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

Author: Dann Corbit

Date: 12:36:15 04/14/04

Go up one level in this thread


On April 14, 2004 at 15:24:53, Andrew Williams wrote:

>On April 14, 2004 at 14:13:04, Dann Corbit wrote:
>
>>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.
>>
>When I first started with MTD (aeons ago now), i tried MTD(bi) and it just
>didn't work. It looks like it should, but doesn't. I then switched to using
>MTD(bi) after something similar to what Bo posted above. That didn't work
>either. YMMV of course.

Have you tried it with an aspiration window around the last search score?

I think the big problem with MTD(bi) is that you will do a terrible job if you
always search -32767 to +32767.  You are guaranteed to do 16 searches every time
and so it is going to be a loser if your standard MTD(f) averages less than
that.  But suppose that you use an 8 bit window of 256 centipawns?  Then you
will need only 8 searches (unless the window fails).



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