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Subject: Re: new thoughts on verified null move

Author: Uri Blass

Date: 14:57:28 11/23/02

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On November 23, 2002 at 15:36:45, Martin Giepmans wrote:

>On November 23, 2002 at 14:06:05, Uri Blass wrote:
>>On November 23, 2002 at 13:00:34, Martin Giepmans wrote:
>>>On November 23, 2002 at 12:10:02, Uri Blass wrote:
>>>>On November 23, 2002 at 11:37:25, Martin Giepmans wrote:
>>>>>On November 23, 2002 at 08:48:36, Omid David Tabibi wrote:
>>>>>>On November 23, 2002 at 08:45:00, Uri Blass wrote:
>>>>>>>On November 23, 2002 at 08:11:37, scott farrell wrote:
>>>>>>>>Just after other people's thoughts.
>>>>>>>>I think Omid's work overlooked the adapative null move searching many of us do,
>>>>>>>>ie. transitioning from r=3 to r=2.
>>>>>>>>I think adaptive null move tries to GUESS where to use r=2 to reduce the errors
>>>>>>>>that R=3 makes. I guess it depends on how often this GUESS is correct, the cost
>>>>>>>>of the verification search, and how long it takes the adaptive searching to
>>>>>>>>catch the error at the next ply.
>>>>>>>>Has anyone looked at setting the verification search to reduced depth of 2
>>>>>>>>(rather than 1)? obviously to reduce the cost of the verification search.
>>>>>>>Omid checked it but you also reduce the gain.
>>>>>>>I think that I will look for good rules when to do the verification search so
>>>>>>>the cost will be significantly smaller but the gain is going to be the same in
>>>>>>>at least 99% of the cases.
>>>>>>I'm currently working on other variations. The initial results are promising.
>>>>>I have done some tests with your method at greater depths.
>>>>>At depth 12 vrfd R=3 still had an overhead (in terms of treesize) of about
>>>>>25% compared to pure R=3.
>>>>>(my engine uses a simple Q-search that shouldn't give problems here)
>>>>>So the question is if your expectation that the treesize of R=3 and vrfd R=3
>>>>>converge at greater depths (> 11) really holds.
>>>>>Needs more testing, I think.
>>>>>Another point:
>>>>>I would expect that vrfd R=3 becomes less safe at greater depths.
>>>>>The subtrees in which you don't verify nullmove (after the verification) become
>>>>>deeper and I see no reason - on logical grounds - why this shouldn't give safety
>>>>>Even if R=3 and vrfd R=3 converge in terms of treesize, the safety (or rather
>>>>>the lack of it) might also converge ...
>>>>>In any case, thanks for sharing.
>>>>I see reasons why the safety does not converge.
>>>>A common problem with null move is cases when the first move has a threat but
>>>>the threat can only be seen at big depth.
>>>>With verified null move pruning you are going to see the threat clearly earlier.
>>>>When the threat is in a move that is not close to the root then the danger of
>>>>being wrong in detecting the threat is smaller because you can miss one threat
>>>>but another threat may give the same result.
>>>I think it will be clear if you replace the somewhat confusing expression "close
>>>to the root" by "distance to the leaves".
>>I do not see the confusion because distance to the leaves has opposite meaning
>>to distance to the root.
>>In terms of distance to the leaves(remaining depth) I say the following:
>>It is more important to detect threats when the distance to the leaves is big.
>>The point is that when the distance to the leaves is small even if you are wrong
>>in detecting threats there is a good chance that it is going to change nothing
>>in the final score.
>>I also have examples when the algorithm save me 2 plies when the depth is big.
>In the subtrees after verification the remaining depth becomes bigger every
>iteration. For instance (not counting extensions for the sake of simplicity)
>if you search 14 ply and you do verification at ply 2 you get a subtree
>of 12 ply. In that tree nullmove with R=3 is done without verification.
>This is risky. If you search 16 ply the same tree will be 14 ply. More risk ...
>The question is if errors in a subtree will easily propagate to the root
>and influence the score and the best move.
>It is possible that the verification acts as a kind of barrier that blocks
>some or most errors and keeps them away from the root.
>I don't think so, but maybe ...
>>Here is one of them.
>>Latest movei need 11 plies when previous movei needs 13 plies and a long time to
>>solve the fail high
>>[D]r1bq1rk1/3pbppp/p1n1p3/4P3/2B1NP2/PP5Q/1B4PP/3R1R1K w - - 0 1
>>depth=11 +1.20 f4f5 e6f5 e4d6 e7d6 e5d6 d8g5 f1f5 g5g6 h3h5 g6h5 f5h5 c8b7
>>Nodes: 15025408 NPS: 158378
>>Time: 00:01:34.87
>>depth=11 +1.21 e4f6
>>Nodes: 18075970 NPS: 158979
>>Time: 00:01:53.70
>>depth=11 +3.72 e4f6 e7f6 e5f6 d7d5 c4d3 h7h6 f6g7 f8e8 h3h6 f7f5 d3f5 e6f5 h6c6
>>Nodes: 69601416 NPS: 156492
>>Time: 00:07:24.76
>>I did not analyze the reason for the difference but it is possible that movei
>>saw no threat after Nf6+ gxf6 exf6 and verification search helped it to see the
>This is interesting.
>After Nf6 gxf6 exf6 null fxe7 Qxe7 white has regained the lost material
>but blacks kingside is damaged. So the nullmove at ply 4 should probably
>not result in a cutoff (fxe7 is really a threat).
>Unless .. you don't evaluate kingsafety in terms of pawnshield and I seem to
>remember that you don't do that in Movei.

Movei of today does not evaluate king safety.

I do not like to evaluate king safety in terms of pawnshield because by that
logic I may evaluate things wrong because there are cases when one side has no
pawns near the king but has no problem of king safety.


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