Author: Robert Hyatt
Date: 08:16:35 01/31/01
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On January 31, 2001 at 05:39:41, Tony Werten wrote: >On January 30, 2001 at 21:10:12, Robert Hyatt wrote: > >>On January 30, 2001 at 17:46:28, Severi Salminen wrote: >> >>>Hi! >>> >>>What is the definition of BF and how is it calculated (this has probably been >>>discussed N times, but...)? Now I'm calculating like this: >>> ______________________ >>>BF=(DEPTH-1)|(Nodes/root_move_count) >>> >>>So it means (DEPTH-1)th root. >>> >>>Example (from Vincent's position): DEPTH=9 plies, Nodes=2'000'000 and we have 38 >>>legal moves in root. >>> ____________ >>>BF=8|2'000'000/38 = 3.9 >>> >>>Could this be close to the truth? >>> >>>Severi >> >> >>There are two terms that are intermixed often: >> >>1. "branching factor" usually means the average number of branches from a >>node, which for chess averages 35-38 (depending on who you believe) for the >>whole game. >> >>2. "effective branching factor" is roughly calculated as you mentioned. IE >>nodes for iteration N-1 divided into nodes for iteration N. Or you can use >>the time. >> >>This is really "effective branching factor" because there is _nothing_ that >>can reduce the true branching factor other than pure forward pruning, where >>some moves are thrown out at each and every ply with no searching of any >>kind. IE for Crafty, EBF is roughly 3.0... which is actually impossible >>on the surface since alpha/beta reduces the branching factor by roughly >>the square root of it. square root of 38 is just over 6, which is a >>reasonable branching factor for alpha/beta. To get lower, in the case of >>crafty, rather than culling some moves by not searching them at all, I simply >>search many of them to a lower than usual depth because of the null-move R >>value... >> >>numbers around 3 are very good IMHO. I only wish mine stayed there all the >>time. But it can easily run up to 10 or more in oddball positions where >>the move ordering breaks down at deeper depths... > >Is this only in odd positions or do you notice ebf going up with dept as well ? > >In my program it seems impossible to keep ebf below 3 if dept becomes bigger >than say 12. ( except for endgame positions ) > >Tony You are probably running into a hashing problem. If you effectively disable the hash table (which can happen at very large depths due to overwriting) then the EBF can rise. It also rises each time you change your mind on the best move, since you search way more nodes each time you change. And it can also rise when move ordering breaks down, which can also happen in lots of positions.
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