Author: Robert Hyatt
Date: 07:29:49 06/13/98
Go up one level in this thread
On June 13, 1998 at 05:55:44, Vincent Diepeveen wrote: > >On June 12, 1998 at 12:38:12, Don Dailey wrote: > >>On June 12, 1998 at 03:49:36, Vincent Diepeveen wrote: >> >>>Hello, >>> >>>Some time ago someone mentionned Deep Search extensions. >>>I looked in archives but could not find anything about it. >>> >>>What exactly are deep search extensions, >>>is it extending after the n-th extension more and more? >>> >>>Greetings, >>>Vincent >> >>Hi Vincent, >> >>Deep search extensions are a by-product of the null move search. >>Essentially they are extensions given when the program fails a >>null move search. The assumption (when this happens) is that >>something interesting must be going on (it's usually just some >>direct attack of something) and warrants an extension. >> >>The typical implementation is to only grant this on 1 or 2 levels >>near the leaf nodes of the main search. > >Doesn't this mean we extend everything, because we get again >leafs after we extend? > >Are there extra conditions like score of position static evaluated >eval >= alfa, or does the definition leave that free? > >So > if( nullmovefail && eval >= alfa ) > then extend a ply. > >>- Don this isn't exactly how it works. Here's what happens: if you use PVS, you have a more difficult problem, if you don't use PVS (or other nega- scout like algorithms) it is easier. You don't just extend if the null move search fails low, you extend if it fails low by a significant margin. In my case, when I did this a couple of years ago, I had to do a second null-move search, with the window lowered by 1/2 pawn. The idea is this... you find what appears to be a good move (one that fails high, or one that just became part of the PV) but before you accept the score, you do the null-move test. If the offset null-move fails low, that means that doing nothing gets you killed, which might mean that the current move is a move that just barely holds off this threat. And you extend. It can find some things pretty well, it fails on others, and the cost is non- trivial. If you are going to try that, you might as well try full singular extensions, as this is just a very weak subset of that...
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