Author: Vincent Diepeveen
Date: 15:13:21 07/13/01
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On July 13, 2001 at 16:50:26, Uri Blass wrote: >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 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... >> >>So you expend quite a bit of effort, to eliminate one small part of the total >>problem... > >If you search to clearly reduced depth(for example before normal search with >null move pruning to depth d when d>=6 you search without null move to depth >d/2-2) then you may be less than 1% slower. > >I believe that it is a good deal to be 1% slower in order to avoid not seeing >simple zunzwangs. > >I guess that you may earn 3 elo from not falling in some zunzwangs and lose only >1 elo from being slightly slower. > >Uri Double nullmove wasn't invented for this math. Double nullmove was invented at a time when many people doubted whether nullmove was a smart way to search. Double nullmove you can proof that at depth == n you get the same best move as at depth n-i fullwidth (where i >= 0). Nowadays the discussion is completely different. Nullmove has proven to be such a winner that most even don't care for correctness of search. They just nullmove *always*, some nullmove only if some kind of static score >= beta. That's fine with me. It only shows the success of nullmove and that searching fullwidth is nowadays outdated.
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