Add compcode to blosc_cbuffer_versions

[FrancescAlted]

It would be interesting to add a way to know which compressor was used to compress a buffer. The clear candidate for this is blosc_cbuffer_versions whose current signature is:

void blosc_cbuffer_versions(const void *cbuffer, int *version, int *versionlz);

and with the suggested change, the signature would become:

void blosc_cbuffer_versions(const void *cbuffer, int *version, int *compcode, int *versionlz);

which introduces an API change. However, as blosc_cbuffer_versions() should not be widely used, I think this is a reasonable change (although it may require a jump in the minor version).

Opinions?

[esc]

I think this is a reasonable choice. Need to make sure though, that the version, versionlz, and compcode are clearly documented. Those variables are quite similar so we should seek to avoid confusion.

• version is the format version of the Blosc metaccodec itself
• compcode is the code for the real/internal/implementation codec library ( NOT the codec implementation, but see fix codec enumeration (in README) #45) used.
• versionlz is the format version of the real/internal/implementation codec used.

Not sure yet about the correct terminology to use to describe the real/internal/implementation codec.

[FrancescAlted]

Hmm, after putting some serious thought on this, I am leaning now to add a new API call:

int blosc_cbuffer_compcode(const void *cbuffer);

Where it returns the compressor code for the buffer. That avoids the API change, and although it makes the API a bit larger, I think it is nice to be explicit. Also, when in the future several filters would be allowed, a new API call could be added too:

int blosc_cbuffer_filtercode(const void *cbuffer);

What do you think?

Also, I agree that current variables in blosc_cbuffer_versions() must be better documented.

[esc]

I think it would fit nicely with the function for setting the complib, some good symetry. Also, the cost of avoiding the API change in this case is not very large so it is preferable.

[FrancescAlted]

Fixed in rev 365f819. I finally decided to go with a blosc_cbuffer_complib() because the most you can do is guessing the compression library, and not the actual compressor used. This only actually applies to LZ4, where both lz4 and lz4hc compressors map to the LZ4 library, but in the future there could be similar cases for other libraries.