LibHac/tests/LibHac.Tests/FullCycleRandom.cs

using System;
using System.Numerics;

namespace LibHac.Tests;

/// <summary>
/// Simple, full-cycle PRNG for use in tests.
/// </summary>
public class FullCycleRandom
{
    private int _state;
    private int _mult;
    private int _inc;
    private int _and;
    private int _max;

    public FullCycleRandom(int period, int seed)
    {
        // Avoid exponential growth pattern when initializing with a 0 seed
        seed ^= 0x55555555;
        _max = period - 1;
        int order = BitOperations.Log2((uint)period - 1) + 1;

        // There isn't any deep reasoning behind the choice of the number of bits
        // in the seed used for initializing each parameter
        int multSeedBits = Math.Max(order >> 1, 2);
        int multSeedMask = (1 << multSeedBits) - 1;
        int multSeed = seed & multSeedMask;
        _mult = (multSeed << 2) | 5;

        int incSeedBits = Math.Max(order >> 2, 2);
        int incSeedMask = (1 << incSeedBits) - 1;
        int incSeed = (seed >> multSeedBits) & incSeedMask;
        _inc = incSeed | 1;

        int stateSeedBits = order;
        int stateSeedMask = (1 << stateSeedBits) - 1;
        int stateSeed = (seed >> multSeedBits + incSeedBits) & stateSeedMask;
        _state = stateSeed;

        _and = (1 << order) - 1;
    }

    public int Next()
    {
        do
        {
            _state = (_state * _mult + _inc) & _and;
        } while (_state > _max);

        return _state;
    }
}