Author: Guido Schimmels
Date: 08:01:24 06/18/98
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On June 17, 1998 at 22:25:12, Roberto Waldteufel wrote: > > >On June 15, 1998 at 12:41:26, Ren Wu wrote: >> > >>I used to use PVS but switch to NegaScout some time ago, but after that > > >Please would you explain what is the difference? I was under the impression that >PVS and Negascout were one and the same algorithm, namely search first move with >window [alpha,beta], then search remaining moves with window [alpha, alpha+1] >with a research if necessary. If this is wrong, I would like to know. > >Best wishes, > >Roberto Well, *almost* the same. PVS comes from Schaeffer et al., NegaScout is from Reinefeld. PVS: value = PVS(-(alpha+1),-alpha) if(value > alpha && value < beta) { value = PVS(-beta,-alpha); } NegaScout: value = NegaScout(-(alpha+1),-alpha) if(value > alpha && value < beta && depth > 1) { value2 = NegaScout(-beta,-value) value = max(value,value2); } The difference is how they handle researches: PVS passes alpha/beta while NegaScout passes the value returned by the null window search instead of alpha. But then you can get a fail-low on the research due to search anonomalies. If that happens NegaScout returns the value from the first search. That means you will have a crippled PV :-( Then there is a refinement Reinefeld suggests which is to ommit the research at the last two plies (depth > 1)- but that won't work in a real program because of search extensions... NegaScout is slightly an ivory tower variant of PVS (IMHO). Look at http://www.uni-paderborn.de/pc2/people/homes/ar/r_nsc.htm for the whole story. You will also find there a link to Reinefeld's ICCA Journal article, ready for download. - Guido
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