Optimize `Random.GetHexString`

[skyoxZ]

Random.Shared.GetHexString can be optimized by:

1. Getting 2 (instead of 1) hex chars from every random byte.
2. Taking advantage of the magic in HexConverter.

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkRunner.Run<Benchmarks>();

public class Benchmarks
{
    [Params(2, 5, 8, 16, 32, 64)]
    public int HexStringLength;

    [Benchmark]
    public char Main()
    {
        Span<char> buffer = stackalloc char[HexStringLength];
        Random.Shared.GetHexString(buffer);
        return buffer[0];
    }

    [Benchmark]
    public char Opt()
    {
        Span<char> buffer = stackalloc char[HexStringLength];
        Random_GetHexString(buffer);
        return buffer[0];
    }

    private static void Random_GetHexString(Span<char> destination, bool lowercase = false)
    {
        // Scale the string length up to the nearest multiple of 16.
        // For production code, we should slice destionation into small pieces first in case it's large.
        Span<byte> source = stackalloc byte[((destination.Length + 15) & -16) / 2];
        Random.Shared.NextBytes(source);
        if (destination.Length % 2 == 1)
        {
            destination[^1] = GetHexChoices(lowercase)[source[^1] & 15];
        }
        source = source[..(destination.Length / 2)];
        _ = lowercase ? Convert.TryToHexStringLower(source, destination, out _)
            : Convert.TryToHexString(source, destination, out _);
    }

    private static ReadOnlySpan<char> GetHexChoices(bool lowercase) =>
        lowercase ? "0123456789abcdef" : "0123456789ABCDEF";
}

Method	HexStringLength	Mean	Error	StdDev
Main	2	8.284 ns	0.0285 ns	0.0252 ns
Opt	2	7.438 ns	0.0108 ns	0.0101 ns
Main	5	10.838 ns	0.0071 ns	0.0066 ns
Opt	5	9.590 ns	0.0476 ns	0.0397 ns
Main	8	10.979 ns	0.0118 ns	0.0099 ns
Opt	8	7.262 ns	0.0813 ns	0.0721 ns
Main	16	17.104 ns	0.0292 ns	0.0228 ns
Opt	16	8.030 ns	0.0201 ns	0.0188 ns
Main	32	26.957 ns	0.1203 ns	0.0939 ns
Opt	32	11.416 ns	0.0169 ns	0.0141 ns
Main	64	57.944 ns	0.3560 ns	0.3330 ns
Opt	64	17.249 ns	0.0897 ns	0.0839 ns

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[PranavSenthilnathan]

From original PR #112162:

We can evaluate based on perf testing whether it'd be worthwhile to do so, but my expectation is the uses for GetHexString will involve shorter sequences where the perf differential will not huge.

Seems like the perf ratio is significant enough at even 8 chars.

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[stephentoub]

The implementation today is a one-liner. The performance improvements would have to be very meaningful to significantly increase the amount of code required (noting that the repro code in the OP couldn't be used as is and would need to be even more involved), not just comparing new to old, but in terms of the scenarios in which the API will be used.

[skyoxZ]

In my projects there's a helper UniqueId.New that returns a simple 32-length random hex string for years and it's heavily used (though technically it's wrong - most scenario should have been cryptographically-secure). It was not easy to get a random hex string years ago. I would be excited to see that it can be done with simple code and good performance using modern .NET.

[stephentoub]

though technically it's wrong - most scenario should have been cryptographically-secure

RandomNumberGenerator.GetHexString has existed since .NET 8, is cryptographically-secure, and does use HexConverter.