You can only copyright the text of the source code, as if it was literature. Write your own source code to implement the same algorithm using the same efficient techniques, and you're fine (as long as it isn't patented.)

so... rename the variables?

On a more serious note, this:

https://en.wikipedia.org/wiki/Clean_room_design

It depends on the jurisdiction. e.g. Such a thing would not be copyrightable under Australian law. That could be a way around it.