Used the minimax algorithm. Experimented with lots of heuristics and methods including but not limited to alpha-beta pruning, iterative deepening, adaptive move reordering, position hashing and an advanced position evaluation function.
I was a competitive chess player at the time and my program was able to play as good as I did during the opening and the middlegame. It wasn't competitive during the endgame because the search space grows a lot when there are only a couple highly-mobile pieces left on the board and a huge end-game database is required for it to be competitive. Creating and storing that end-game database was going to be expensive so I ended up skipping that part. Had so much fun developing this.