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

Author: Robert Hyatt

Date: 08:25:15 10/02/03

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On October 02, 2003 at 05:35:16, Tord Romstad wrote:

>On October 01, 2003 at 19:02:43, Robert Hyatt wrote:
>>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.
>>OK.  It is just a restricted "singular extension".  If a move is singular
>>at two consecutive plies for the same side, it gets extended.
>Huh?  I don't understand what you are saying here, I'm afraid.  Unless
>I am missing something, the extension seems totally unrelated to singular
>extensions.  Where are the singular moves?

The "singular move" is the move that is refuting the null-move.  It isn't
exactly the same thing, obviously, but it fits here pretty well.

>>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...
>This problem can be reduced (though of course not completely eliminated)
>by a minor modification to the move ordering.  In my implementation, I
>search moves with the same target as the refutation move from two plies
>earlier directly after the move from the hash table (this move ordering
>trick is only done at nodes where the move leading to the node was a null
>move, of course).
>So far, it seems to work rather well, at least in my program.

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