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Subject: Re: Null-Move: Difference between R = 2 and R = 3 in action

Author: Vincent Diepeveen

Date: 02:28:29 07/14/02

Go up one level in this thread

On July 14, 2002 at 02:44:17, Christophe Theron wrote:

In my draughtsprogram i tried a zugzwang detection with
a normal search depth-2. The overhead from this is
*huge* though.

*no one* is using this overhead in his chessprogram,
for obvious reasons.

double nullmove is allowing you to have both a zugzwang
detection as well as use nullmove in an efficient combination.

It's a very elegant solution.

most important however, is that double nullmove was invented
to proof that searching with nullmove is a correct way to search.

Nowadays we don't have this discussion a lot anymore, though
Schaeffer at world champs was still defending deep blue seeing more
than other chessprograms, even though they searched 11 to 12 ply
at nominal search depth, but i remember huge discussions here
from dudes mentionning that fullwidth 12 ply was seeing more
*always* than 12 ply with nullmove.

Well, i can imagine someone coming from the checkers area, to
defend the principle that 12 ply fullwidth is better. In draughts/checkers
not detecting zugzwang is suicide, because the whole game goes about
it. This whereas the last time he had a chessprogram was in the 80s
where a 6 ply search always beated a 4 ply search and a 4 ply search always
beated a 2 ply search. And the 8 ply search from deep thought/chiptest
always beated the 6 ply searches from others.

double nullmove therefore was in first place something to show that
using double nullmove IS a correct way to search in chess. Only *sometimes*
you need a lot of plies more. To detect a zugzwang you need 2R-2 ply
more than fullwidth.

For more than 1 zugzwang and searching SMP i need to mention you usually
are lucky that 1 or more processors have seen nonsense lines and by
means of transposition solve it then. If not, then you simply need
a bunch of plies more. for each additional zugzwang.

That's a price to pay, but a small price.

>On July 13, 2002 at 21:26:30, Omid David wrote:
>>On July 13, 2002 at 11:52:36, Vincent Diepeveen wrote:
>>>On July 13, 2002 at 11:09:19, Omid David wrote:
>>>>On July 13, 2002 at 10:33:19, Vincent Diepeveen wrote:
>>>>>On July 13, 2002 at 04:47:16, Omid David wrote:
>>>>>>On July 13, 2002 at 02:39:38, Dann Corbit wrote:
>>>>>>>On July 13, 2002 at 02:22:00, Omid David wrote:
>>>>>>>>On July 13, 2002 at 02:07:17, Vincent Diepeveen wrote:
>>>>>>>>>I still do not understand which positions you talk about which R=2
>>>>>>>>>is finding and R=3 isn't.
>>>>>>>>I read your other post, that's also my point: Although at fixed depth, R=2 is
>>>>>>>>much better than R=3 (see also "adaptive null-move pruning" Heinz 1999), in
>>>>>>>>practice R=3 performs about the same as R=2 since on many occasions it finds the
>>>>>>>>correct move one ply later with lower search cost.
>>>>>>>By the way, if you have not found Vincent's post on double null move you should
>>>>>>>look it up.  It is a clear win for sure.
>>>>>>Yes it's a nice idea. But the main null-move pruning deficiency is its tactical
>>>>>>weakness due to horizon effect. Zugzwangs are not a major problem, and as
>>>>>>Vincent points out, he invented the double null-move idea just to show that
>>>>>>null-move pruning is OK. Now nobody doubts effectiveness of null-move pruning at
>>>>>>all, the only discussion nowadays is the depth reduction value.
>>>>>I'm missing any position where you have a problem though. Seems to me
>>>>>your thing is incredible weak, and or doing other dubious things which
>>>>>gets looked up in hashtable, after which it weakens your program.
>>>>>In DIEP i don't have all these problems.
>>>>>  - no dubious forward pruning
>>>>>  - no futility
>>>>>  - no razoring or any of these techniques.
>>>>>  - checks in qsearch
>>>>>Just PVS with nullmove R=3 and a bunch of extensions. That's it.
>>>>>Means that after a nullmove i don't get transpositions to positions
>>>>>where you have stored a score which is based upon a dubious score.
>>>>>Best regards,
>>>>Why do you think there is a problem?! All the results I got are natural. I'm
>>>>sure even in DIEP, R=2 works better under "fixed theoretical" conditions.
>>>No it works worse, because i search at least a ply less deeply. If i search
>>>a ply deeper that doesn't only mean i get a ply more. Because the depth
>>>is already pretty decent it also means all extensions might get triggered
>>>a ply extra (like singular extensions).
>>>>However in practice you don't search to fixed depth and thus R=3 might be better
>>>>in practice.
>>>>My only point is that "R=3 might be better than most people consider it." (Take
>>>>DIEP as a successful use of R=3)
>>>>Have you published anything regarding double null-move?
>>>I simply posted in CCC and RGCC. the thing is real easy.
>>>allow 2 nullmoves in a row always, but not 3.
>>>Exception is if both sides only have pawns (of course you
>>>can solve a few testset positions sooner by saying that
>>>if either side has only pawns you don't allow nullmove
>>>FOR BOTH SIDES, but that's in reality not so smart to do).
>>>Apart from that the normal conditions that i don't nullmove
>>>when in check.
>>>This in fact results in nullmove not missing zugzwangs anymore.
>>>Of course for more than 1 zugzwang the extra depth needed is
>>>pretty big.
>>>Best regards,
>>I'd rather see such articles as "double null-move pruning" in ICCA than the
>>usual pure-theoretical-non-practical articles!
>Double null-move sounds elegant, but actually it's a zugzwang detection search
>at depth D-2R-2 (D=remaining depth, and R is the classic null move reduction
>It is less flexible than a zugzwang detection search because:
>1) the depth is fixed to D-2R-2, so you can't adjust it to a better value (and
>D-2R-2 is not an optimal value).
>2) it's less easy to declare the conditions determining when you want to do (or
>avoid to do) the zugzwang detection (but this can be discussed).
>    Christophe

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