In case there are any doubts, consider this code and its description public domain.

But then I'm not sure how much code is enough to be considered copyrightable. Is "2*2" copyrightable? Clearly not, because it's too trivial. Where is the line?

Patent != copyright. You can patent an algorithm (e.g., Adaptive Replacement Caching, which was scheduled to go into public domain this year but unfortunately got renewed successfully) but when it gets to the level of an actual specific implementation, it's a matter of copyright law.

It's why black-box clones where you look at an application and just try to make one with the same externally-observable behavior without looking at the code is legal (as long as you don't recycle copyrighted assets like images or icons) but can be infringing if you reuse any of the actual source code.

This was an issue that got settled early on and got covered in my SWE ethics class in college, but then more recently was re-tried in Oracle v Google in the case of Google cloning the Java standard library for the Android SDK.

I have no idea how copyright applies here. StackOverflow has a rule in their terms of use that all the user-generated content there is redistributable under some kind of creative commons license that makes it easy to reuse. Perhaps HN has a similar rule? Not that I'm aware of, though.

This is a copyright question not patent one.