But you can copyright the most efficient implementations.

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.

As mentioned above, some algorithms are so simple that there's only one canonical way to write them. (Take 'looping through an array' for example - in each language there's generally one canonical way to do it.) You could argue that the text of that canonical way is copyrightable.

(It's similar to the early days of heat engines, where a guy called James Pickard managed to patent the crank, preventing other engine manufacturers from using cranks and forcing them to use a sun-and-planet gear arrangement instead: https://en.wikipedia.org/wiki/Sun_and_planet_gear )