Author: Robert Hyatt
Date: 18:39:04 01/02/03
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On January 02, 2003 at 12:05:31, Steve Maughan wrote: >Has anyone documented all the factors that affect a program's branching factor? >I notice that Tiger 15 and Shredder 7 seem to be approaching BF = 2, which IMO >is amazing. Ed also makes reference to a BF of 2 on his webpage. I remember >that a BF of 6 was the norm 15 years ago. Then hash tables came in and if fell >to 4. Then null move and it fell to 3 and now we seem to be at 2. The common >thread to all of these elements is forward pruning i.e. hash tables cut off >whole branches and null move the same. So is BF simply a function of >selectivity and the ability of a program to forward prune or are there other >elements? > >Your thoughts please, > >Steve First, "branching factor" is being used wrongly by many. It is a _static_ value for chess, regardless of the search method used, because it means "how many moves are possible at a position, on average?" "effective branching factor" is what you are really asking about. It is the measure of "after my selectivity stuff, my alpha/beta algorithm searches the tree as if it had a branching factor of what?" Since the tree is exponential, each additional ply multiplies the tree size by EBF, which for minimax is about 38, for alpha/beta, it is about 6-10, and for selective searches, it can get very low... Null-move is one thing that can drop it down to below 2.5 quite easily, as can forward pruning or other types of search extensions or reductions.
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