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Subject: Re: nullmove and tactics

Author: Tord Romstad

Date: 00:50:37 03/26/04

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On March 26, 2004 at 03:42:00, Roberto Nerici wrote:

>On March 26, 2004 at 03:02:04, Tord Romstad wrote:
>
>>On March 26, 2004 at 02:39:29, Johan de Koning wrote:
>>
>>No, I do lots of pruning besides null-move, and reach rather high search
>>depths even without it.  Here are the number of nodes I need to search
>>n plies (n from 1 to 13) with and without null move from the opening
>>position:
>
>I think this makes a big difference to the result of the null move versus
>non-null move test. Most people when first writing an engine (this includes me)
>add null-move to a search that has no other pruning at the time and this makes a
>huge difference.

Yes, certainly.  And I should perhaps have pointed out that I did not
disable what I call "static null move pruning", but only the recursive
null move searches.  When the remaining depth is low or it seems very
clear that the side to move is winning, and the static eval minus the
biggest statically detected threat is above beta, I return a fail high
score immediately.  This makes the effect of the removal of recursive
null move pruning less dramatic than it otherwise would have been.

>People here have pointed out several times that pruning schemes overlap; any one
>scheme is usually more effective on its own that in combination with others.
>
>>Plies   Nodes (null move off)   Nodes (null move on)
>> 1                 55                      55
>> 2                228                     261
>> 3                965                     439
>> 4              3,741                   2,318
>> 5             13,213                   6,638
>> 6             35,056                  18,964
>> 7             37,398                  36,095
>> 8             77,226                  60,240
>> 9            117,274                 109,939
>>10            239,355                 237,775
>>11            555,661                 412,969
>>12          1,942,523               1,084,089
>>13         12,060,312               3,531,279
>>
>>I have no good explanation for the bizarrely irregular effective
>>branching factor of my non-nullmove search here.  I wonder how it is
>>possible that hardly any nodes are needed in the 7th iteration.
>
>Two possibilities:
>1) there's something wrong at the 7th iteration
>2) you've stumbled onto something amazing at your 7th iteration and just need to
>make it do the same thing on the later ones... :->

The most likely explanation is:

3) I have yet another bug.

Considering all the serious-looking bugs I constantly discover in my
search and eval, it is amazing that this thing is able to play even
a single game without lots of serious blunders.

Tord



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