Chess AI Idea

So, I had this idea using a genetic algorithm to optimize the parameters of a program that plays chess. Now, the genetic algorithm wouldn’t be used at all during a game, but rather to optimize the board evaluation parameters beforehand. I don’t know much about writing board game AI programs, as I have only written a few of them for fun (tic-tac-toe, connect 4, Pente). For this chess program, I am taking a simple approach because I am more interested in seeing the genetic algorithm at work than seeing the chess playing AI do well against other chess AI or people.

The program would search the game tree using the minimax algorithm, with some possible optimizations added afterward. Tree searching is just a matter of generating all possible moves and looking at them. The hard part is the board evaluation function, which evaluates the a particular board’s score based on the arrangement of the pieces. Parameters to this evaluation function would be, for example, the piece values. The pawn would be locked in at a value of 1, which anchors the other values and provides a base unit to work from.

Again, the parameters would not change during a game. We use the genetic algorithm ahead of time to determine the parameters.

For the genetic algorithm, the set of parameters strung together makes up a single chromosome. We maintain a pool of different chromosomes, i.e. different sets of parameters, and breed these chromosomes together to improve our parameter sets. We start out with a random pool made of parameters that are most likely pretty terrible.

To evaluate the chromosomes, we need a fitness function, which evaluates each chromosome for its level of “fitness” deciding if it breeds or not. To do this we simply play the chromosome we are evaluating against some base chromosome, which may just be parameters chosen intuitively by the programmer. Or, the base chromosome could be random too. Starting with a better base chromosome would be a good head start, though. The fitness of the chromosome is how often it wins against the base chromosome in, say, a few hundred games.

The most fit chromosomes are bred by taking a few parameters from each to make a new chromosome. Mutations are occasionally added in order to keep the chromosome pool from getting stuck in a local maximum. A mutation involves changing one or more parameters in a chromosome slightly in some random way. Mutations are rare and will usually be detrimental to the chromosome quickly killing it off, but will occasionally cause a good change that will be spread to other chromosomes in the next generation, improving the gene pool.

We iterate this until either the maximum fitness level in the pool is stuck for several iterations (we aren’t getting anywhere and mutations aren’t helping), or the chromosomes are so good they always beat the base chromosome, making the fitness algorithm meaningless. When this happens, we replace the base chromosome with the best chromosome in the pool and start over from scratch again with a random, or mostly random, pool.

As you would expect, I have looked into parallelizing this process to take advantage of a cluster. This is easy for several reasons. First, evaluating chromosomes can be done simultaneously. No evaluation depends on another chromosome’s evaluation. Second, the minimax game tree search can be parallelized so that several different processes search the game tree and give their results back to the parent process. This works very well because the data being sent back to the parent will be a single integer. No need to send large amounts of data around the network.

I spent an afternoon hacking at this, but its still too crude to share yet. I got the non-parallel version of the chess engine built but I am still working on the evaluation function. The genetic algorithm hasn’t been started. The only parameters at the moment are piece values. The board evaluation function just adds up the piece values on the board completely ignoring their positions. This makes the computer play extremely aggressively, capturing the opponent’s pieces whenever it can. This makes for a somewhat interesting bloodbath where the board goes empty after just a few moves.

My problem right now is finding a good way to represent piece movements so that it can be recycled. That is, I want to represent movements when generating my search tree, verifying the legality of a move, and evaluating the board all the same way so that I don’t have to program in piece movements several times.

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Chris Wellons (PGP)
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