Author: Ed Schröder
Date: 12:26:25 11/29/03
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On November 29, 2003 at 03:48:51, Tord Romstad wrote: Interesting post Tord, I will investigate if there is something to gain for me. Just one remark, if I were you I would make an exception for the endgame, that is, always use R=2. R=3 in the endgame I consider quite destructive. My best, Ed ============= >I apologise for the provocative subject line. With all those endless List >threads going on, I >have to do something drastic in order to get attention. :-) > >I don't really have any idea whether the following idea will work at all for >other engines, but >so far it seems quite effective in Gothmog. > >One of the well-known problems with null move pruning is that it sometimes hides >horizon >problems. The classic case is when white has a pawn on f6 and a queen on h6 >attacking >the undefended square g7 in front of the black king, it's black's move, and the >remaining >depth is less than R. Unless the engine does checks in the qsearch, there is a >risk that >the null move will fail high even though there is no defence against mate. >Similar problems >may occur when the side to move has a pinned piece or when the other side has an >unstoppable passed pawn. > >Checks in the qsearch helps to reduce this kind of problems, but they are still >there. For >instance, in the above scenario with a mate on g7, it could happen that the >white queen >is not yet on h6, but that black cannot prevent it from reaching that square. > >The point of classic adaptive nullmove pruning is to use a smaller value of R >when the >remaining depth is low. My idea is to instead use the evaluation function as >well as the >remaining depth when deciding what value of R to use. The idea will work only >for engines >which call the evaluation function at all interior nodes. > >Here is what I do: > >1. If the side to move is in check or has a big hanging piece, or if the side >not to move >threatens to give mate or promote a pawn, avoid the null move search. > >2. If the side to move has an insecure king position or a pinned piece, if the >last move >of the opponent increased the pressure on the king, or if the opponent has at >least one >unblocked advanced pawn, there is a serious danger that the opponent has some >kind >of serious threat. Choose R=1 or R=2 depending on how serious it looks. > >3. In all "normal" positions, use R=3. > >4. If the side to move has a clear advantage, no hanging, pinned or overloaded >pieces and >good king safety, and the opponent has no advanced passed pawns, it is probably >safe to >use an even bigger value of R. I simply return the score of the static eval >(and hence do >static forward pruning) if the remaining depth is less than or equal to 5. If >the remaining >depth is bigger than 5, I set R=max(remaining_depth/2, 4). > >For me, this works better than all other schemes I have tried (plain R=3, plain >R=2, >classic adaptive null move pruning, and verified null move). > >Tord
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