Author: Andrew Williams
Date: 15:00:01 07/18/99
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On July 18, 1999 at 17:27:12, William Bryant wrote: >I have created a SEE routine for my program (following the general plan of >Swap() >from Crafty) and now have a question about where it should be implemented. > >I initially implemented it in the move generator. When generating captures, if >the Captured_Piece - Capturing_Piece <0, I would uses a SEE score of < 0 to >discard the move, never searching it. > You can't do this. What if your only way out of check is a losing capture? The way I use SEE in my move generation is to help me in ordering moves. In other words, if the SEE for a particular capture is >= 0, I add this to a large constant, so that when I sort the moves before searching them, this one is considered early on. If the SEE score is negative, I don't add the large constant. This means that my sorting gives me winning captures then non-captures then losing captures. But I don't discard anything at move generation time. Note that most people don't use SEE in move generation at all. They just use a trick called MVV/LVA (Most Valuable Victim/Least Valuable Attacker), which means take the value of the attacker away from the value of the victim to discover the significance of the capture. When I used this, I used my normal material values for the victim (100, 330, 330, 500, 900 etc), but the values: 1, 3, 3, 5, 9 for the attacker. You then add the result of the subtraction to a large constant (whose purpose, as above, is to ensure that captures will be considered before other moves when you come to search). >This created an overall speed up but leaves a number of questions. > >Some positions require a sacrifice to find the winning line. These positions >are never found because the sacrifice move is always discarded and never >searched. > >Is this the tradeoff of this heuristic? > >Is there a better way to apply the SEE rather than to all capture moves as they >are generated? > I do the discarding trick you suggest, but only in the qsearch. If there's a line that needs such a capture to be found, I rely on reaching sufficient depth in the main search to find it. >Are there other heuristics that allow these discarded moves to be found in the >right positions? > >Here is an example position: >1rr3k1/4ppb1/2q1bnp1/1p2B1Q1/6P1/2p2P2/2P1B2R/2K4R w - - >It is from a list of mating positions posted by Dan > >I can find the winning mate in 0:32 without the SEE, >and never with the SEE. > >Thanks in advance. > >William >wbryant@ix.netcom.com Hope this is helpful. Andrew Williams
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