It really doesn't help that it's written in ancient K&R C, but if you spend ten or so minutes just staring at it, familiar shapes and patterns start to appear. (Give it a try!)

Incidentally, it's in line with how APL code looks like an alien artifact at first, but you get used to it fast if you have spatial reasoning to wrap your head around reshaping and transposing.

Once I put the effort to understand code like this and it turned out it's straightforward once you learn the conventions:

https://news.ycombinator.com/item?id=19421524