Author: Robert Hyatt
Date: 11:33:53 02/05/04
Go up one level in this thread
On February 05, 2004 at 14:25:24, Anthony Cozzie wrote:
>On February 05, 2004 at 13:31:56, Robert Hyatt wrote:
>
>>On February 05, 2004 at 13:03:59, Anthony Cozzie wrote:
>>
>>>On February 05, 2004 at 12:47:26, Robert Hyatt wrote:
>>>
>>>>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.
>>>
>>>There's no need to be so defensive. I haven't read the Crafty source in a while
>>>as I've pretty much found all your secrets already, so I forgot a few things.
>>
>>wasn't trying to be "defensive". Just "informative". You had made a statement
>>that wasn't correct about Crafty, and I was just pointing that out to not leave
>>a wrong impression.
>>
>>
>>>Note also that you are handling only the kingsafety for rook lift here. If I
>>>have WP@B2, WR@B3, BP@B7, BR@B8, Zappa will give white an advantage and crafty
>>>will not.
>>
>>yes, but if the bp is at b6, there isn't much of an advantage. I prefer to pick
>>up the "advantage" by seeing the file half-open-up at some point.
>>
>>
>>>
>>>Anyway, the point remains: You are taking rook mobility, breaking out what you
>>>feel are 4-5 key cases, and implementing those. Advantage: quantization and
>>>speed. Disadvantage: doesn't catch all cases. Its simply a tradeoff for the
>>>programmer to make.
>>>
>>>anthony
>>
>>I disagree. your approach doesn't catch all cases either. The one I gave
>>previously is one example. with pawns at f7 g6 and h7, a rook on the g-file is
>>not a serious threat unless you push your own f and h pawns to try to attack
>>that g-pawn. But once it goes away we both notice it is good. So we both are
>>going to miss some things, and it is just a matter of picking which you like
>>best. I didn't choose to not do mobility for rooks because of the cost. I
>>didn't like some of the moves it produces. IE unnecessary pawn advances, or not
>>liking rooks side-by-side because they interfere with mobility, etc...
>>
>>
>>I don't think you have to count squares to do adequate mobility. With today's
>>search depth, if you have a rook on an open file, most likely you can see deep
>>enough, once the file gets opened or half-opened on the enemy king, so that you
>>can get the rook over there within the normal search horizon.
>>
>>Other pieces can use mobility better. Bishops are a good example and I do
>>mobility for them since it is not an expensive term at all with table lookups
>>available. however, I happen to believe that often mobility is a _result_ of
>>something good happening, not the "cause".
>>
>
>Zappa's mobility is somewhat more complex than just counting squares.
>
>anthony
So was the mobility in Cray Blitz, where the "mobility value" for every
reachable square was computed as well...
however, for rooks in particular, it rarely made any difference, because all the
old praxims about "rooks on open files" and so forth are just another way of
expressing mobility, until you see the exceptions. IE rook on an open E file
but your opponent has a bishop at e4 supported by pawns at f5 and d5 and you
have nothing to drive it away... I do miss those now where I didn't years ago.
But I have not seen it much of a problem and don't deal with it as of yet...
>
>>>
>>>>
>>>>>(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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