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Subject: Re: Question: Null Move Fiddling in Fritz6 and in general

Author: Vincent Diepeveen

Date: 11:20:19 11/29/99

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On November 29, 1999 at 12:06:57, William Bryant wrote:

>A thought occured to me (which can be a dangerous thing).
>
>When a null move fails high (produces a cutoff), this value is stored in the
>hash table.  Should the depth of the hash table entry be the current depth
>of the search, or the depth used for the Null search (ie depth - null
>reduction).

In contradiction to Fail High reductions you can store it in the hashtable
without problems. Fail High reductions return the score of the
same search with a smaller depth.

However nullmove returns the score of a smaller depth where THE OTHER
SIDE has the move, so that's something quite different, as the other
side gets the move twice!

The last few plies nullmove is a pure threat detection (depending upon
how well your quiescencesearch is), however
the plies before that it's also positionally seeing everything
as it does a normal search. The room for error is that it is using
a reduced search depth. However considering a certain side may move
twice there is already a lot less room for error as when someone
is threatening something, making 2 moves in a row is *definitely*
speeding things up a lot.

The pseudo code which is completely correct:

   int alfabeta(int alfa,int beta,int depth,...) {
     if( depth <= 0 )
       return(qsearch(alfa,beta));

     .. NULLMOVE:
     nullmovescore = -alfabeta(-beta,-beta+1,depth-1-R,..);
     if( nullmovescore >= beta ) {
       StoreHash(alfa,beta,nullmovescore,depth);
       return(nullmovescore);
     }
     ..
     ..
   }

the only thing we can fight about is what the qsearch should see
and what it should not see. The better the qsearch the more you see
the last few plies.

Suppose you don't do checks in qsearch, then you won't detect a matethreat
last 1+R plies!

How important is it to see a matethreat the last few plies?

Everyone must answer that for himselve.

>I have traditionally used depth, and am now testing with a depth - null depth to
>see if it helps/hurts.
>
>I would appreciate comments from others who have tried or considere this.
>
>William
>wbryant@ix.netcom.com



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