Author: Fabien Letouzey
Date: 03:34:54 07/27/04
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On July 26, 2004 at 19:47:21, Peter Alloysius wrote: >What's the different between negascout and PVS ? They look like the same >algorithm to me. Somebody who has the two original articles can correct me if I'm wrong: PVS was published first. It combines the scout idea by Pearl with alpha-beta pruning. The definition uses two functions: PVS() for the PV nodes and TEST() for the scout searches. I am not sure about the TEST part, which comes from the original SCOUT. Algorithmically, TEST is equivallent to a fail-hard NWS (AlphaBeta with null window, as used in the MTD framework). NWS is sometimes called MWS or even ZWS in on-line documents. NegaScout re-formulates PVS with only one function. TEST is replaced with AlphaBeta, which enables the use of known improvements such as fail-soft. The re-search window is reduced as I mention in http://www.talkchess.com/forums/1/message.html?378029. There is also a "trick" near the leaves which can (should!) be ignored. It can only avoid re-searching a handful of nodes at best, and is anyway incompatible with most refinements used in real-world programs such as quiescence search, lazy eval, etc ... Knowledgeable readers, please correct me as I tried to guess from references only. Also among those differences, I am not sure which distinguishes PVS and NegaScout the most. If it is the two-function vs. one-function property, it could be argued that most scout-based chess programs use some form of NegaScout. I prefer to think in terms of algorithms (as in the tree that is actually searched) rather than implementation, and the re-search window strikes me as the main characteristic. Fabien.
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