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Subject: Re: Confusion on Null Move

Author: Robert Hyatt

Date: 05:53:14 02/10/99

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On February 10, 1999 at 01:54:25, KarinsDad wrote:

>On February 09, 1999 at 22:59:52, Robert Hyatt wrote:
>>On February 09, 1999 at 20:09:12, KarinsDad wrote:
>>one of the big 'time savers' is what happens _below_ a sacrifice for nothing.
>>Qxp pxq.  And now no matter what two moves white plays in a row, black's
>>position is still 'good' and everything below such moves gets trimmed away
>Ok. So in a given position (ply 0), there may be 5 captures (maybe even 10) that
>are real losers (ply 1). For each of these, I would quickly find a response (ply
>2, e.g. pxq). To do a null move, I would have to check about 36 reponses (on
>average, not endgame) by white followed by 36 responses by white (i.e. all legal
>moves at ply 3 and all legals moves as if it was still white's move at ply 4)
>for a total of 36 + 1296 or 1332 generated legal moves (37 calls of the legal
>move generator) and 1332 evaluations (but possibly not a full evaluation).
>If any of the "ply 3" or "ply 4" null moves lead to check, do you have to search
>on? And what do you do with the "ply 3" check where you are doing null move, so
>that black cannot move out of check before the "ply 4" null move? Do you just
>ignore it?
>In a situation without the check (not knowing the answer yet), you pruned 10
>lousy moves (although there was no guarantee that qxp, pxq didn't lead to mate
>in 3 or more) and there are 26 left which are also null prune checked, but they
>do not fail, so their children can be searched and pruned normally. Correct?

the search below a 'null-move' is just like any other search.  It is subject
to the normal extensions, additional null moves may be tried (although not on
the _next_ ply as two nulls in a row are not allowed) and so forth.  Null move
is simply an algorithm that recognizes when a part of the tree is hopeless for
one side or the other, and it causes such sub-trees to be searched very quickly.

>>IE with a full-width search, there are _many_ captures that are losers.  Null
>>move reduces the depth below these moves and make dismissing them quick and
>>>KarinsDad :)

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