Author: Robert Hyatt
Date: 09:47:26 02/05/04
Go up one level in this thread
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
You didn't look far enough. Crafty handles this but at another place in the
evaluation. Also I do more than "open files" There are half-open files. I
also catch the rook lift directly as I look in _front_ of the rook to see if I
hit any of my own pawns. If not, I like it. And if it bears on the opponent's
king, I like it even more.
I'm not sure where your "only 4 patterns" came from. Unless you just looked at
the comments alone. IE this:
/*
************************************************************
* *
* determine if the rook is on an open file. if it is, *
* determine if this rook attacks another friendly rook, *
* making it difficult to drive the rooks off the file. *
* *
************************************************************
*/
trop = 7;
if (!(file_mask[file] & tree->all_pawns)) {
score += ROOK_OPEN_FILE;
trop = FileDistance(square, tree->b_kingsq);
} else {
if (tree->pawn_score.open_files) {
unsigned char rankmvs = AttacksRank(square) >> (56 - (square & 0x38));
if (!(rankmvs & tree->pawn_score.open_files))
score -= ROOK_OPEN_FILE >> 1;
}
if (!(file_mask[file] & WhitePawns)) {
score += ROOK_HALF_OPEN_FILE;
trop = FileDistance(square, tree->b_kingsq);
} else if (!(plus8dir[square] & WhitePawns)) {
trop = FileDistance(square, tree->b_kingsq);
}
}
It does a _lot_ more than just open files in that block of code...
I don't like rook mobility myself. I used to do it and it is not very expensive
(IE I do bishop mobility at present already). I think the concept of open files
and half-open files is just another way to express mobility, as is rook on the
7th. however I don't blindly go for rook on the 7th as some do, it has to have
a reason for being there or it can be pointless.
>(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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