it's the antithesis of maintainable code. But given that one is unlikely to want to change from calculating the inverse square root to inverse cube root or any other variation....
"// what the fuck? "
You'd be hard pressed to write an inverse square cube without basically rewriting the whole function. There's nothing that can be reused. The only saving grace is that it is side-effect free, so replacement is trivial, unlike a lot of other code that's not maintainable.
You mean inverse cube root? I believe the geometry ain’t easily upgradeable this way.
As the designer of the code, you would understand that the inverse square root is a standalone problem.