I don't think it should be infringement. You can't copyright an algorithm, and Carmack's function is like six lines of code. If you just read Carmack's source code, then rewrote the same algorithm with different variable names, it would clearly not be infringement. Is it really so bad if you keep his precise variable names, comments, and indentation? How does it hurt Carmack to reproduce this tiny snippet of code exactly, rather than with a small rewrite?