Oh God, I just realized looking at that code (and then Wikipedia) that I've re-invented atan2 from first principles (using regular old atan) like 20 times over the years, each time as painful as the last, simply because I didn't know that atan2 existed.

I don't know whether to be angry at my past self or relieved that I'll never have to deal with that bullshit again.

It took me a little while to figure out how the code worked, and it's really the fault of the variable names. Once it clicked for me that the array of coordinates was for the BASE of the line segment, and the parameters to dragSegment passed in the coordinates for where the HEAD should be, the algorithm becomes quite straightforward. (Of course, the "head" and "tail" terminology doesn't make much sense in the context of your wisemen toy, since there isn't an actual line separating them.) It is quite a lovely effect, for how simple it is.