Author: Robert Hyatt
Date: 22:28:52 09/12/02
Go up one level in this thread
On September 12, 2002 at 11:54:11, Dieter Buerssner wrote:
>On September 12, 2002 at 09:28:28, Robert Hyatt wrote:
>
>>The gotcha is to use fail-soft. And this probably requires a few changes
>>to how you back up scores. IE you might do something like this:
>>
>> v=Search(-beta, -alpha, etc);
>> if (v > alpha) {
>> if (v > beta) return beta;
>> alpha=v;
>> }
>>
>>That return beta is not fail-soft. it should be changed to
>>
>> if (v > beta) return v;
>
>This is not enough. You need a new variable, for example best (intitialized to
>some very bad score)
>
> v=Search(-beta, -alpha, etc);
> if (v > best) {
> best = v;
> /* And to the PV discussion: here the "semi-PV" must be updated */
> if (v > alpha) {
> if (v > beta) return v;
> alpha=v;
> }
> }
>
>and out of the search loop:
>
> return best;
>
>Your code snippet (Crafty, too) is essentially fail hard. In the parent node,
>after you returned best, you end up returning alpha in most cases (which is
>almost the same, as you would return beta in the fail high node).
>
>Regards,
>Dieter
Right. This was one of _many_ changes I did when I tried mtd(f) a couple of
years back. I left a bit of the "fail-soft" code in place, because it didn't
make any real difference, but I did take the piece you mentioned above out
(starting alpha off very low).
I ended up with a pretty decent implementation, but it was only efficient
in positions where the score didn't change a lot. Which was a pretty big
percentage, generally. However, when I had those big jumps, it reall did
badly, enough so that the "average" performance simply never quite caught
back up to normal PVS...
But I haven't "given up", just "put on the back burner" further testing...
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