Copyright just doesn't protect such cases. There's a funny exaggeration that is very illustrative: copyright protects the bugs in the code. I.e. the specific way in which code was written. Reading it and getting inspired was never meant to break copyright.

What protects particular solutions is patents. For example if someone were to obtain a patent for computing GCD of large integers the usual fast way, well then everyone else would have to use a different solution.

This analogy to someone reading a book, perhaps peppered with lots of legalese to the point of being hardly recognizable, will definitely be used in courts at some point. And I can't see how it wouldn't stand as a valid argument.