I came across source code for an AI that plays some classic video games. It's licensed as:
GNU LESSER GENERAL PUBLIC LICENSE Version 2.1,
I understand the Gnu license pretty well and if I were to port or include the AI in a game I'm working on I'd need to include the source in my release, and make it free.
But what if I use it as a reference for design paradigms? Basically, I'd have my source code on my right screen, building it as I go, and on the left screen I'd have the AI published under GPL 2.1 for a reference. Technically, not a single line of the author's code would make it into my code.
But, in all fairness, the original author has solved a lot of the hard problems already and I'm using the solutions that they came up with through hours of experimentation and development. This seems to be a bit of a gray area and I haven't found an answer online to this question.