Author: Georg v. Zimmermann
Date: 04:05:19 12/07/00
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On December 06, 2000 at 20:03:21, Scott Gasch wrote: >>First a question about history heuristics: Should I clear or decrease the >>counters after a search. Now I do nothing. I just add history value to all non >>capturing moves and increase the counters by depth if I find a move whose >>score>alpha. > >You certainly need to clear the history table sometime. What happens when one >entry overflows? If you don't ever clear your history table your move ordering >is going to get really funky. > >History is a move ordering heuristic... because a move made 20 turns ago was >good has nothing whatsoever to do with that move being good now. Keep the >history table between iterative calls to search, sure... but don't keep it >between moves, in my opinion. It is probably a bit better to decrease not clear the history table after a move. History from 20 moves ago is certainly not worth much, but from 2 moves ago ... yes. Most increase counter by depth*depth btw. For me history doesn't help at all ... > >>How do killers work? More precisely when do I replace a killer move by another? >>If I have two killers at each ply with counters should I replace allways the >>lowest one? And after the search should I make all ply 1 killers ply 0 killers >>and so on? > >A killer move is a great move at this ply. Like promoting a pawn or checking >the king. THe idea is that you want to try this move again at the same depth in >another branch of the tree earlier (and therefore cutoff earlier in the other >branch). I do not make capture moves killers because, as Bob Hyatt said, >captures are often not repeatable at different branches of the tree. I make any >non-capture that results in a beta cutoff a killer move for its ply. I keep 2 >killer moves and order these first before all other non-capture moves. I clear >killer moves between top-level calls to Think (iterative search). > Just one more note: if you use 2 killers and get a new one make the 2nd 1st and replace the second one. >Your numbers for an 8 ply search of that position look sane. > >Good luck, >Scott Regards, Georg
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