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Subject: Re: NULL MOVE

Author: Vincent Diepeveen

Date: 05:09:34 02/25/99

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On February 24, 1999 at 15:53:49, Don Dailey wrote:

>On February 24, 1999 at 13:49:49, Will Singleton wrote:
>>On February 24, 1999 at 07:44:06, Vincent Diepeveen wrote:
>>>On February 24, 1999 at 02:30:21, Will Singleton wrote:
>>>>On February 24, 1999 at 01:16:56, Sylvain Lacombe wrote:
>>>>>I just finished implementing the null move. At first, i thought it wasn't saving
>>>>>much time. Then, i reallize that it does save alot, but only at deeper plys. On
>>>>>the first few plys, i think, it evens slow me down. Is that normal? Am i doing
>>>>>something wrong?
>>>>>I don't use the null move at the first ply, only at the second. It save about
>>>>>40% reaching deep 6. But it takes about 10% more for reaching deep 3 and 4.
>>>>>Hope, you can help.
>>>>You're probably doing something wrong. But get a second opinion. :)
>>>>My prog will use about half as many nodes early on, like up to ply 5, then the
>>>>effect becomes more pronounced, and in plys above 7 or 8 can show 2-3x
>>>>reductions.  But it depends on the position.  If there's a single best move,
>>>>there's more reduction than if the position is pretty quiet.  So try it on a
>>>>couple of different test suites, like wac vs Bratko-Kopec.  Or, use the
>>>>positional BK positions measured against the tactical ones (12 each).
>>>>Of course you wouldn't use null on the first ply, that would make it kind of
>>>>hard.  Are you using a reduction factor of 2?  That's the most popular, it
>>>>seems.  I'm testing R=3 now, seems to work OK.  But stick with 2 to start.
>>>>Also, make sure you don't do 2 nulls in a row.  And, for testing purposes, you
>>>>should probably limit a null move to one per search (I mean, once the null has
>>>>been done, don't do it again below that node).  Then when it's working better,
>>>>test out multiple nulls.
>>>>Null moves are susceptible to zug positions, so just disable them in the
>>>>endgame.  You can try out better things later.
>>>>Don't do null when in check!
>>>>There's an open question about allowing nulls in the pv, so look at that.  And
>>>>make sure to clear the ep flag after a null.
>>>This is no open question. Of course always do nullmove.
>>>If it doesn't give cutoff then you still can see whether you need a
>>>mating extension.
>>>It saves for me a lot anyway. ALWAYS do nullmove, just take care you
>>>don't improve your alfa with it if it fails. Some do that, but i'm
>>>not a big fan of that.
>>Yes, just recently I took the alpha update out, since it seemed to be causing
>>problems on occasion.  I don't know that for certain, but in my case, it's
>>working better.
>My null move is a "test" search, it always has a zero width window.  All
>I care about is whether you get a beta cutoff and seems to be a slight
>improvement.  I also had occasional trouble with the alpha updates when
>I used to use them, but the zero width window solves this problem and
>is seems to be slightly faster too!
>Your statement about not using two null moves in row shouldn't matter.
>I had this rule in my program and Don Beal asked me, "why?"  I took
>it out and the program worked fine.   I think a few years ago
>my implementation of null move needed this rule to prevent infinite
>recursion, but I cannot remember for the life of me why this was so.
>The worst that can happen is that you do more  depth reduced
>searches which is such a tiny fraction of the whole you will not be
>able to measure the difference in time.   But even this won't happen
>if you do not do a null move search when the "stand pat" score is
>already below beta.  Some programs do the null move selectivity anyway,
>or they do it if the score is CLOSE to beta.  However I decided to
>ignore any minor speedups this gave because it also introduces
>some risk.  I really doubt you can prove one is better than the
>other and my current program doesn't even register a speedup for

I'm not interested in what gives me another 0.5% speed up, but
how to implement nullmove in such a way that it makes my search correct.

One can see that nullmove is a correct form of search if one
doesn't allow the third nullmove in a row; *only* forbid the third
nullmove if there were 2 nullmoves before me.

Then you get into the same position and are forced to search with the
same color, so you detect zugzwangs then.

However to overcome the reduction factor, i reduce my reduction factor
after the first nullmove. This is kind of tricky (as you
might have bad luck with hashtables giving you a cutoff for R=3,
where you already reduced one time with R=3, so then the second nullmove
is also R=3 although you want it to be R=2 or R=1), but it works cool.

I'm sure that my nullmove eats up more nodes than always doing a nullmove.

Note that i nullmove with the window [-beta,-alfa], i have found no
evidence that this eats more nodes, but be warned. My move ordering
is quite well, so alfa is usually beta-1.

Using MTD i have big problems doing this, as 50% chance that a cutoff
is no longer a cutoff, because when i search with mtdvalue=1, then
usually all my cutoffs are 2, and if i then research with
mtdvalue =10, then suddenly the 2 cutoffs are not seen as a cutoff,
so i need a research then which eats up more nodes.

>- Don

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