Author: Ulrich Tuerke
Date: 08:12:23 07/08/98
Go up one level in this thread
On July 08, 1998 at 09:55:58, Robert Hyatt wrote: >On July 08, 1998 at 06:20:18, Inmann Werner wrote: > ... > >I think the idea is one that has "been tried and found wanting". It has >problems for a couple of reasons: > >1. many programs use PVS or Nega-scout algorithms, which means that 99.999% >of all nodes searched have alpha=beta-1, making it difficult to return a >value "very much lower than alpha" to trigger this. > >2. once you try it for a while, you find it has simply cost you a lot of >time. I don't know of anyone using this method any longer, including >Chrilly... IMHO, another draw-back of the originally suggested method is that the extensions depend on the current alpha-beta window. So, it could happen that a verification search resulting from such kind of extension does not reproduce the condition for the extension (as result of the shifted alpha-beta window). A unstable search could result (fail-low in fail-high verification search, ...). The corresponding modification for mate values only is free from this draw-back. > >But there is one useful thing you can do... at ply N+1, you *can* return >an accurate score, namely "mate". And this can be useful. Bruce was the >first I know of that used this method, and it works well. If you do nothing, >and you get mated, then there is obviously a "threat" in the position and it >is worth searching deeper. I do this too, but *only* for mate, because I >can't detect when the score is "well below alpha" because it can't happen in >PVS.
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