Downlevel implementation of RandomNumberGenerator.GetInt32

randomnumbergenerator.getint32.cs

// Adapted from https://github.com/dotnet/corefx/pull/31243
// Please see license information at https://github.com/dotnet/runtime/blob/main/LICENSE.TXT

using System;
using System.Security.Cryptography;

public static class RandomNumberGeneratorExtensions
{
    public static int GetInt32(this RandomNumberGenerator csprng, int fromInclusive, int toExclusive)
    {
        if (fromInclusive >= toExclusive)
            throw new ArgumentException("Bad from / to values.");

        // The total possible range is [0, 4,294,967,295).
        // Subtract one to account for zero being an actual possibility.
        uint range = (uint)toExclusive - (uint)fromInclusive - 1;

        // If there is only one possible choice, nothing random will actually happen, so return
        // the only possibility.
        if (range == 0)
        {
            return fromInclusive;
        }

        // Create a mask for the bits that we care about for the range. The other bits will be
        // masked away.
        uint mask = range;
        mask |= mask >> 1;
        mask |= mask >> 2;
        mask |= mask >> 4;
        mask |= mask >> 8;
        mask |= mask >> 16;

        byte[] bytes = new byte[sizeof(uint)];
        uint result;

        do
        {
            csprng.GetBytes(bytes);
            result = mask & BitConverter.ToUInt32(bytes, 0);
        }
        while (result > range);

        return (int)result + fromInclusive;
    }
}