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Mixing code under different licenses

I am attempting to make a game for the month of August. I want to release the source under the GPL. Up until I started coding, I didn't think it would be very complicated.

I already have game engine code from something I wrote for Game in a Day #13, but I found a tutorial for a game engine that I really liked.

The author's source is under the ZLib license, which is GPL-compliant. So far, so good.

Still, I have no experience mixing code under different licenses. I took his Main.cpp and modified it very slightly to make it my own main.cpp, so obviously I would need to keep the ZLib notice in the source file while specifiying that I had changed it. But what about when I write my own version of the GameEngine class? If I add a line or two to my own code, do I need to signify that those lines come from the ZLib-licensed code? Do I just add a comment above it like:

// Copyright 2005 Other_Author

If I mix a certain percentage of his code with mine, those comments would really dirty up the source listing, I would think. Do I just keep the ZLib notice in a separate license.txt file and mention in a README that I use code from the other project? I would think that it would be too vague to let people know which license specific code is under.

Also, when is it reimplementation as opposed to copying? Some of the functions are trivial to write, but I don't want to just assume that I can write a version that looks only slightly different and call it my own.

I have a greater appreciation of the problem with license proliferation. I'm just borrowing from one project, but if a larger project borrows from multiple sources, that could be a legal nightmare.

That is, unless I am making a mountain out of a molehill here.  Am I?

Thanks in advance,

GBGames' Blog: http://www.gbgames.com/blog