Author: Robert Hyatt
Date: 08:08:10 10/30/01
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On October 30, 2001 at 10:37:04, Dan Andersson wrote: >I agree. That logic is like evaluating a KRPkrp endgame by looking first at The >corresponding KRPkr EGTB. And seeing only a draw for white, thus concluding that >black must be better. Bad things are sure to happen then, since that extra black >pawn may hurt black. You can't even be certain of it if the corresponding KRkrp >is won for black. Chess don't work this way (subgames are not additive), and I >remember quite a few endgames where I would gladly have given a pawn away for >free. To secure a draw. Faulty logic. > >MvH Dan Andersson That is the _wrong_ way to look at this. The question is probability. How many positions in KRB vs KRPP do you think are _won_ for the KRB side? And how many are not? If you believe, as I do, that the vast majority _are_ draws, and this is provable pretty easily, then I certainly want my program to avoid such a position if it has the KRB, because it isn't going to win. IE if crafty has a KRBP vs KRPPP, it is _not_ going to trade away that last pawn to reach a KRB that is most likely drawn. While many programs will do this (I see this rule kick in with reasonable frequency on ICC). When I see an exception that causes it to misevaluate and lose, I certainly modify the code to recognize the exception. But so far, it hasn't happened that I have seen. It is very difficult to make evaluations 100% perfect, which is getting overlooked here. IE most programs are probably under 75% in evaluating king safety correctly. That is going to have a much more significant effect on how the program does than this simple ending heuristic. Meanwhile I don't have to suffer through the embarassment of seeing my program trade into a KRB vs KR with an evaluation of +3, or into a KRB vs KRPP with an evaluation of +1. Particularly when _before_ the trade it really _was_ winning. If you strive for 100% accuracy, you are doomed to failure from the beginning, until your program can search to a forced mate from the starting position. I haven't suggested that anyone copy this rule if they don't want to. I didn't suggest that anyone copy the bishop trapped at a7/h7 stuff either, but when I first did it, I busted most every program around with that trap, over and over, until they decided it was time to fix it too. And I can show you exceptions where the rule doesn't work. But they are rare enough that the risk of mis-evaluating is much lower than the risk of not evaluating that at all..
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