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Subject: Re: Double Nullmove

Author: Robert Hyatt

Date: 09:54:48 03/30/01

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On March 30, 2001 at 12:04:36, Andrew Dados wrote:

>On March 30, 2001 at 11:17:22, Robert Hyatt wrote:
>>On March 30, 2001 at 09:04:25, David Rasmussen wrote:
>>>Anyone who have tried double nullmove (I know you have, Vincent :), what are the
>>>_drawbacks_ of double nullmove?
>>There are two sides to this:
>>1.  you use the double null-move to detect zugzwang positions.  Because if
>>a position fails high after the second null-move, it would have failed high
>>after the first as well and that is a strong indication of a zugzwang position.
>>And since the second null search fails high and returns beta, the first null
>>search will fail low and it won't be used.
>>So you get zug detection and you can be more relaxed in where you try
>>null moves.  Note that many null-move failures are _not_ zugzwang positions.
>>They are simply positions that look won to a less-than-normal search depth.
>>But in reality, a normal search would reveal they are dead lost.  Double nulls
>>don't handle this at all, so you need some other protection.  A classic is to
>>let your opponent get a pawn stuck at f6.  If he gets a queen to h6, the mate
>>threat might be unstoppable on g7.  But after playing Qh6, <null> you might
>>hit your q-search and never notice that Qg7 is mate.
>>2.  The double null move search is not free.  It is a tree search that will
>>be used to signal (on a few occasions) that a previous ply null-search should
>>not be trusted due to zugzwang.  The downside is this is pretty expensive.
>How expensive can double nullmove be?
>For bf of 3 you get about 1/3^3=1/27 more nodes.
>For endgames and bf of 2 you'll see estimated 1/8 more nodes.
>Actually my program uses double null in endgames and it seems 10% more nodes is
>maximal overhead I saw (compared to 'normal nullmove').

that is the number I saw, roughly.  Although on occasion it is much higher.
IE the second null-move search can explode with extensions in the right

>However in those low material situations when other programs disable nullmove
>totally (except for pawn endings) double nullmove is an obvious winner.

yes it can be.  Or you can be more selective and not disable null-move when
you have a single piece left, just be more careful about where you try them...

>>On one hand, you catch zugzwang positions.  On the other hand, you make the
>>tree larger.  Which is better?  It is just another compromise decision where
>>you win some and lose some because of it.

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