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Subject: Re: Nullmove: when to avoid it?

Author: Miguel A. Ballicora

Date: 22:35:53 02/28/01

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On February 28, 2001 at 16:52:05, Robert Hyatt wrote:

>On February 28, 2001 at 14:49:19, Miguel A. Ballicora wrote:
>>On February 28, 2001 at 13:22:41, Robert Hyatt wrote:
>>>On February 28, 2001 at 11:10:30, Miguel A. Ballicora wrote:
>>>>On February 28, 2001 at 05:56:36, Leen Ammeraal wrote:
>>>>>I am not sure about when to avoid nullmoves.
>>>>>I omit it:
>>>>>a. when in check
>>>>>b. when there are less than 5 pieces (including pawns) on the board
>>>>>c. when the last move was a nullmove
>>>>>d. at the root node
>>>>>Should I also omit it in some other cases,
>>>>>for example, when any hashmove (even with a low draft) was found,
>>>>>or when beta = alpha + 1?
>>>>>Thanks in advance for any help.
>>>>Hi Leen,
>>>>Regarding b, I do not know whether what I am doing now is correct but I think
>>>>that works for me:
>>>>When either black or white had no "long range" pieces (bishop, rook or queen)
>>>>I disable null move. The rationale is that one side cannot waste
>>>>a tempo in a given position having pawns, king and/or knights making the
>>>>position prone to have a zugswang.
>>>That seems dangerous.  you are white, with a bishop on d5.  I am black and I
>>>have a pawn on a7 and g7.  The bishop is zugged here.  If your king can't move,
>>>you lose even though you have a long-range slider on the board.  And null move
>>>will fail high here naturally as not moving is better than having to move and
>>I think that you meant a3 and g3?
>Sorry.  I am white, you are black trying to stop both of my pawns.
>> If that's so I got your point
>>and you're right. However, I disable nullmove when _either_ side lacks a
>>long-range slider. In your example, it will be disabled because you do not
>>have a bishop. If you do have a bishop, it won't be disabled (both sides got a
>>slider) but at least I don't have "mutual" zugswangs which are the nastiest (I
>>think). At least, with a slider per side the mutual zugswangs are more difficult
>>(of course not impossible but I have to draw a line somewhere).
>That only makes it worse.  So I have a bishop and two pawns threatening to
>promote.  You have the bishop as above.  You are _still_ zugged.  I don't
>see why you would limit null move based on _both_ sides.  You should only
>limit it if the side on move can be zugged.  But in any case, it still fails
>if we both have a bishop.

The idea is to limit the nullmove when there is a zugswang that matters.
In other words, there are zugswangs where it does not matter if I move or not.
The example that you give could be one as pointed out by R. Gibert. If you move
you lose because you are zugswang if you don't I win anyway advancing one of the
pawns. Let me give an example trying to keep the spirit of your criticism
so I can illustrate the idea:

[D]2k5/1p6/1K5p/2p4P/2P5/1P2b3/8/4B3 w - - 0 1

White plays Bg3 and Black is zugswang. Black king can move without losing a pawn
and Bishop can move defending c5 and at the same time avoiding Bf4.
This is not a mutual zugswang, because if white is on the move still win wasting
another tempo with Bh2 thanks to the slider.
I think that is relevant for nullmove. For instance, nullmove won't fail high
since "passing" gives no extra advantage to black, just keep the material.. so
score won't be >= beta. Besides, in the nullmove line white plays again Bh2 and
black still loses.
What I think it is terrible is the mutual zugswangs because they always fail
high. So, the idea is that if both sides have at least, say, a bishop, Q or R,
mutual zugswangs are mode difficult (not impossible of course).
That's where I draw the line.

How do you limit your nullmove? you said a rook. One of the sides?



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