Computer Chess Club Archives




Subject: Re: MTD: an observation and a question

Author: Dave Gomboc

Date: 00:28:21 09/13/02

Go up one level in this thread

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).
>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...

Maybe I've misunderstood... I thought Dieter was saying that even in your
current PVS implementation, you'd do better to have that fail-soft code in


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