I'm well aware of how the code works. I was mostly interested in why it was written like that. nwiswell's comment[1] gave me the hint I needed to find that yes, this was part of an obfuscated C challenge[2].

I know you mean well but LLMs are the very last resource I'd turn to for help. Those things make crap up all the time.

[1]: https://news.ycombinator.com/item?id=36364283

[2]: https://www.ioccc.org/1986/bright/bright.c