That's awesome. I'm looking at the source code but I can't seem to grasp it. What's a simple explanation of how it works?
Sorry, the code is a little unkempt.
The basic idea is minimax search. Googling that will get you started, but basically the algorithm plays out the game and keeps a score of the position after every move. Then it just makes the move that leads to the best score.
The "score" here is basically a count of how many free squares there are (with a little extra to keep things aligned if possible).
One major thing with these search algorithms is that the game tree grows exponentially as you move forward. To combat that, implemented alpha-beta pruning and a heuristic to only search the nastier computer moves rather than all of the possibilities.