Author: William Bryant
Date: 12:14:26 05/15/99
Go up one level in this thread
Thanks to both Bob and Edward.
There was a very nice but difficult to notice bug. It attribute it to a
compiler error, the compiler did what the source said to do, not what I "though"
it said to do. :) I forgot to negate the score returned from the null search.
ie
x = search(-beta, -beta+1, depth-NullRDepth, false);
instead of
x = -search(-beta, -beta+1, depth-NullRDepth, false);
You miss these things at 1 am.
Interestingly, the search was much faster (but wrong) on some positions doing
it the other way. I now only get to 9 ply in 2 minutes (actually 1:36) from the
1.e4 starting position instead of 10 ply, more work to be done.
I still have a question on the Null move.
If at the root, you fail high and set alpha to -infinity so that your new search
window is -Infinity, beta, then with the first call to search,
alpha becomes -beta, beta becomes -alpha which becomes Infinity.
I assume under these circumstances, it is better to skip the null move at this
ply since there is not point is searching -beta, -beta+1 (-Infinity,
-Infinity+1). Is this correct?
Finally,
on your last comment
>The most common bug with null move is a fail high, and on the research where
>beta has been relaxed, you get a fail-low. _that_ can be a serious problem if
>you don't ignore the false fail-high at the root. You will occasionally play a
>totally garbage move.
I had this problem when I implemented my aspiration window search. I would fail
high, then fail low and get stuck. What I have done is this, if I fail high, I
change the search to
alpha = beta -1;
beta = INFINITYSCORE;
and flag that I have failed high.
If fail low, I change the search to
alpha = -INFINITYSCORE;
leaving beta unchanged unless I have already failed already on this search,
then I change beta; beta = INFINITYSCORE;
Is there a better way to handle this problem.
Thanks again,
William
wbryant@ix.netcom.com
BTW, here is my Null move code to see what I have done. Please feel free to
comment.
//No Hash table cutoff
//Now lets try a Null move
#ifndef _No_NULL_
if (beta < INFINITYSCORE) {//Avoid doing a Null move just after fail low
if ((!check) && doNull && (depth - NullRDepth >= 0) &&
(GetPieceMaterial(side) >= NULL_Threashold)) {
gNullMoveAttemps++;
makeNULLmove();
if (depth - NullRDepth) //if depth remaining
score = -search(-beta, -beta+1, depth-NullRDepth, false);
else if (depth - NullRDepth == 0)
score = -quiesce(-beta, -beta+1);
takebackNULL();
if (OutOfTime)
return(0L);
if (score >= beta) { // Null move cutoff
gNullCutoffs++; // counter for successful null moves
UPDATEHASHRECORD(mHash, theHashRecord, CurHashSig, age, 0L,
score, depth, hash_Lower);
return score; // alternatively, return beta
}
else if (score < -MateScore)
if (ply <= kAbsoluteMaxSearchDepth)
depth++; // may wish to handle null move scores that
//suggest a mate by
// increasing depth and researching
}
}
#endif
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