[API] Add ASCIIEncoding.IsEquivalentOrdinal(ROS<char>, ROS<byte>)

[benaadams]

From dotnet/aspnetcore#8374 (comment)

Cross encoding compare

namespace System.Text
{
    public partial class ASCIIEncoding : Encoding
    {
        public static bool IsEquivalentOrdinal(
            ReadOnlySpan<char> chars, 
            ReadOnlySpan<byte> asciiBytes);
    }
}

This is method to test whether a sequence of ascii bytes is the same as a string; without having to first convert the bytes to a sting and allocate and then use string equality. (e.g. compare ascii bytes to a UTF-16 string)

Used in dotnet/aspnetcore#8374 to drop steady state string allocations to zero (as previous materialized strings can be reused when they match the on-wire bytes)

e.g.

Could extend with an bool ignoreCase = false if desired, however that is not required for this particular use case.

/cc @GrabYourPitchforks @davidfowl @halter73

[Wraith2]

Couldn't you rent a char buffer scatter the ascii bytes into it and then compare with the existing ROS char comparison operations?

[benaadams]

Bit slow compared to loading into vectors, widening and comparing them*, also limited in concurrency by the number of ArrayPool buffers (will allocate outside that number); but that's implementation rather than api.

* i.e. the write back to memory for the ArrayPool and then re-read for comparision doesn't need to happen; as you can compare with the first read

[svick]

Why is str a string and not a ReadOnlySpan<char>?

[benaadams]

Why is str a string and not a ReadOnlySpan<char>?

Changed

[GrabYourPitchforks]

For API reviewers: I probably wouldn't put this on ASCIIEncoding for the same reason we didn't add extra static methods to UTF8Encoding (namely, the behavior would be surprising if somebody subclassed and customized the Encoding type). But there's nothing inherently wrong with the underlying scenario.

[gfoidl]

Why is str a string and not a ReadOnlySpan<char>?

Changed

@benaadams please also update the title to reflect this.

[GrabYourPitchforks]

Possibly covered by https://github.com/dotnet/corefx/issues/34144?

[benaadams]

Yes, think it would be covered by Equals(ReadOnlySpan<byte> left, ReadOnlySpan<char> right) ?

[GrabYourPitchforks]

Tracked by #28230.