CryptographicRandomSource.cs

using System;
using System.Runtime.Intrinsics.X86;
using System.Security.Cryptography;

public sealed class CryptographicRandomSource : IRandomSource, IDisposable
{
    private readonly RandomNumberGenerator generator;

    public CryptographicRandomSource()
    {
        generator = RandomNumberGenerator.Create();
    }

    public int ChooseIndex(int optionCount)
    {
        if (optionCount < 1) throw new ArgumentOutOfRangeException(nameof(optionCount), optionCount, "There must be at least one option.");

        var maxValue = (uint)optionCount - 1;
        var maxValueLeadingZeroCount = (int)Lzcnt.LeadingZeroCount(maxValue);

        var result = default(uint);

        var minimumNumberOfBytesToFill = sizeof(uint) - (maxValueLeadingZeroCount / 8);

        Span<byte> resultBytesToFill;
        unsafe
        {
            resultBytesToFill = CreateSpanForLeastSignificantBytes(&result, minimumNumberOfBytesToFill);
        }

        var maskToRemoveBitsGuaranteedToExceedMaximum = uint.MaxValue >> maxValueLeadingZeroCount;

        while (true)
        {
            generator.GetBytes(resultBytesToFill);
            result &= maskToRemoveBitsGuaranteedToExceedMaximum;
            if (result <= maxValue) return (int)result;
        }
    }

    private unsafe static Span<byte> CreateSpanForLeastSignificantBytes(uint* memory, int byteCount)
    {
        return new Span<byte>(
            (byte*)memory + (BitConverter.IsLittleEndian ? 0 : sizeof(uint) - byteCount),
            byteCount);
    }

    public void Dispose()
    {
        generator.Dispose();
    }
}