Author: Robert Hyatt
Date: 12:44:31 09/28/01
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On September 28, 2001 at 14:37:32, Alvaro Jose Povoa Cardoso wrote: >Suppose I'm doing a 12 ply search and at ply 2 I have a promotion or a pawn at >the 7th rank. >Should I extend? Or should I extend these cases only at frontier nodes / >qsearch? >My reason is that if we are a ply 2 we don't need to extend because we have 10 >ply remaining to search. >But if we are at fontier nodes or at quiescence search then we need to search >deeper these and other cases. >So, could we only do extensions at frontier nodes and at qsearch? >Perhaps my idea/concept of extensions is incorrect. >Could someone comment on this? > >Best regards, >Alvaro Cardoso The only purpose of extensions is to avoid the "horizon effect". If you are doing a 10 ply search, and you try a move and discover that I have a strong 9-ply threat I can spring on you, then you want to avoid that threat. There are two ways. One is to directly address the threat and eliminate it. This might cost you a pawn. The other is to do something that pushes the threat deep enough that your search can't see it. That costs you nothing. Of course, it only postpones the threat, but doesn't eliminate it. One easy example is to trade queens. If you play QxQ, then I don't have time to spring my threat to win a pawn yet, because I have to first recapture your queen. After you make any move, I only have 7 plies to work my trap and that isn't enough. So you just "solved" it (but only until after you and I make those two moves where my threat is back.) Extensions are designed to prevent that by making sure that forcing moves don't reduce the depth enough that threats are hidden... That is why you do them _everywhere_. Otherwise forcing moves at the root hide deep tactics that you can't afford to overlook..
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