Replace string creation with static strings for Regex options conversion to string

[paulomorgado]

The BsonRegularExpression(Regex) constructor uses string concatenation to create the options string.

If there's none only one option, it will result on a static interned string. Otherwise, it will create new strings. One less than the number of options.

Using a buffer to create the string is only better when there are 4 options.

However, using precomputed strings (because they aren't that much) beat every option in time and memory consumption.

Benchmarks

[HideColumns("Error", "StdDev", "RatioSD")]
public class RegexOptionsEnumToBsonRegularExpressionStringBenchmark
{
    [Benchmark(Baseline = true)]
    [Arguments(RegexOptions.None)]
    [Arguments(RegexOptions.IgnoreCase)]
    [Arguments(RegexOptions.IgnoreCase | RegexOptions.Multiline)]
    [Arguments(RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline)]
    [Arguments(RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline | RegexOptions.IgnorePatternWhitespace)]
    public string WithStringConcatenation(RegexOptions regexOptions)
    {
        var options = "";
        if ((regexOptions & RegexOptions.IgnoreCase) != 0)
        {
            options += "i";
        }
        if ((regexOptions & RegexOptions.Multiline) != 0)
        {
            options += "m";
        }
        if ((regexOptions & RegexOptions.Singleline) != 0)
        {
            options += "s";
        }
        if ((regexOptions & RegexOptions.IgnorePatternWhitespace) != 0)
        {
            options += "x";
        }

        return options;
    }

    [Benchmark]
    [Arguments(RegexOptions.None)]
    [Arguments(RegexOptions.IgnoreCase)]
    [Arguments(RegexOptions.IgnoreCase | RegexOptions.Multiline)]
    [Arguments(RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline)]
    [Arguments(RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline | RegexOptions.IgnorePatternWhitespace)]
    public string WithoutStringConcatenation(RegexOptions regexOptions)
    {
        if (regexOptions == RegexOptions.None)
        {
            return string.Empty;
        }
        var options = new char[4];
        var o = 0;
        if ((regexOptions & RegexOptions.IgnoreCase) != 0)
        {
            options[o++] = 'i';
        }
        if ((regexOptions & RegexOptions.Multiline) != 0)
        {
            options[o++] = 'm';
        }
        if ((regexOptions & RegexOptions.Singleline) != 0)
        {
            options[o++] = 's';
        }
        if ((regexOptions & RegexOptions.IgnorePatternWhitespace) != 0)
        {
            options[o++] = 'x';
        }

        return new string(options, 0, o);
    }

    [Benchmark]
    [Arguments(RegexOptions.None)]
    [Arguments(RegexOptions.IgnoreCase)]
    [Arguments(RegexOptions.IgnoreCase | RegexOptions.Multiline)]
    [Arguments(RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline)]
    [Arguments(RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline | RegexOptions.IgnorePatternWhitespace)]
    public string PreComputed(RegexOptions regexOptions)
    {
        switch (regexOptions & (RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline | RegexOptions.IgnorePatternWhitespace))
        {
            case RegexOptions.None:
                return string.Empty;
            case RegexOptions.IgnoreCase:
                return "i";
            case RegexOptions.Multiline:
                return "m";
            case RegexOptions.Singleline:
                return "s";
            case RegexOptions.IgnorePatternWhitespace:
                return "x";
            case RegexOptions.IgnoreCase | RegexOptions.Multiline:
                return "im";
            case RegexOptions.IgnoreCase | RegexOptions.Singleline:
                return "is";
            case RegexOptions.IgnoreCase | RegexOptions.IgnorePatternWhitespace:
                return "ix";
            case RegexOptions.Multiline | RegexOptions.Singleline:
                return "ms";
            case RegexOptions.Multiline | RegexOptions.IgnorePatternWhitespace:
                return "mx";
            case RegexOptions.Singleline | RegexOptions.IgnorePatternWhitespace:
                return "sx";
            case RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline:
                return "ims";
            case RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.IgnorePatternWhitespace:
                return "imx";
            case RegexOptions.IgnoreCase | RegexOptions.Singleline | RegexOptions.IgnorePatternWhitespace:
                return "isx";
            case RegexOptions.Multiline | RegexOptions.Singleline | RegexOptions.IgnorePatternWhitespace:
                return "msx";
            case RegexOptions.IgnoreCase | RegexOptions.Multiline | RegexOptions.Singleline | RegexOptions.IgnorePatternWhitespace:
                return "imsx";
            default:
                return string.Empty;
        }
    }
}

BenchmarkDotNet v0.13.8, Windows 11 (10.0.26100.2152)
13th Gen Intel Core i9-13900K, 1 CPU, 32 logical and 24 physical cores
.NET SDK 9.0.100-rc.2.24474.11
[Host] : .NET 8.0.10 (8.0.1024.46610), X64 RyuJIT AVX2

Method	regexOptions	Mean	Ratio	Gen0	Gen1	Allocated	Alloc Ratio
WithStringConcatenation	None	0.1918 ns	1.00	-	-	-	NA
WithoutStringConcatenation	None	0.0331 ns	0.17	-	-	-	NA
PreComputed	None	0.0155 ns	0.08	-	-	-	NA
WithStringConcatenation	IgnoreCase	2.1721 ns	1.00	-	-	-	NA
WithoutStringConcatenation	IgnoreCase	9.8270 ns	4.53	0.0030	0.0000	56 B	NA
PreComputed	IgnoreCase	1.1709 ns	0.54	-	-	-	NA
WithStringConcatenation	IgnoreCase,Multiline	9.0000 ns	1.00	0.0017	0.0000	32 B	1.00
WithoutStringConcatenation	IgnoreCase,Multiline	9.6087 ns	1.07	0.0034	0.0000	64 B	2.00
PreComputed	IgnoreCase,Multiline	0.9576 ns	0.11	-	-	-	0.00
WithStringConcatenation	IgnoreCase,Multiline,Singleline	15.3435 ns	1.00	0.0034	0.0000	64 B	1.00
WithoutStringConcatenation	IgnoreCase,Multiline,Singleline	9.3241 ns	0.59	0.0034	0.0000	64 B	1.00
PreComputed	IgnoreCase,Multiline,Singleline	0.9865 ns	0.06	-	-	-	0.00
WithStringConcatenation	IgnoreCase,Multiline,Singleline,IgnorePatternWhitespace	20.9242 ns	1.00	0.0051	0.0000	96 B	1.00
WithoutStringConcatenation	IgnoreCase,Multiline,Singleline,IgnorePatternWhitespace	9.0079 ns	0.43	0.0034	0.0000	64 B	0.67
PreComputed	IgnoreCase,Multiline,Singleline,IgnorePatternWhitespace	0.9218 ns	0.04	-	-	-	0.00