ZSTD_CCtx_setCParams

[Cyan4973]

Inspired by @terrelln in #3395,
offer a new capability to set all parameters defined in a ZSTD_compressionParameters structure with a single symbol invocation to improve user code brevity.

[embg]

LGTM, just one small nit and one potential issue that might require changes (hence, I am requesting changes). Feel free to land if changes aren't required.

[embg]

Hm... I just thought of a possible issue. You are assuming all the parameters in ZSTD_compressionParameters are authorized for mid-frame updates. However, isn't windowLog not authorized?

[Cyan4973]

That's a very good point @embg .
windowLog can't change during compression.
And I actually assumed that windowlog's stickiness was taken care of somewhere later in the pipeline.

That's indeed the case :
https://github.com/facebook/zstd/blob/dev/lib/compress/zstdmt_compress.c#L1072

That being said, I agree this situation is not clear for users, so adding a code comment to specify this behavior would be helpful.

[embg]

That's indeed the case : https://github.com/facebook/zstd/blob/dev/lib/compress/zstdmt_compress.c#L1072

Thanks for the pointer. Another question for my own understanding: how does this work for single-threaded compression? E.g. if ZSTD_CCtx_setCParams() is called in the middle of a frame, during streaming compression?

[Cyan4973]

Another question for my own understanding: how does this work for single-threaded compression? E.g. if ZSTD_CCtx_setCParams() is called in the middle of a frame, during streaming compression?

If (allowed) parameters are changed during single-threaded streaming compression, actually nothing changes for the current compression session.

Changes are merely registered into requestedParameters, while ongoing compression uses appliedParameters.