Question/Proposal: Zstd level 5-22 (a C lib supports)

[prochac]

The C lib supports compression level up to 22
https://facebook.github.io/zstd/zstd_manual.html

 The library supports regular compression levels from 1 up to ZSTD_maxCLevel(),
  which is currently 22. Levels >= 20, labeled `--ultra`, should be used with
  caution, as they require more memory. The library also offers negative
  compression levels, which extend the range of speed vs. ratio preferences.
  The lower the level, the faster the speed (at the cost of compression).

Is it something that could be easily implemented? I'm not familiar with the Zstd algorithm os it might be multiple runs, or totally new approach.

[klauspost]

4 fast and well-tested levels are plenty.

Having 10 in deflate is vastly overkill since practically nobody uses anything between fastest/default/smallest.

[prochac]

I will maybe describe my XY problem: I'm trying to rewrite a code from Python to Go, and it's hard to check various steps and compare binary data. And as Go's iota/enum isn't equal to C lib levels, I don't know what would be the equivalent.
Maybe a better question is: are the algorithms equivalent and deterministic?
The documentation mentions "roughly equivalent", what makes me think that not

// SpeedDefault is the default "pretty fast" compression option.
// This is roughly equivalent to the default Zstandard mode (level 3).
SpeedDefault

[klauspost]

Maybe a better question is: are the algorithms equivalent and deterministic?

No, they are rewritten and Yes (they are deterministic within a version of the library on the same platform)

It's hard to check various steps and compare binary data

Newer compare compressed data. Formats are only determined by their decompression, not by their compressed representation, which can change between versions (which is also true for the c library).