Author: Tord Romstad
Date: 02:40:53 04/02/04
Go up one level in this thread
On April 02, 2004 at 04:34:45, Vasik Rajlich wrote:
>On April 02, 2004 at 02:11:48, Tord Romstad wrote:
>
>>On April 02, 2004 at 00:41:48, Dann Corbit wrote:
>>
>>>On April 02, 2004 at 00:37:22, Artem Pyatakov wrote:
>>>
>>>>Dann,
>>>>
>>>>That was a very nice reply - I will incorporate the feedback.
>>>>
>>>>>If you keep a table of the counts of attack by piece type for each square, you
>>>>>don't have to bother with quiescence. The question in that case is whether it
>>>>>is better to do the work of updating the tables or to do the work of >quiescence.
>>>>
>>>>Is this really true? Can you explain how? Because I do have attack tables in my
>>>>program and I don't understand how this can help me avoid qsearch. Thanks!
>>>
>>>You need more than the attack tables. You need counts of attacks by piece type
>>>for every square.
>>>
>>>The strongest attack is a pawn attack. If you have 2 white pawn attacks and one
>>>black pawn attack, the square is won by white no matter what other pieces attack
>>>it (of course if you move one of the pawns to the square then the pawn attacks
>>>decreases by one).
>>>
>>>So do this for every square under attack:
>>>(white pawn attacks) - (black pawn attacks) {if zero, you are concerned about}
>>>(white piece attacks) - (black piece attacks) {knights and bishops}
>>>(white rook attacks) - (black rook attacks)
>>>(white queen attacks) - (black queen attacks)
>>>(white king attacks) - (black king attacks)
>>>
>>>And the same for pins (you will want to track the pinned pieces and xrays)
>>>
>>>If you track it for not only the attacked pieces but also the attacked squares,
>>>then you can know if an empty square is safe to step on for a given piece type.
>>
>>The problem with substituting a qsearch with something like this is that
>>you only consider the purely material consequences of captures and exchanges.
>>It is not at all unusual that an exchange dramatically changes some of the
>>positional components of the evaluation function. Such changes are hard
>>to estimate without actually making the captures and evaluating the resulting
>>positions.
>>
>>In the past, some engines tried to save processor time by not calling the
>>full evaluation function in the qsearch. Instead, they just counted the
>>change in material and added this to the eval at the horizon depth. Most
>>authors have abandoned this approach today, because the loss in positional
>>understanding was too big. Substituting the whole qsearch with a very
>>sophisticated SEE would have similar bad consequences, I think.
>>
>>Tord
>
>Tord,
>
>I thought you do this yourself (ie SEE rather than qsearch) when the position at
>the start of qsearch meets certain criteria. (ie is quiet enough)
I do in some cases, but only when a fail high or fail low is almost certain.
My evaluation function does not only return a score, but also locates hanging
pieces for both sides. If the static eval is not high enough to cause an
immediate fail high, but the opponent has a hanging piece of sufficiently
high value that the capture of it is likely to bring the score *really far*
above beta, and the side to move has no hanging pieces, I return a fail high
score. Similarly, if the static eval + biggest hanging piece for the opponent
is very far below beta, I sometimes return a fail low score without any
search.
In both cases, I use huge safety margins. I am not even close to eliminating
the qsearch completely.
Tord
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