congruential Generator, and 64-bit output via "xorshift low (bits),
/// random rotation" output function.
///
/// This is a 128-bit LCG with explicitly chosen stream with the PCG-XSL-RR
/// output function. This combination is the standard `pcg64`.
///
/// Despite the name, this implementation uses 32 bytes (256 bit) space
/// comprising 128 bits of state and 128 bits stream selector. These are both
/// set by `SeedableRng`, using a 256-bit seed.
#[derive(Clone, PartialEq, Eq)]
#[cfg_attr(feature = "serde1", derive(Serialize, Deserialize))]
pub struct Lcg128Xsl64 {
    state: u128,
    increment: u128,
}

/// [`Lcg128Xsl64`] is also officially known as `pcg64`.
pub type Pcg64 = Lcg128Xsl64;

impl Lcg128Xsl64 {
    /// Construct an instance compatible with PCG seed and stream.
    ///
    /// Note that PCG specifies default values for both parameters:
    ///
    /// - `state = 0xcafef00dd15ea5e5`
    /// - `stream = 0xa02bdbf7bb3c0a7ac28fa16a64abf96`
    pub fn new(state: u128, stream: u128) -> Self {
        // The increment must be odd, hence we discard one bit:
        let increment = (stream << 1) | 1;
        Lcg128Xsl64::from_state_incr(state, increment)
    }

    #[inline]
    fn from_state_incr(state: u128, increment: u128) -> Self {
        let mut pcg = Lcg128Xsl64 { 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 Lcg128Xsl64 {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "Lcg128Xsl64 {{}}")
    }
}

/// We use a single 255-bit seed to initialise the state and select a stream.
/// One `seed` bit (lowest bit of `seed[8]`) is ignored.
impl SeedableRng for Lcg128Xsl64 {
    type Seed = [u8; 32];

    fn from_seed(seed: Self::Seed) -> Self {
        let mut seed_u64 = [0u64; 4];
        le::read_u64_into(&seed, &mut seed_u64);
        let state = u128::from(seed_u64[0]) | (u128::from(seed_u64[1]) << 64);
        let incr = u128::from(seed_u64[2]) | (u128::from(seed_u64[3]) << 64);

        // The increment must be odd, hence we discard one bit:
        Lcg128Xsl64::from_state_incr(state, incr | 1)
    }
}

impl RngCore for Lcg128Xsl64 {
    #[inline]
    fn next_u32(&mut self) -> u32 {
        self.next_u64() as u32
    }

    #[inline]
    fn next_u64(&mut self) -> u64 {
        self.step();
        output_xsl_rr(self.state)
    }

    #[inline]
    fn fill_bytes(&mut self, dest: &mut [u8]) {
        fill_bytes_impl(self, dest)
    }

    #[inline]
    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
        self.fill_bytes(dest);
        Ok(())
    }
}


/// 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, also known as
/// `pcg64_fast`.
/// 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.
#[derive(Clone, PartialEq, Eq)]
#[cfg_attr(feature = "serde1", derive(Serialize, Deserialize))]
pub struct Mcg128Xsl64 {
    state: u128,
}

/// A friendly name for [`Mcg128Xsl64`] (also known as `pcg64_fast`).
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 = u128::from(seed_u64[0])  |
                    u128::from(seed_u64[1]) << 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 {
        self.state = self.state.wrapping_mul(MULTIPLIER);
        output_xsl_rr(self.state)
    }

    #[inline]
    fn fill_bytes(&mut self, dest: &mut [u8]) {
        fill_bytes_impl(self, dest)
    }

    #[inline]
    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
        self.fill_bytes(dest);
        Ok(())
    }
}

#[inline(always)]
fn output_xsl_rr(state: u128) -> u64 {
    // 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(always)]
fn fill_bytes_impl<R: RngCore + ?Sized>(rng: &mut R, dest: &mut [u8]) {
    let mut left = dest;
    while left.len() >= 8 {
        let (l, r) = { left }.split_at_mut(8);
        left = r;
        let chunk: [u8; 8] = rng.next_u64().to_le_bytes();
        l.copy_from_slice(&chunk);
    }
    let n = left.len();
    if n > 0 {
        let chunk: [u8; 8] = rng.next_u64().to_le_bytes();
        left.copy_from_slice(&chunk[..n]);
    }
}