I once wrote a "chess engine" when I was much younger, however as I'm not much of a chess player, it wasn't very good. I encoded all the movement rules for each piece and then programmed the opponent to choose random valid moves. It would pick a random piece, pick a random square to move it to, and if it was considered valid, it would make the move, or pick another square. I think later I reprogrammed it to select 10 random valid moves, and select the one that allowed it to capture the highest value piece.

The great thing was that it worked, and it could "play chess". The sad thing was, I am such a bad chess player that it could still actually beat me around 50% of the time by choosing random moves.