Author: Andrew Dados
Date: 09:04:36 03/30/01
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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'). However in those low material situations when other programs disable nullmove totally (except for pawn endings) double nullmove is an obvious winner. -Andrew- > >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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