https://github.com/zlib-ng/zlib-ng/blob/42aa81beafa2ea14cd780a9acf3359a0a09ca44b/functable.c#L89

same goes for all other so called stub functions that are resolved fully at compile time once sse2 is assumed to be available.

ZSTD expanded their range of compression levels to include negative integers. The current "max" (-5) pushes it into LZ4 territory. I would make a pull request, but I'm not sure how many additional levels you want to test by default.

It should be quite obvious for first time users to see some simplest APIs (to compress/uncompress buffer without dealing with any file io). Strangely, lizard has links to file format, but no clues to basic api use (is it in examples, which one is the "hello world"?. I've been using zlib for more than a decade, and I've never needed to know its file format, perhaps same for 90% of its users.

IMO

ÖbEngine currently uses a homemade argument parser which isn't really well-made nor feature-complete.
Here is a list of possible candidates :

• https://github.com/adishavit/argh
• https://github.com/sailormoon/flags
• https://github.com/mmahnic/argumentum
• https://github.com/jamolnng/argparse
• https://github.com/vietjtnguyen/argagg
• https://github.com/p-ranav/argparse

Your default levels for ZSTD seem a little wonky:

• TURBO: ZSTD is missing, it should be -5 | --fast=5 (the fastest level currently available). This should push it into LZ4 territory.
• EFFICIENT should probably include 3, ZSTD's default level.
• OPTIMAL (meaning best trade off?) should probably be 20, as 22 balloons memory usa

The pthreads based multi-threading code should be improved (IMHO). I see two options:

1. keep the internal 'explicit' interface more or less like it is but switch from pthreads to c++11 threads.
2. remove the 'explicit' interface, which should simplify the code significantly and switch to OpenMP where applicable.

I would strongly prefer the second option as is would reduce the complexity of

It is gaining popularity and it seems that OCaml bindings are abandoned: https://github.com/ygrek/ocaml-zstd

See format description https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md

And decoding testing tool https://github.com/facebook/zstd/tree/v1.3.4/tests#decodecorpus---tool-to-generate-zstandard-frames-for-decoder-testing