Author: blass uri
Date: 03:05:31 03/12/99
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On March 11, 1999 at 18:12:48, Heiner Marxen wrote: >Some time ago, when I listened to a discussion of evaluating >positions vs. evaluating moves (or pathes), I started to >realize that there may be real problems with the usual way >to evaluate positions at the tips of the search tree and >backing them up to the root moves. > >Clearly, evaluation of the path (leading to a position) >_must_not_ go into the value of the position, at least with >respect to the transposition table (TT). [For simplicity I >will omit complications with draw by repetition or the >50-move rule.] > >In principal, I feel it is the right thing to evaluate >positions, because this is the way game theory defines >mini-max and justifies alpha-beta. > >As an example let's assume white is to move, material is >balanced, and white can immediately take a (hung) pawn (with >a rook, say) and really win the pawn. Say, we do a 9-ply >search, and two moves come up with an eval of +1. One is >the pawn eater, and the other is a Q-move checking the black >king. That second move is the problem (my opinion): the >search sees a lot of king hunting with check moves where >at the different depths the Q can continue to issue yet >another check, or the rook could now eat the pawn. So, both >moves come up with the same eval for the same reason: they >both win a pawn (the same pawn). > >Let's further assume, for some reason, the Q-move is chosen >(e.g. because searched first). Its value indicates that >white will reach a position with value +1. The black king >moves out of check, and now white is facing a position >essentially the same as 2 plies before. Again, white >selects the Q-move etc. > >I.e. the search finds white can force to win a pawn, but >does not really make progress towards that win. > >The danger is that eventually black may make some progress >such that the pawn eating is not possible, any more, since >the search now can see that black would win by a forced >mate (say). > >Such a problem with making progress has already come up with >forced mates (IIRC Bob Hyatt reported some program to >continue with threatening mate in 2). That is easily solved >by forcing the search to seek for a shorter mate the next >time, thus making progress. > >Shouldn't we try to make some progress in the general case, >not just for forced mates? >What is the state of the art with this? >Does it happen, at all, in real games? >Or am I way off, and this is not a problem, at all? >Or is it solved, already? > >Another view: We have two equally good best moves, along >with their PVs (when searched first), one of which wins a >pawn in ply 1, and confirms this win with a 8-ply search, >while the other PV does 8 plies of something, followed by >winning a pawn, and confirmes this win with a quiescence >search, only. As a human I would clearly take the pawn, >first. Intuitively I'd say the program should do so, also, >because it is more "sure" about it. > >eval > ^ >+1 + * * * * * * * * * taking the pawn, first > | > 0 *------------------ > 1 2 3 4 5 6 7 8 9 depth > >eval > ^ >+1 + * hunting the king, first > | > 0 *-*-*-*-*-*-*-*-*-- > 1 2 3 4 5 6 7 8 9 depth > >Thinking more about it, I occurs to me that iterative >deepening, together with a TT (both are pretty standard), >may solve the problem, as shown above, since a 1-ply search >would prefer the pawn eater and store it as "best move" into >the TT, and in the further searches start with this move. >May be, that is enough, already. Dunno. > >Opinions? as a human I would prefer not to take the pawn because I cannot see progress after taking the pawn(the evaluation is the same for many plies and I do not see even a minor improvement) If it is a good pawn to take then I expect 0,1,1.02,1.05,... and not 0,1,1,1 Uri
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