Expand small value sets to all case permutations in SearchValues<string>

[MihaZupan]

Implements #98791 (comment)

If we have a set of values like ["ab", "c!"], we can expand it to ["ab", "Ab" "aB", "AB", "c!", "C!"] and switch to case-sensitive searching.
As long as we're making use of buckets that would otherwise have been empty, this is going to be an improvement as the prefix search loop is a bit simpler due to not needing to deal with casing. In the below benchmark, it means eliminating this step from the loop.

This optimization is a bit niche (unlikely to be applicable often), but it's really cheap to check whether it could apply, and can help cases with many non-letter characters in their prefixes.

publicclassIgnoreCaseToOrdinal{privatestaticreadonlySearchValues<string>s_values=SearchValues.Create(["ab","c!"],StringComparison.OrdinalIgnoreCase);privatereadonlystring_text=new('\n',1000);[Benchmark]publicintIndexOfAny()=>_text.AsSpan().IndexOfAny(s_values);}

Method	Toolchain	Mean	Error	Ratio
IndexOfAny	\main\corerun.exe	82.42 ns	0.596 ns	1.00
IndexOfAny	\pr\corerun.exe	67.83 ns	0.369 ns	0.82

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Issue Details

---

Implements #98791 (comment)

If we have a set of values like ["ab", "c!"], we can expand it to ["ab", "Ab" "aB", "AB", "c!", "C!"] and switch to case-sensitive searching.
As long as we're making use of buckets that would otherwise have been empty, this is going to be an improvement as both the prefix search loop and the verification steps are a bit simpler due to not needing to deal with casing.

This optimization is a bit niche (unlikely to be applicable often), but it's really cheap to check whether it could apply, and can help cases with many non-letter characters in their prefixes.

publicclassIgnoreCaseToOrdinal{privatestaticreadonlySearchValues<string>s_values=SearchValues.Create(["ab","c!"],StringComparison.OrdinalIgnoreCase);privatereadonlystring_text=new('\n',1000);[Benchmark]publicintIndexOfAny()=>_text.AsSpan().IndexOfAny(s_values);}

Method	Toolchain	Mean	Error	Ratio
IndexOfAny	\main\corerun.exe	82.42 ns	0.596 ns	1.00
IndexOfAny	\pr\corerun.exe	67.83 ns	0.369 ns	0.82

Author:	MihaZupan
Assignees:	MihaZupan
Labels:	area-System.Buffers
Milestone:	9.0.0