Author: Robert Hyatt
Date: 17:42:21 10/05/01
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On October 05, 2001 at 19:15:19, Paul wrote: >On October 05, 2001 at 14:59:28, Robert Hyatt wrote: > >>On October 05, 2001 at 12:33:06, José Carlos wrote: >> >>> Due to recent posts about SEE, I've decided to implement it. I have a doubt. >>>In this position: >>> >>>[D]6rk/3n4/6p1/8/8/3B4/P2R4/7K w - - 0 1 >>> >>> white has only a capture: Bxg6. If I understood right, SEE looks only for >>>captures in one square, so it will try: Bxg6 Rxg6 and I'm losing a bishop for a >>>pawn. So SEE will tell me Bxg6 loses, and I'll discard a move that actually >>>gives me a pawn for free. Is this correct? >>> >>> José C. >> >> >>That is correct. But notice this is only "discarded" somewhere in the >>q-search (in my case.) I _never_ throw moves out in the normal search. >>This will be searched near the end, because it looks like it loses a >>piece for a pawn. But it _will_ be searched. The q-search will be wrong. >>But then it is possible that your RxN can't be played either because your >>rook might be pinned. That gets too complicated for this kind of simple/static >>analysis... >> >>The q-search is _full_ of errors, because it is by definition not exhaustive >>at all. Which means it overlooks pins. forks, skewers, etc already. What's >>one more small error tree in that forest? :) > >Bob, could you explain what you mean by 'it overlooks pins'? > >Groetjes, >Paul Sure... the typical SEE code (at least that implemented in Crafty, and what I did in Cray Blitz) looks at one specific target square for exchanges. It doesn't pay any attention to absolute or regular pins (ie if a piece is pinned on my king by your rook, I can _still_ use that piece in the sequence of captures. It is less accurate, but the errors in the q-search are so large, this is "noise" basically. In the example given at the start of this thread, we have a discovered attack that SEE misses as well. BxP, and when the opponent plays something takes bishop, he has a piece hanging to the discovered attack exposed when we moved the bishop to take the pawn. Other things include overloaded pieces. The point is that _all_ that is considered is "which pieces bear on the target square, whether they are pinned, overloaded, or anything else?"
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