Saturday, November 30, 2013

Drawish pawn structures

Many moves later the games ends 1/2 : 1/2
One of the things I had on my todo list was related to pawn structures. I thought that some pawn structures have a more drawish character than others.

Especially blocked or symmetrical positions tend to be more drawish than open ones. 

My idea was to build up a statistics over a lot of games and calculate the probability that a game ends as draw depending on the pawn structure that's on the board. If my engine later encounters a pawn structure with a high probability for a draw the score is pulled towards 0.

So if my engine is ahead it tries to avoid such structures (e.g. not lock its pawns) and if behind it tries to play towards them.

The first step was to collect a lot of positions from games and categorize them depending on the game outcome of the game they were taken from. I assembled a collection of games that lasted at least 60 moves as a base.
  • I saved the position at move number 50 into either a WIN or DRAW file.
  • I then processed the positions from the WIN and DRAW files. 
  • In all positions I only considered the specific pawn structure for the files B - G
Index: 111101 111011 = 3.963
  • For a white pawn at the file I recorded a 1 in the lower 6 bits and for a black pawn a 1 in the higher 6 bits. This number served as a index where I stored the number of games that ended as DRAW or as WIN when this structure was present.
Example: In my statistics the above pawn structure was found in 2.286 games. 857 were drawn and 1.429 were won. This resulted in a draw probability of 37% pct. The overall draw probability over all games and structures was 36%. So this pawn structure is a tiny bit more drawish than the average.

I did this for every possible combination. If there is no white and black pawn present on the B - G files the structure is very drawish (62%), symmetrical combinations had values of about 45%.

Finally I modified my evaluation function to scale the score up or down depending on the pawn structure. It used a lookup table where it stored all the 4096 possible scaling values.

Unfortunately it did not work. The tests showed the modified engine as equal or even slightly weaker then the original engine. I used also different indexing schemes like only counting files with blocked pawns etc... Nothing worked. So the patch was removed from the code again.

It was worth a try.

Hopefully I have more luck with the next idea.