Author: Aivaras Juzvikas
Date: 05:39:41 07/16/04
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On July 16, 2004 at 07:24:28, Fabien Letouzey wrote: >On July 16, 2004 at 06:33:20, Vasik Rajlich wrote: > >>There is one interesting caveat in comparison to PVS. A PVS engine has the >>choice to treat null move as any normal move, or to require that the null move >>fails high. (ie within bounds, but not fail high, is not enough) The latter is >>somewhat logical, and IIRC this is what Crafty does. > >I think it's much more than somewhat logical!!! > >IIUC, you are talking about allowing null moves in the PV (in PVS). > >Imagine a PV node where null move gets the best score (inside of the >window). That means passing is the best option, which is the >characteristic of Zugzwang positions. They are exactly the positions >in which you don't want to try null move :) > >One of the reasons why null-move pruning is safe is that it is only >"kept" in incorrect lines, never in the PV. that sounds logical, however have you tested this in the games? for me the version with nullmoves allowed in pv nodes plays better.. > >>An MTD (f) engine must treat a null move as a normal move. I couldn't come up >>with a way to circumvent this restriction. It seems to be an inherent >>restriction of the MTD (f) search. > >>Another way to look at this: an MTD (f) engine must accept null moves along the >>"pv", where "pv" is defined as following the fail-high moves, and >>highest-scoring fail-low moves. > >I think it's more complex than this. Eventually the PV is made of two >"semi-PVs" (one failing high and one failigh low) that must agree on >the score. There can't be a null move at the same place in both, >since the null move can only fail high in its own node (contrary to normal >moves). > >>Vas > >Note that I am not a MTD(f) expert, but very interested in reasoning >concerning the PV. > >Fabien.
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