Author: Bruce Moreland
Date: 09:58:05 05/19/98
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On May 18, 1998 at 22:05:07, Robert Henry Durrett wrote: >I noticed a recent post about tablebases, plus also the below query >about this same topic. > >Would someone please tell everybody what tablebases are and whether or >not they play any significant role in modern chess software? Is it not >enough to just have a nice table and good hashtables? If tablebases are >used, what do they offer or "buy you"? Assume the position is king and queen versus king. You can figure out the "mate in X" values for every conceivable position, and store them in a big table. When you encounter the position in a game, you just look it up and say "mate in 8", or whatever. To figure out what move to make, you look at all of the legal moves from this position and choose the one with the highest score from the table. This particular table is like 40K bytes, it has three pieces in it, counting the kings. In order to make tables with more than three pieces, you need more space. The tables with 5 pieces are pretty huge. If you have a lot of them, there are ending classes that you can always win from any won starting position, and always draw from any drawn starting position. And if you have an advantage in a drawn position, you can apply other techniques to force your opponent to play precisely, or you will capitalize on the slightest mistake and win. In practice you see a lot of cases where you convert into a won table ending, or escape into a drawn one. As an example, if you have KRNP against KRNPPP, your program might figure out a way to sacrifice the knight for two of the pawns and trade the other pair, leading to a drawn ending, since you have a KRN vs KR table. This might seem easy to a human, but it can be a little harder for a computer. There are other cases with more practical benefit, for instance if you have KRPP vs KR, and your pawns are h- and f- pawns, this can be extremely hard to win, even for grandmasters. But using a KRP vs KR table you can sometimes win this by knowing when to sacrifice a pawn, and if you are playing the weaker side of it, you have a better chance of drawing it because your opponent won't be able to sacrifice into a won KRP vs KR. Another common example involves sacrifices into drawn KR vs KP endings. The eval function says +4 or -4, depending upon which side you are on, but the table says "draw". In practice it's not that big a deal, but you will score a few extra half-points if you have tables hooked up. bruce
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