AuroraRuntime/Source/RNG/WELL.cpp

/*
 * Copyright (C) 2008, Chris Lomont
 *               2021-2020, Rocks
 * Public Domain
 *
 * http://lomont.org/papers/2008/Lomont_PRNG_2008.pdf
 */
#include <Source/RuntimeInternal.hpp>
#include "WELL.hpp"
#include "mtwister.hpp"

// WELL should provide us reasonably 'secure' and, more importantly, fast RNG for
// general purpose use. It isn't fundamentally broken unlike MT, I dont think 512
// or 16 samples is going to break anything unlike MT with 624 or so. WELL has
// demonstrable better statistical properties and 32-bit performance. I think it 
// makes for a better defacto backend than MT. Generic xorshiftrots might be alright
// for improved speed at the cost of what little reliability PRNGs have.

// Redunced seed entropy should not matter to the extent that MT can make up for it
// WELL is used in the fast rng backend, and i feel as though it would be unreasonable
// to go above UInt64 seeds. For toy purposes, U32/64 seeds are probably fine. Treat
// WELL like MT without 624 prediction and performance issues. 

static auline void WELL_SeedRand64(WELLRand *rand, AuUInt64 seed)
{
    MTRand mtrand  = MT_SeedRand(seed & 0xffffffff);
    MTRand mtrand2 = MT_SeedRand(seed >> 32);
    
    AuMemset(rand->state, 0, sizeof(rand->state));

    // Let's mess with the entropy in case there is an attempt to reverse the base seed from
    // iterations of the WELL_NextLong_Unlocked function. Variance between mtrand and mtrand2
    // increases with each iteration up to a hopefully unknown value
    AuUInt8 confuser = (MT_NextLong(&mtrand2) & 0xFF) / 5 + 5;
    
    for (int z = 0; z < confuser; z++)
    {
        for (int i = 0; i < z; i++)
        {
            MT_NextLong(&mtrand);
        }

        for (unsigned int i = 0; i < AuArraySize(rand->state); i++)
        {
            rand->state[i] = MT_NextLong(&mtrand) ^ MT_NextLong(&mtrand2);
        }
    }
}

static auline void WELL_SeedRand(WELLRand *rand, AuUInt32 seed)
{
    MTRand mtrand = MT_SeedRand(seed);

    AuMemset(rand->state, 0, sizeof(rand->state));

    AuUInt8 confuser = (MT_NextLong(&mtrand) & 0xFF) / 5 + 5;

    for (int z = 0; z < confuser; z++)
    {
        for (int i = 0; i < z; i++)
        {
            MT_NextLong(&mtrand);
        }

        for (unsigned int i = 0; i < AuArraySize(rand->state); i++)
        {
            rand->state[i] = MT_NextLong(&mtrand);
        }
    }
}

/**
 * Creates a new random number generator from a given seed.
 */
WELLRand WELL_SeedRand(AuUInt32 seed)
{
    WELLRand rand {};
    WELL_SeedRand(&rand, seed);
    return rand;
}

WELLRand WELL_SeedRand64(AuUInt64 seed)
{
    WELLRand rand {};
    WELL_SeedRand64(&rand, seed);
    return rand;
}

WELLRand WELL_SeedRandBig64(const AuArray<AuUInt8, 64> &seed)
{
    WELLRand rand {};
    static_assert(64 == sizeof(rand.state));
    AuMemcpy(rand.state, seed.data(), sizeof(rand.state));
    return rand;
}

/**
 * Generates a pseudo-randomly generated long.
 */
auline AuUInt32 WELL_NextLong_Unlocked(WELLRand *rand)
{
    AuUInt32 a, b, c, d, ret;

    a = rand->state[rand->index];
    c = rand->state[(rand->index + 13) & 15];
    b = a ^ c ^ (a << 16) ^ (c << 15);
    c = rand->state[(rand->index + 9) & 15];
    c ^= (c >> 11);
    a = rand->state[rand->index] = b ^ c;
    d = a ^ ((a << 5) & 0xDA442D24UL);
    rand->index = (rand->index + 15) & 15;
    a = rand->state[rand->index];
    rand->state[rand->index] = a ^ b ^ d ^ (a << 2) ^ (b << 18) ^ (c << 28);
    ret = rand->state[rand->index];
    return ret;
}

/**
 * Generates a pseudo-randomly generated long.
 */
AuUInt32 WELL_NextLong(WELLRand *rand)
{
    AU_LOCK_GUARD(rand->lock);
    AuUInt32 ret = WELL_NextLong_Unlocked(rand);
    return ret;
}

void WELL_NextBytes(WELLRand *rand, void *in, AuUInt32 length)
{
    AuUInt i;
    AuUInt8 *base;

    i    = 0;
    base = reinterpret_cast<AuUInt8 *>(in);

    AU_LOCK_GUARD(rand->lock);

    for (; i < length; i += 4)
    {
        AuUInt32 rng = WELL_NextLong_Unlocked(rand);
        AuMemcpy(base + i, &rng, 4);
    }
    
    if (i > length)
    {
        i -= 4;
        AuUInt32 padRng = WELL_NextLong_Unlocked(rand);
        AuMemcpy(base + i, &padRng, length - i);
    }
}