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Subject: Re: a question to Tord about detecting threats in null move

Author: Uri Blass

Date: 11:43:24 10/03/03

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On October 03, 2003 at 13:27:20, Anthony Cozzie wrote:

>On October 03, 2003 at 12:47:23, Uri Blass wrote:
>
>>On October 03, 2003 at 12:13:13, Anthony Cozzie wrote:
>>
>>>On October 03, 2003 at 12:03:49, Uri Blass wrote:
>>>
>>>>
>>>>I can detect every big threat by adding a special search after making null move.
>>>>
>>>>For example if I like to detect threats of at least 2.2 pawns I can do
>>>>val=-alphabeta(depth-4,-beta+219,-beta+220,...)
>>>>
>>>>if (val<=beta-220)
>>>>threatmove[ply-1]=1;
>>>>//ply-1 because I still did not undo the null move.
>>>
>>>small error here: beta should be replaced by eval, or you will have massive
>>>instability problems.
>>
>>If I am a queen down in the search and I threat to win a bishop then I do not
>>consider it as a threat  because a threat is a threat relative to beta.
>>
>>I believe that Tord does the same(he replied that your example of threating the
>>queen twice by sacrificing material is solved by extending only big threats so
>>if you sacrifice a rook and a bishop the threat on the queen is not a big
>>threat).
>>
>>I think that the idea that an extension should not be dependent on beta is a bad
>>idea.
>>It is better to have stronger engine with stability problems and not
>>weaker engine without them.
>>
>>I prefer even not to care about using hash tables for pruning because my
>>experience told me that I cannot get significant gain there easily(I have a lot
>>of stuff that means that pruning or extension is not defined only by the
>>position).
>>
>>Uri
>
>in your code:
>
>Suppose the BM extension succeeds, and the node fails low.  In other words, the
>program was using delaying moves to push a threat beyond the horizon, the BM
>extension stopped it, and it realized it was in trouble (a reasonable scenario).
>
>Then beta will be (say) 200 less than it was previously, and the threat might
>not be 200 less than beta any more, and your new re-search w/out the extension
>will fail high.  etc.
>
>Being a queen down in the search (may) have very little to do with beta: example
>WAC#141, where beta is a mate score even though white is down a rook and a
>queen, and beta for black is -mate score even though black is up a rook and a
>queen.
>
>anthony

I can agree with you that using beta may be a mistake because of the fact that
beta is changed after fail high but using the static evaluation is also
a bad idea.

In the case of Wac141 I want to use instead of beta the last calculated score of
the position that is almost equality that has nothing to do with the fact that I
lost a queen and a rook.

If I replace beta by eval I do not get it because I understand eval as the
evaluation of the position that I have(I evaluate every node)

Uri



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