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