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Subject: Re: Deep Search Extension

Author: Robert Hyatt

Date: 12:29:39 01/22/98

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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
>>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
>>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* ... :-(
>It made my search tree explode too. I will try putting in the Dark
>limitations of depth = 2 and the first 2 * iteration-number plies.
>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
extension.  It helped, but not enough.  however I didn't limit it to
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

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