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Subject: Re: Deep Search Extension

Author: Stuart Cracraft

Date: 13:17:35 01/22/98

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On January 22, 1998 at 15:29:39, Robert Hyatt wrote:

>On January 22, 1998 at 13:50:10, Stuart Cracraft wrote:
>
>>On January 22, 1998 at 05:03:38, Ernst A. Heinz wrote:
>>
>>>On January 21, 1998 at 21:56:27, Robert Hyatt wrote:
>>>
>>>>On January 21, 1998 at 18:52:44, Heiko Mikala wrote:
>>>>
>>>>>
>>>>>On January 19, 1998 at 04:21:26, Ernst A. Heinz wrote:
>>>>>
>>>>>>On January 18, 1998 at 18:53:15, Bruce Moreland wrote:
>>>>>>>On January 18, 1998 at 10:37:59, Robert Hyatt wrote:
>>>>>>>
>>>>>>>>(...) The idea was to shift the null-move window downward, and
>>>>>>>>then notice whether the null-move search fails high or low.  If it fails low,
>>>>>>>>(...) you extend by 1 ply.
>>>
>>>No, Bob, the idea as mentioned in Donninger's paper is different. Here
>>>is
>>>the reference for all who do not know the article.
>>>
>>>Donninger, C. (1993).
>>>Null move and deep search: Selective search heuristics for obtuse chess
>>>programs. ICCA Journal, Vol. 16, No. 3, pp. 137--143.
>>>
>>>Donninger's idea is to extend the search one ply if a null move near the
>>>horizon (e.g. at depths <= 3) does not fail high and the null move score
>>>plus a constant margin (e.g. minor piece value) is <= alpha while the
>>>static evaluation at the node is >= beta (i.e. fails high). In order to
>>>get meaningful results for the null move score, you need to do it with a
>>>full alpha-beta window instead of a zero window (this is a known error
>>>in
>>>Donninger's original article).
>>>
>>>Citing from my article about how "DarkThought" plays chess:
>>>
>>>In order to avoid possibly explosive growth of the search tree as caused
>>>by excessive deep search extensions in the case of repeated mutual
>>>mating threats, "DarkThought" restricts them to null moves at depth = 2
>>>in the
>>>first "2 * iteration-number" plies on all paths.
>>>
>>>>>I tried Bruce's mate threat extension (everyone did, I guess...), and it
>>>>>works fine.
>>>
>>>I just gave it a quick shot but then put it on my to-do list because the
>>>quick implementation made our search trees *explode* ... :-(
>>>
>>>=Ernst=
>>
>>It made my search tree explode too. I will try putting in the Dark
>>Thought
>>limitations of depth = 2 and the first 2 * iteration-number plies.
>>Thanks
>>for the tip!
>
>
>the 2*iteration number won't be enough by itself for sure.  You can find
>this limit in early versions of crafty sources when I was doing the
>threat
>extension.  It helped, but not enough.  however I didn't limit it to
>near
>the tips either, because it was also useful nearer to the root where it
>is just as profitable to extend when there is a threat.  But it is also
>expensive...

How did you get a score to use for this extension? The original article
advocates using the evaluation function at the interior tree nodes!

I think this is far more expensive than the extension.

--Stuart



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