Author: Roberto Waldteufel
Date: 10:34:04 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 Hi KD, Basically you are taking a gamble that the position is *not* zugzwang whenever you use null move. The speed-up is substantial, but if you do hit a zugzwang then your search is broken completely (unless you are lucky and he PV is not affected). This gamble is fairly safe in the opening and middlegame, but very dangerous in the endgame when zugzwang is common, so normally we disallow null moves when the material falls below some critical threshold. Hope this makes it clearer. Merry Christmas Roberto
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