SeedableRng to include from_seed_const

[phughk]

Background

I am making a single-allocation game (no heap, no alloc) in a nostd environment.
The allocation is fairly big for embedded devices (currently at 18kb).
Since this is a moderately large allocation, I would like it to be allocated outside the stack (not necessarily heap).
To achieve this, I have a const constructor.
The RNG needs to be included in the constructor (I am using Lcg64Xsh32).

What is your motivation?

I would like to initialise the rng internally of a const constructor.

What type of application is this? (E.g. cryptography, game, numerical simulation)

Embedded systems game with c-type interface.
Targets GameBoy Advance, Wasm, Desktop, maybe others.
Also targets machine learning models that can read from the entire allocation and learn to play.

Feature request

Add a const variant constructor for SeedableRng please

impl SeedableRng for ... {
    type Seed = [u8; 16];

    fn from_seed(seed: Self::Seed) -> Self {
        ...
    }

    const fn from_seed_const(seed: Self::Seed) -> Self {
        ...
    }
}

Happy to work on this if the feature request is agreed to be helpful

[newpavlov]

For a proper implementation this requires the unstable const_trait_impl language feature. So we probably will not add support for it until it's stabilized.

We could add inherent const methods, but I am not sure if it's worth the trouble. Usually PRNG initialization is a relatively cheap operation, so it can be handled fine by a lazy static in most cases.

[phughk]

Fair point about the language feature - can't get around that.

I gave lazy_static a go now, and the issue is that I would ideally like an owned value - that would require cloning the value (also not const). It's a good idea though; I might be able to work around that.

An alternative idea may be for rand_pcg structs to have this implemented without a trait.

I am going to go with Option<Pcg32> for now and initialise lazily. Not ideal, but it solves the problem.

[phughk]

Actually, I ended up modifying the implementation. Sharing in case anyone would like a copy.

// 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

use core::fmt;
use rand::{RngCore, SeedableRng};
use rand_pcg::rand_core::{impls, le};

// This is the default multiplier used by PCG for 64-bit state.
const MULTIPLIER: u64 = 6364136223846793005;

/// A PCG random number generator (XSH RR 64/32 (LCG) variant).
///
/// Permuted Congruential Generator with 64-bit state, internal Linear
/// Congruential Generator, and 32-bit output via "xorshift high (bits),
/// random rotation" output function.
///
/// This is a 64-bit LCG with explicitly chosen stream with the PCG-XSH-RR
/// output function. This combination is the standard `pcg32`.
///
/// Despite the name, this implementation uses 16 bytes (128 bit) space
/// comprising 64 bits of state and 64 bits stream selector. These are both set
/// by `SeedableRng`, using a 128-bit seed.
///
/// Note that two generators with different stream parameter may be closely
/// correlated.
#[derive(Clone, PartialEq, Eq)]
pub struct Lcg64Xsh32 {
    state: u64,
    increment: u64,
}

/// [`Lcg64Xsh32`] is also officially known as `pcg32`.
pub type Pcg32 = Lcg64Xsh32;

impl Lcg64Xsh32 {
    /// Multi-step advance functions (jump-ahead, jump-back)
    ///
    /// The method used here is based on Brown, "Random Number Generation
    /// with Arbitrary Stride,", Transactions of the American Nuclear
    /// Society (Nov. 1994).  The algorithm is very similar to fast
    /// exponentiation.
    ///
    /// Even though delta is an unsigned integer, we can pass a
    /// signed integer to go backwards, it just goes "the long way round".
    ///
    /// Using this function is equivalent to calling `next_32()` `delta`
    /// number of times.
    #[inline]
    pub fn advance(&mut self, delta: u64) {
        let mut acc_mult: u64 = 1;
        let mut acc_plus: u64 = 0;
        let mut cur_mult = MULTIPLIER;
        let mut cur_plus = self.increment;
        let mut mdelta = delta;

        while mdelta > 0 {
            if (mdelta & 1) != 0 {
                acc_mult = acc_mult.wrapping_mul(cur_mult);
                acc_plus = acc_plus.wrapping_mul(cur_mult).wrapping_add(cur_plus);
            }
            cur_plus = cur_mult.wrapping_add(1).wrapping_mul(cur_plus);
            cur_mult = cur_mult.wrapping_mul(cur_mult);
            mdelta /= 2;
        }
        self.state = acc_mult.wrapping_mul(self.state).wrapping_add(acc_plus);
    }

    /// Construct an instance compatible with PCG seed and stream.
    ///
    /// Note that the highest bit of the `stream` parameter is discarded
    /// to simplify upholding internal invariants.
    ///
    /// Note that two generators with different stream parameters may be closely
    /// correlated.
    ///
    /// PCG specifies the following default values for both parameters:
    ///
    /// - `state = 0xcafef00dd15ea5e5`
    /// - `stream = 0xa02bdbf7bb3c0a7`
    // Note: stream is 1442695040888963407u64 >> 1
    pub fn new(state: u64, stream: u64) -> Self {
        // The increment must be odd, hence we discard one bit:
        let increment = (stream << 1) | 1;
        Lcg64Xsh32::from_state_incr(state, increment)
    }

    #[inline]
    const fn from_state_incr(state: u64, increment: u64) -> Self {
        let mut pcg = Lcg64Xsh32 { state, increment };
        // Move away from initial value:
        pcg.state = pcg.state.wrapping_add(pcg.increment);
        pcg.step();
        pcg
    }

    #[inline]
    const fn step(&mut self) {
        // prepare the LCG for the next round
        self.state = self.state.wrapping_mul(MULTIPLIER).wrapping_add(self.increment);
    }

    pub const fn seed_const(seed: [u8; 16]) -> Self {
        let mut seed_u64 = [0u64; 2];
        // le::read_u64_into(&seed, &mut seed_u64);
        let mut i = 0;
        while i < seed_u64.len() {
            let mut v = 0u64;
            let mut p = 0;
            while p < 8 {
                v |= (seed[i * 8 + p] as u64) << (p * 8);
                p += 1;
            }
            seed_u64[i] = v;
            i += 1;
        }

        // The increment must be odd, hence we discard one bit:
        Lcg64Xsh32::from_state_incr(seed_u64[0], seed_u64[1] | 1)
    }
}

// Custom Debug implementation that does not expose the internal state
impl fmt::Debug for Lcg64Xsh32 {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "Lcg64Xsh32 {{}}")
    }
}

impl SeedableRng for Lcg64Xsh32 {
    type Seed = [u8; 16];

    /// We use a single 127-bit seed to initialise the state and select a stream.
    /// One `seed` bit (lowest bit of `seed[8]`) is ignored.
    fn from_seed(seed: Self::Seed) -> Self {
        let mut seed_u64 = [0u64; 2];
        le::read_u64_into(&seed, &mut seed_u64);

        // The increment must be odd, hence we discard one bit:
        Lcg64Xsh32::from_state_incr(seed_u64[0], seed_u64[1] | 1)
    }
}

impl RngCore for Lcg64Xsh32 {
    #[inline]
    fn next_u32(&mut self) -> u32 {
        let state = self.state;
        self.step();

        // Output function XSH RR: xorshift high (bits), followed by a random rotate
        // Constants are for 64-bit state, 32-bit output
        const ROTATE: u32 = 59; // 64 - 5
        const XSHIFT: u32 = 18; // (5 + 32) / 2
        const SPARE: u32 = 27; // 64 - 32 - 5

        let rot = (state >> ROTATE) as u32;
        let xsh = (((state >> XSHIFT) ^ state) >> SPARE) as u32;
        xsh.rotate_right(rot)
    }

    #[inline]
    fn next_u64(&mut self) -> u64 {
        impls::next_u64_via_u32(self)
    }

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

[dhardy]

I think we could add this const fn on the RNG directly, but as @newpavlov says, not on the trait (for now). So make a PR (against the rand_pcg crate) if you like.