Author: Robert Hyatt
Date: 16:05:30 12/24/98
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On December 24, 1998 at 11:39:23, KarinsDad wrote: >A sudden thought occured. > >How do you handle zugzwang in a null move search? > >I could see how it would be a problem: > >Player A = computer >Player B = opponent > >A is examining a move Ply x. >B can counter with Ply x+1. >B is given a null move; effectively, but not really Ply x+2. >Every response by A at Ply x+3 results in the Ply x+2 move having a lower score >than Ply x+1, hence we ran into zugzwang. > >However, how does one handle a more complex case such that zugzwang is not >detected until Ply x+4 or Ply x+6? Zugzwang is often associated with only a few >number of moves on the board (although that is not always the case). If the >zugzwang would not be detected until Ply x+4, it seems that this would defeat >the entire purpose of using a null move search (i.e. pruning the tree of moves >that would be bad anyway). > >And how does this work when you do not run into zugzwang? Do you check for >zugzwang by ignoring the results at Ply x+2 where that move results in either a >piece capture or a piece promotion (and hence the score will jump materially >towards the move being good as opposed to bad) and only look at the other moves? > >This is somewhat unclear and confusing. Can anyone explain it in a clear manner? > >Thanks, > >KarinsDad You don't, because that is exactly what breaks the null-move search in the first place. Most turn nulls off when material is reduced to the point where zug can happen. Nobody uses it in pawn-only endings that I know of since zug is the most common theme there...
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