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

Author: Anthony Cozzie

Date: 10:27:20 10/03/03

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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



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