Document whether RollingHash is thread-safe

[Rabadash8820]

Hello and thanks for maintaining this project! The API docs for the rollingHash param of the VcEncoder.Encode method state:

Manually provide a VCDiff.Encoders.RollingHash instance that can be reused for multiple encoding instances of the same block size. If you provide a VCDiff.Encoders.RollingHash instance, you must dispose of it yourself

However, I can't tell for sure whether RollingHash can be used by multiple encoding instances that are running in parallel. Is RollingHash thread-safe? It would be great to document this either in the API docs or somewhere in this repo.

[chyyran]

RollingHash is never mutated once created so it should be safe to use in parallel.

[Rabadash8820]

Sounds good; thanks for getting back to me. The main field I was concerned about is v_shuf. I see it referenced on this line (shown below) and wasn't sure if that mutates v_shuf (I don't know much about AVX2).

c_v = Avx2.PermuteVar8x32(c_v, v_shuf);

Also, to be clear, this Issue was about documenting whether RollingHash is thread-safe, as I'm sure other users of this library might wonder if it can be used in a multi-threaded environment. Pinning this Issue is a start, but it would be good to have this info in the Readme.

[chyyran]

No worries, I'm a bit backed up on things at the moment so I'll be updating the documentation when I get the chance.

For reference, Avx2.PermuteVar8x32 is a shim for _mm256_permutevar8x32_epi32, which doesn't mutate its arguments in place.

[Rabadash8820]

Sounds good, thanks for your help!