Author: Andrew Williams
Date: 05:26:35 03/04/99
Go up one level in this thread
On March 03, 1999 at 14:33:04, James Robertson wrote:
>
>My question is: my qsearch is just alpha-beta, with moves that lose material by
>SEE being pruned without searching. Are there any refinements to this method? I
>read somewhere that EXchess does something with somevariable called delta, but
>that is all I know. I am hoping that the more I cut down my qsearch, the more
>advantage the non-bug version will have over the bugful version.
>
>Thanks for any help.
Hi James,
This is what my program does before generating captures in the qsearch:
ev = evaluate();
if(ev >= beta) { /* Do we need to bother? */
return ev;
}
In other words, if it's our turn and we're already doing well (above beta)
we don't need to bother with this node. I *think* this sort of thing is
common to most programs (maybe with checking for check).
If we get past that test, before trying my SEE on a move, I try this:
/* If this capture isn't going to get us back close to or above alpha,
don't bother with it.
*/
if((indexValues[bd[mv.tosq].index] + ev + 100) < alpha ) {
mvNum++;
continue;
}
This says, "if I add the value of the piece I'm about to take to the
evaluation I did before and I'm still more than a Pawn worse than alpha,
I might as well not bother with this capture." So if you're down a Queen,
it's not worth thinking about taking a Bishop etc.
You can vary the 100 as you see fit. I think this is what is meant by "delta",
but I'm not sure. In the past, I have varied the 100 depending on whether
the piece I'm about to grab is a Passed Pawn or not.
If the move passes this test, I then try SEEing it. If the SEE suggests it's
not going to lose material, I do the recursive call to quiesce.
There are many other considerations - I generate captures + promotions to Q,
even if the promotion isn't a capture. And I don't ever prune promotions -
some would call this superstition.
Regards
Andrew
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