but what if I publish an algorithm in my book that just happens to be the same as code you've written, say, because we both had the same professor in school, or that it's the obvious solution to the problem.

once you've written a few lines of code as part of a larger project, is the rest of the world prohibited from writing the same code unless they agree to the terms of your license?

Copyright doesn't punish incidentally matching content. It's specifically right to copy or transform content. To make a case for copyright violation, you have to make the case that it was actually copied.

If you want to make a point about things that incidentally match making people who independently reinvent the same thing, you're criticizing the function of software patents, not copyright.