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Subject: Re: Double Nullmove

Author: Robert Hyatt

Date: 08:17:22 03/30/01

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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.

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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