Author: Anthony Cozzie
Date: 09:38:20 02/05/04
Go up one level in this thread
On February 05, 2004 at 12:22:44, Bob Durrett wrote: > >Bob Hyatt: > >I was going through the older CCC bulletins to make sure I didn't miss anything >important and noticed the thread begun by Russell, >http://www.talkchess.com/forums/1/message.html?345569. After checking Russell's >reference, I saw something you wrote cited below. This made me really curious >about how the alpha/beta algorithm might be impacted by improvements in the >position evaluation code. It seems to me, intuitively, that accurate assessment >of positional [and other non-material] factors in a position, along with the >correct assessment of material factors, would give >values which would change the interpretations of failing alpha or beta tests. >It seems that this would significantly alter the way searching would proceed. > >If this is unclear, I can try to be more detailed if you wish. [I never claimed >to be a Pulitzer Prize winning author.] > >Bob D. Hey, you are starting to realize why it is so hard to write a good chess program :) One of the reasons Crafty gets good search depth is that it keeps a lot of the piece eval simple. For example, Rooks in crafty have just 4 patterns: open file, 7th rank, behind (friendly|enemy) passed pawn. The advantage here is that the eval is very quantized [0 | 20 | 40]. In Zappa, I do a true (and fairly complex) mobility calculation. The advantage is that this catches a lot of cases that crafty does not, for example a rook lift (R@B3 P@B2 BP@B7) or a rook on the 8/6th ranks (which can also be powerful). The disadvantage is that the eval is much less quantized. [0 | 1 | 2 ... | 40]. This means that move ordering is worse, and so I search less deeply with mobility on than with mobility off (not to mention the speed loss). I believe the depth I lose is worth Zappa playing a somewhat more natural game, but it is a tradeoff that everyone has to make for themselves, of course. anthony >http://groups.google.com/groups?q=%22The+meaning+of+Alpha+and+Beta%22&hl=en&lr=&ie=UTF-8&selm=a6d9ho%24899%241%40juniper.cis.uab.edu&rnum=1 > >Referenced by: > >http://www.talkchess.com/forums/1/message.html?345569 > >> An alpha cutoff is what happens when you search the second move, >>> and you prove that if you play that move, your opponent has a move >>> he can play that will produce a score less than your "lower bound" >>> you established for the first move. There is no need to search >>> further. >>> >>> For example, after that +1 on the first move, you try the second >>> move and after trying the first move the opponent has in reply to >>> that move, you discover you _lose_ a pawn. The score is -1.0... >>> There is no need to search other opponent moves to produce a >>> score even lower than -1.00, because you already know this move >>> is at _least_ -1.00 and possibly worse, while the first move is >>> +1.00. You stop searching this move and move on to your third >>> choice...
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