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Subject: Re: The "same threat extension" as effective way to resolve horizon prob

Author: Robert Hyatt

Date: 16:02:43 10/01/03

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On October 01, 2003 at 14:01:52, Tord Romstad wrote:

>On October 01, 2003 at 13:50:49, Robert Hyatt wrote:
>>
>>It seems to me that Chrilly's paper on null-move search included this
>>idea as the "threat extension".
>
>It is not the same.  The "deep search extension" (the term Chrilly used,
>IIRC) extends in all positions where the null move fails low by a
>sufficiently high margin, as you explain.  The Botvinnik-Markoff
>extension is different.  It extends when the null move fails low at
>ply n _for the same reason_ as the null move failed low at ply n-2.
>
>I have never seen this idea before.  It seems original and interesting.
>
>Tord

OK.  It is just a restricted "singular extension".  If a move is singular
at two consecutive plies for the same side, it gets extended.  The "for the
same reason" seems wrong, since all you are looking at is the refutation move.
Note that the _best_ move is not needed to produce a cutoff, just "a good enough
move".  That means it is more than just barely likely that two different moves
will cutoff and cause the null-move search to fail low at two consecutive
plies for the same side.  This could happen when history totals change, killer
arrays/counts change, one gets a hash hit the other doesn't, etc...





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