Author: Ulrich Tuerke
Date: 03:30:37 10/28/99
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On October 27, 1999 at 15:38:54, Frank Schneider wrote: >Hi all, > >this is a question about the searchalgorithms used by other programs. >Many programs and also Gromit seem to use PVS or MDT(f) which both use >nullwindow-searches. >Is there anyone using a search that doesn't do nullwindow searches? >Why? Why not? I am sure that there are some programs not using minmal window searches, also among the top programs (Hiarcs ?). In principle (AFAIK), the AB standard algorithm works correctly, PROVIDED the result of the search does not depend on the choice of alpha and beta. However in practice, this condition may be violated in most cases. Think for instance of evaluation window techniques, null move cuts, futility cuts, extensions, ... . The violation of this precondition can make the search instable, e.g. you may get a fail-low in a fail-high verification search, because alpha & beta have changed when doing the re-search and the conditions for your tactical extensions will not be reproduced in some cases. When it now comes to a minimum window search, you have generally many more verification searches with shifted aspiration window. Thus, the above described instabilities may become far more dramatic. Of course, this happens not only on the root of the search, where it becomes obvious but also internally (in PVS) all the time. In my prog, I observed that I do not get stable evaluations at all when using PVS, though it fastened up the search a little bit. I think that this was a consequence of the very strong alpha-beta dependencies in my prog. Therefore, i decided not to use PVS. Uli > > >Frank
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