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Subject: Re: An MTD(f) question about NULL MOVE searching

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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