Author: Robert Hyatt
Date: 14:56:39 01/30/06
Go up one level in this thread
On January 30, 2006 at 16:51:05, David B Weller wrote: >When a move fails high, check the null-move search on that same position to see >if it fails low (using a lowered window for safety). If it fails low, something >is up here as playing move X fails high, standing pat fails low, so probably >move X is holding off some serious threat by the opponent. Extend and search >again to see if you can now see what is happening with the increased depth. > >So, > >set flag if NM fails low by a certain margin >then later, while searching real moves, if *any* one fails high >&& flag was set by NM earlier, research node with extended depth > >Have I understood this correctly? I did it the other way. Search until you get a fail-high. But just before you return the value for the fail high, do the null-move threat test. If it fails low, you don't return the fail-high value. You do another normal search on the fail-high move alone, but you extend the search by some value (one ply unless you use fractional extensions and want to try something more or less than that.) > >If this is correct, I am still a little unsure of what this actually describes >in a game tree. > >What does the fail high prove/indicate? The search fails high on a move that appears to be good enough to refute the opponent's move at the previous ply. So it is definitely good. But if it is the _only_ good move, would that give you any information you don't have? Yes. Because I'd rather have several good moves to choose from rather than just one that seems to do the trick. Now I know I have a good move. If I do a null move search on a downward-offset window, and it fails low, I learned something new. Namely that if I do nothing, I get crushed, while if I play the fail-high move, I appear to be doing just fine. But I might worry about that since it now looks like my opponent has some threat that I have only one way to meet. And if I only have one way to meet it, I want to make sure there isn't some horizon-effect or insufficient-depth issues with that move since I am going to be forced to depend on it if the search comes this way. It is somewhat akin to singular extensions, but not exactly the same. In most normal cases, the offset null-move search will fail high since the alpha/beta window is above the window used for that search. So you have isolated a particular circumstance where one move fails high, and if you "pass" you get crushed. That's a "fishy circumstance" and you had better make sure that one saving move will really save you by searching it deeper. > > > >BTW, where might I find older Crafty Sources? There are some on the ftp box. I will look to see what is there. Most were lost quite a few years ago in a disk crash, but I have slowly gotten a few old versions sent to me over time by those that saved them for whatever reasons...
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