Monday, May 23, 2011

Draw recognizer with pawns involved

I try to build a endgame draw recognizer for all 4 man endgame positions that are not easily evaluated. So I don't build one for king and 2 rooks against king as this make no sense. The engine will mate the enemy king and avoid stalemate traps without any additional help.

King and bishop vs king and pawn is more interesting. Usually it is a draw but there are quite a few winning positions for the side with the pawn and even a few for the side with the bishop.

I verify the recognizer by running all possible positions through it and verify its decision (draw / not draw) against a table base. This verification process takes much longer when pawns are involved as I have to test much mor positions. Without pawns I only test 10 squares for the white king, all other positions can be matched by mirroring or flipping to one of the 10 tested squares. With pawns that does not work, so I have to test 32 squares for the white king and so it takes three times as long.

So the whole recognizer implementation takes quite some time and I looking forward when it is done so I can move to more interesting stuff.

Sunday, May 8, 2011

More on drawish endgames

After implementing a draw recognizer for the KRKR endgame I decided to implement a recognizer for the endgame of rook vs. knight and rook vs. bishop. I thought it is easier because only the side with the rook has winning chances but in fact it turned out to be much more difficult.

Even in the rook vs knight endgame there are a few positions where the side with knight wins.
 Black to move - Mate in 1

And in both endgames there are positions that look like a draw (you cannot easy see a forced win) but are in fact a forced win. In the knight vs. rook endgame some wins require up to 40 moves.

So at the end I implemented the draw recognizer but in order to not let them report false draws they rule out a lot of stuff and miss a lot of draw positions. The worst is knight vs. rook, bishop vs. rook works a bit better.

Anyway it helps the engine to play those endgames a lot

White moves and Mates in 29! (Ke1 !)
In this position it finds the winning move at ply 4 instantly. It sees the mate at ply 31 after 55 seconds.

position fen 8/8/8/8/8/2R5/8/3K1bk1 w - - 0 1
go depth 31
info depth 1 seldepth 2 time 0 nodes 37 pv d1d2  nps 36999 score cp 125 hashfull 0 tbhits 0
info depth 2 seldepth 3 time 0 nodes 95 pv d1d2 g1f2  nps 94999 score cp 110 hashfull 0 tbhits 1
info depth 3 seldepth 5 time 0 nodes 470 pv d1d2 g1f2 c3c7  nps 469999 score cp 110 hashfull 0 tbhits 15
info depth 4 seldepth 6 time 0 nodes 1141 pv d1e1 f1g2 c3g3 g1h2  nps 1140999 score cp 120 hashfull 0 tbhits 65

info depth 31 seldepth 47 time 11797 nodes 9152530 pv d1e1 g1g2 c3c2 g2g1 c2c6 f1b5 c6g6 g1h2 e1f2 h2h3 f2f3 h3h4 f3f4 h4h3 g6g3 h3h2 f4f3  nps 775835 score mate 29 hashfull 57 tbhits 77627