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Subject: Re: PVS and NegaScout

Author: Yngvi Bjornsson

Date: 12:27:36 01/05/00

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On January 05, 2000 at 14:35:18, Gian-Carlo Pascutto wrote:

>Hi all,
>what is the difference between a Principal Variation Search
>and the NegaScout algorithm ?
>Everytime I look at them, they seem to do exactly the same
>thing, except that NegaScout has a special case if depth > 2,
>but because of extensions, you can't use that in a chess program.
>So, what *is* the difference ?

Search-wise PVS and Negascout are identical (except the deep-cutoffs on the PV
you mention), they are just formulated differently. In Negascout the
same routine is used for searching both the PV and the rest of the tree,
whereas PVS is typically formulated as two routines: PVS (for searching the PV)
and NWS (for the null-window searches).  Negascout and PVS were developed
about the same time in the early '80 (82-83), but independently. I guess, that's
part of the reason we know them by different names.

Personally, I've always found the PVS/NWS formulation the most intuative,
it's easier to understand what's really going on.


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