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

Author: Tord Romstad

Date: 02:35:16 10/02/03

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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.
>>
>>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.

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 "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.

Tord



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