I have my old infinite board (any size may be set) project with several different AIs implemented as plug-ins, including my own AI, that can fight each other in any combinations and vs human players to determine, which one is the best. But for several reasons I won't post it's code. Mine AI is "medium difficulty", as it uses fast heuristics in order to play, but isn't that smart. I have my friend's AI, that is little bit harder. Not sure, if it's weight-based or heuristic-based.