Author: Volker Böhm
Date: 16:00:26 08/26/05
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On August 26, 2005 at 14:21:34, Alvaro Jose Povoa Cardoso wrote: >Hi, >some of you compare the number of times a move failed high to the number o times >the same move failed low in order to decide if a move can be reduced one ply. >I've tested this and also tested using the actual values of the history table >(using of course another history table for fail lows). >I couldn't reach a conclusion though. >What is your experience on this? > >best regards, >Alvaro Hi Alvaro, the first time I tried history based pruning I was allmost sure that this couldn´t work. But now it works fine for me. One point I have to add is that I only count a fail low if there is some move sorted after it that failed high, thus I do not count fails on alpha nodes. I can´t really beleive that there are really many moves that are more often failing high than failing low. But it is the case. There are many parameters you can experiment with: 1. how many points you add to a history-pruning table for a move failing high/low dependant on the current search depth. (example square of depth?) 2. Which moves are not reduced even if the fail high/fail low ratio is "bad" (example: captures, check moves, promotions, ...) 3. At what ratio do you reduce and how many plies do you reduce depending on the current search depth 4. What do you do if a reduced move fails high (search again without reduction?) 5. "Hysterese": Prevent a toggling of pruning/non pruning decisions by adding some points if a move switches from pruning to non pruning. 6. Interaction between moves and pieces (example a rook move that leads to a double rook on a column could be handled different as the same rook move without doubling rooks). I have tried at least 50 different combinations of those parameters. For me I have found one combination that improved strength much. But the combination is far away from the combination I had expected to be best. The only advice I can give for this type of pruning: don´t think to much, just try! And keep it simple! Greetings Volker
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