Author: Vasik Rajlich
Date: 05:26:05 03/25/04
Go up one level in this thread
On March 25, 2004 at 01:56:43, Johan de Koning wrote: >On March 24, 2004 at 11:09:41, Robert Hyatt wrote: > >>On March 23, 2004 at 05:05:56, Vasik Rajlich wrote: > >>>Junior, however, appears to come at the problem of selective search via >>>discussions about this in the CCC archives. Amir has claimed that the best way >>>to search selectively is via extensions. To complete the reductions vs >>>extensions thought from above, an extension strategy will have the profile that >>>most moves have the same basic search depth, while certain special moves will >>>have a higher search depth. The profile of a search based on reductions compared >>>to a search based on extensions will be different. >> >>It is easy to prove that last statement wrong. >> >>You write a program that only does search depth reductions. I write a program >>that only does extensions. I can make mine _identical_ to yours. Where you >>reduce, I do nothing. Where you don't reduce, I extend. IE if you don't reduce >>a check, I extend the check. We search _exactly_ the same tree. > >Indeed, assuming fractional plies, it is rather trivial to build >the same tree using either extensions or reductions. > >But it's better to avoid the term "reductions" since it is confusing. >The real issue is extensions versus *pruning*. > >Assuming Vasik intended "pruning" in that last statement, he is >quite right: different profiles (called search envelopes by Beal). >And *very* different back-up values. > Ok I'm still slightly new to this - but I meant reductions, not pruning. IMO pruning is appropriate for near-leaf positions, but bad moves near the root should be reduced, not extended. Let me try a reformulation. An "extension-based search" is one in which various extension rules are applied, each of which is triggered by a fairly small percentage of the possible moves. (Ditto for a "reduction-based search".) So, Bob's non-check reduction fails this criterion for a true reduction, since it triggers too often. So, practically, an extension-based search would look a bit different than a reduction-based search. My theory is that it's cheaper to decide to extend a small class of moves than to decide to reduce a small class of moves. Junior and Shredder would appear to support this - Junior appears to search very quickly, and it appears to use extensions heavily; while Shredder appears to search slowly, and it appears to use reductions. Not sure about Chessmaster :-) >To add to the confusion an, earlier (snipped) paragraph from Vasik's: >| Of course, in principle there is no difference between >| selective search via pruning and selective search via extensions, the two >| approaches could be equivalent. >IMHO that is the right words but the wrong conclusion. :-) > There I used the wrong word - I meant "reducing" rather than pruning. Practically speaking, I am about to get back into making my search selective. There's a question of framework for doing it. Ie, 1) do you look for moves to extend, or for moves to reduce 2) do you do a complicated static tactical analysis to support the decisions. Re. the first question, theoretically it may end up the same, but the code will look much different, and the evolution from a flat search will be different. Vas >... Johan
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