Computer Chess Club Archives




Subject: Re: Question for the MTD(f) experts

Author: Dann Corbit

Date: 11:13:04 04/14/04

Go up one level in this thread

On April 14, 2004 at 14:09:04, Bo Persson wrote:

>On April 14, 2004 at 13:47:06, Dann Corbit wrote:
>>On April 14, 2004 at 13:33:38, Robert Hyatt wrote:
>>>On April 14, 2004 at 03:30:22, Dann Corbit wrote:
>>>>I decided to toss an MTD(f) search into TSCP, and I've got something wrong, but
>>>>I can't quite see what it is.
>>>There is a lot more to do.
>>>1.  you need to modify the hash table to store 2 bounds, not 1.
>>That was not done yet.
>But not too hard!
>>>2.  the search must be fail-soft.  TSCP isn't.
>>I had already done that.
>>>3.  PV has to be yanked from the hash table and that makes it flakey at times as
>>>has been discussed many times.  There is another way to get the PV, but it is a
>>>special case solution only for mtd...
>>Tord showed a very nice way to do that with a clever "hash only" update.
>>>4.  the convergence has to be accelerated.  IE on a fail high searching v and
>>>v+1 won't cut it.
>I can supply that part, learned from CCC a couple of years ago of course.
>            // Calculate next gamma and Step
>            if (gamma < Beta)
>            {
>               GlobalUpperBound = gamma;
>               gamma = max(gamma - Step, GlobalLowerBound + 1);
>               SteppedDown = true;
>            }
>            else
>            {
>               GlobalLowerBound = gamma;
>               gamma = min(gamma + Step, GlobalUpperBound - 1);
>               SteppedUp = true;
>            }
>            if (SteppedUp & SteppedDown)
>               Step /= 2;
>            else
>               if (Step < (GlobalUpperBound - GlobalLowerBound) / 2)
>                  Step *= 2;
>Here gamma is your f. The idea is to accellerate the stepping, until you have
>over stepped the score twice, once in each direction. Then you have an
>acceptable bound, and can start to zoom in, using smaller and smaller steps as
>you get closer and closer.
>The GlobalUpperBound and GlobalLowerBound should really be global. If you use
>lazy evaluations those are the bounds to use there, not the local alpha/beta
>which are always just 1 point apart.

I have lots of ideas along these lines.

I thought of using an MTD(f) like driver, but in MTD(bi) format, so that I will
binary search.

I thought of doing a parabolic fit of the last few guesses and extrapolating the
next guess.

I thought of doing an aspiration window around the last bound and MTD(bi) search
of that.

Some others besides.  But I want to understand the algorithm through and through
before I venture into that land.

I got reinterested in MTD(f) because Uri mentioned that he would like to be able
to experiment with it in his search.

In case he wants me to give him a hand, I had better understand it well myself.

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