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Subject: Re: Double Null move?

Author: Vincent Diepeveen

Date: 13:17:37 07/13/01

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On July 13, 2001 at 14:25:23, Robert Hyatt wrote:

>On July 13, 2001 at 12:28:01, Steve Maughan wrote:
>
>>I'm thinking of implementing double null move in my program.  Now as far as I
>>know the most conventional way is to do the normal null move search and if there
>>is a cutoff follow it with a normal search at reduced depth to confirm no
>>zugwag.  However I do remember that someone here (Vincent?) outlined a different
>>way of doing double null move.  Is there another way?  If there is, what are the
>>pros and cons of each?
>>
>>Thanks,
>>
>>Steve
>
>
>That's the gist of it.  If the position is a zugzwang position, the second
>null-move search will fail high, which will cause the first to fail low and
>you don't run into the zug problem.
>
>The downside is the cost.  The second null will fail low most of the time and
>just generate wasted nodes.

The downside is not the cost of the second nullmove, but preventing
a third nullmove in a row.

The best nullmove implementation is of course always nullmoving whatever
happens.

Even if a static evaluation is < beta then still nullmoving works way
better:

  a) nullmove is nearly free of cost as it's like 1/bf^4 nodes of
     the current subtree, where bf is the branching factor
  b) perhaps there is a major threat which my opponent can't prevent,
     then i still fail high.

>The other downside is that not all null-positions are zugzwang problems.  In
>fact, most null-move problems are caused by the R-value which bring the horizon
>too close to spot a tactical threat.  Double null won't find any of those...

Double nullmove is a cheap way to use nullmove in the far endgame also
without losing a huge number of plies. The big reduction (R=3 instead
of R=2) factor which we nowadays all use already shows how little of
loss a few ply of search depth is, as long as we replace the search by
the best thing we have: another search.

>So you expend quite a bit of effort, to eliminate one small part of the total
>problem...
>
>That's why I don't do it myself....

The only faster thing as double nullmove is always nullmoving,
this is a small subtree and we talk about a few nodes only.

For that price you search in a correct way (so at a depth n you always
find the solution of a problem).

The only stage of the game where i turn nullmove off is if either
side is in a pawn endgame. I do this dynamically in the search.





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