Computer Chess Club Archives




Subject: Re: Nullmove: when to avoid it?

Author: Severi Salminen

Date: 01:23:51 03/03/01

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>>>>Yep, that's true. As Bob said it is very rare to have beta=INF. Actually
>>>>Beta=INF can occur only in the very beginning of the search until we get our
>>>>first real score (score<=INF-Ply). Remember also that you don't have to know
>>>>anything else about the nullmove but whether its score is >=beta. So you can
>>>>search with (-beta,-beta+1)
>>>An idea I had, is that since about half the time we expect a move to fail low,
>>>it doesn't make much sense to give the opponent an extra move and expect it to
>>>fail high.
>>Well, the point is: how do we know it _before_ trying the null move? Futility
>>(pre) condition is one good indicator: if(depth==1&&material+MARGIN<=alpha).
>We know from where in the search we are, e.g. if the move being considered is
>off tje PV, we expect a fail high, and should try nullmove, but the response to
>this move should fail low, and nullmove is a waste of time.

Yes, now I understand, but if the move is off the PV why should we expect a fail
high? It could be fail low move as well. And in normal implementations we
already get these cases right: we don't try nullmove if we tried it at the
previous ply (except Vincent).


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