Author: Bo Persson
Date: 14:33:33 12/07/00
Go up one level in this thread
On December 05, 2000 at 19:25:55, Scott Gasch wrote: [...} > >Finally, I changed the code to follow Ernst's advice... Before I was storing >depth+extend as the depth in the hash node... now I am only storing depth. I do >not fully understand this, though... if I just got a score X from a search with >depth=depth+extend, is it incorrect to store depth+extend in the hash table? I >would really appreciate some clarification of this concept if someone has time. > The idea is to store the depth/draft you will have when you get back to using the node, which is the remaining depth upon entry to search(). The fact that you extend some branches and/or cut some with nullmoves etc, doesn't change teh *nominal* depth remaining. That is what you will use in the probe, so that is what you have to store! >Let me describe one more thing (which I also learned from one of Bruce's >messages, I think) so that someone can sanity check me if I am doing it wrong. >When I get an exact score that is >= +MATE_IN_N or <= -MATE_IN_N I am converting >it to a bound. I do this because a MATE_IN_N is relative to the depth of the >position in the search tree... and if we come across the same position in the >tree later at a different depth the score will be incorrect. >So convert _exact_ scores >= +MATE_IN_N into lower bounds of MATE_IN_N -- that >is, convert "this node is mate in N moves" into "this node is worth at least a >forced checkmate". Convert exact scores <= -MATE_IN_N into upper bounds of >-MATE_IN_N -- change "this position gets me killed in N moves" into "this >position is a forced mate in some number of moves". I've also seen people >adjust these scores relative to the current ply... I like the bound idea better >because it is simpler. Yes, but the adjustment improves the scores. Again, the idea is to store a score that is independent of the path taken to get there. You probably get a mate-in-N-from-the-root, which you cannot score, as it is path dependent. If you reach a mate-in-7 when you are already 4 plies down in the search, that is stored as a mate-in-3-from-here (exact score). If you later hit the same position 6 plies down from the root, mate-in-3-from-here is now a mate in 9. If you hit the position 2 plies from the root, it is a mate in 5. Bo Persson bop@malmo.mail.telia.com
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