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Subject: Re: Question for Hyatt about Alpha/Beta

Author: Bob Durrett

Date: 09:49:02 02/05/04

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On February 05, 2004 at 12:38:20, Anthony Cozzie wrote:

>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...

Your idea of "playing a more natural game" may be a very good thing in the
context of making engines play very human-like at the amateur level, maybe.

Bob D.



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