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Subject: Re: MTD: an observation and a question

Author: Robert Hyatt

Date: 12:47:51 09/13/02

Go up one level in this thread

On September 13, 2002 at 15:10:47, Dieter Buerssner wrote:

>On September 13, 2002 at 01:28:52, Robert Hyatt wrote:
>>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).
>>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).
>?? Starting alpha very low should make your search rather slow. I believe, one
>really needs a new variable "bestscore" besides alpha.

Right.  Misspoke.


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