Wednesday, December 18, 2013

Queen vs 3 minors

Recently there was an interesting "Lonely queen" discussion at Talkchess where it was claimed that stockfish seems to have trouble to understand positions where a queen is exchanged vs 3 pieces.

iCE also does not have special code to handle this imbalance. I relied here on the mobility (3 pieces have more mobility than a queen, whose mobility in iCE is cut at 14 squares). This imbalance is difficult to tune as it appears in less than 1% of the games.

H.G. Muller suggested a cool method to not tune but to measure the value of the imbalance by playing games that enforce the imbalance and measure the winning chances for both sides. This is what I did.

I created a set of starting positions where the imbalance is present. To make it simpler to count I always gave the 3 minors to Black, but awarded the first move in halve of the starting positions to Black.

I then played a iCE vs iCE tournament. After 1000 games, Black scored 63,75%. So the imbalance has a significant influence. I now have to translate this winning percentage into centipawns as this is the score unit in iCE.

Here I removed for Black also the f7 pawn in the starting positions and played again. Black now scored 46,96%. This made the pawn worth 16,79%.

Now I could calculate the value of the imbalance as 13,75% / 16,79% * 1 pawn = 0,82 pawns.

At Talkchess the value of the imbalance was guessed to be 0,70 pawns. It seems this was an excellent guess.

Next step is to include a material imbalance term in the evaluation. It can go with the material signature and material hash, so it provides almost no additional computational costs. The impact on the playing strength will be tiny as it occurs so seldom.

But if your engine plays in 1% of its games really awkward people will notice. The stockfish case is a good example for that.

Next steps will be the measurement of some more imbalances that also involve a rook.