Added GetInt32 to RandomNumberGenerator.

[vcsjones]

Implement RNG.GetInt32.

/cc @bartonjs @GrabYourPitchforks

Fixes #30873.

[bartonjs]

throw on next line. The next function happens to violate our style (rule 1, at that). If you wanted to clean it up I wouldn't mind.

[bartonjs]

My personal style is to have a blank line before a control flow statement because I find it helps legibility. I'm just offering it out there, our style guide isn't prescriptive on this one way or another.

[bartonjs]

There's a 1 in 4 billion chance this fails. If you want to assert something you should just assert it wasn't int.MaxValue

[bartonjs]

typo: tolerance

[bartonjs]

Typo: Occurred

[bartonjs]

It would be good to see range validity test for negative bounds, and some tests with interesting-looking bounds sizes (really I want to test both the lzcnt implementation and the algorithm. Something annoying like (x, x + 0x0101) so the high bit in the mask value is technically required, but rarely valid. If you ran that 1000 times you "should" have at least one hit on the highest value, and shouldn't have value skew.)

(int.MinValue, int.MinValue + 3)
(-257, -129)
(-100, 5)
(254, 512)
...

Also, roll 1000d6, check for skew. (The existing "coinflip" test sort of does this, but it's bit-aligned in range, so it doesn't test the high value rejection vs mod bias algorithm differential)

[vcsjones]

I'm not fully sure what the failure in d243963 was. It looks like infrastructure? The most information I could glean was a workItemStatus had a failure count of 1, but the test failure count was 0.

Update: "mission control" says it was System.IO.Packaging.Tests.

[vcsjones]

@bartonjs @jkotas thank you for the (quick!) code review. I think I have addressed all feedback.

[danmoseley]

Maybe comment where the 7 comes from?

[vcsjones]

Test failures look unrelated.

[vcsjones]

@GrabYourPitchforks @bartonjs OK. Changed, rebased, squashed.

[khellang]

@terrajobst Candidate for api-candidate-for-standard Marks APIs that are tracked to be included in the next version of the standard. ? 🤔

[puenktchen]

@vcsjones Sorry for misusing this platform for contacting you. For several reasons i'm stuck at Netstandard 2.0 for my project, so i wanted to add your GetInt32 method as an extension to RandomNumberGenerator. Unfortunately i have a problem with "RandomNumberGeneratorImplementation.FillSpan()." Where does it come from? What does it do? Can you shed some light on it please?

[vcsjones]

@puenktchen It's the internal implementation for RandomNumberGenerator in .NET Core that can fill a Span<byte>. .NET Standard 2.0 has no such API, so you'll have to allocate and fill a byte array.

This will mean the GetInt32 will allocate 4 bytes per-invocation, but to keep the implementation straight forward it's probably the best thing to do.

Disclaimer: The snippet below is untested for correctness. The only testing on the code below that I have done is that it compiles for netstandard2.0.

static class Extensions
{
    public static int GetInt32(this RandomNumberGenerator rng, int toExclusive)
    {
        return GetInt32(rng, 0, toExclusive);
    }

    public static int GetInt32(this RandomNumberGenerator rng, int fromInclusive, int toExclusive)
    {
        if (fromInclusive >= toExclusive)
            throw new ArgumentException("Range is invalid");

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

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

        return (int)result + fromInclusive;
    }
}

[puenktchen]

@vcsjones Works perfect for me. Thank you so much!