At least to me, that AI is actually incredibly simple. All its doing is mutating a random code snippet through a genetic algorithm. If you read the comments, the author explains that once it creates a function that it needs to complete the target program (e.g. addition func for its multiplication), it adds the function string to the list of possible mutations to add to the code snippet. It's actually a relatively simple design and I would go so far as to say genetic algorithms are probably not the way that AIs will write code in the future. Its too random and for any significant program, it would take much too many generations and the generations would be increasingly harder to fitness test as you added more and more possible functions. Still a really interesting article though. Thanks for the link.