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Subject: Re: Couple of chess programming questions

Author: Vincent Diepeveen

Date: 13:58:01 09/10/02

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On September 10, 2002 at 15:54:09, Dann Corbit wrote:

>On September 10, 2002 at 15:48:40, Vincent Diepeveen wrote:
>
>>On September 10, 2002 at 13:11:02, Dann Corbit wrote:
>>
>>>On September 10, 2002 at 12:07:37, Vincent Diepeveen wrote:
>>>[SNIP]
>>>>>(3) Reading Aske Plaat's search & re-search paper, it really seems like mtd(f)
>>>>>is something of a magic bullet.  But I note it seems that more programs don't
>>>>>use it than do (for example Crafty).  What is wrong with mtd(f) which Plaat
>>>>>doesn't say?
>>>>
>>>>There is nothing wrong with it. However a softwareprogram must have certain
>>>>conditions to let it work well (in hardware other issues play a role)
>>>>  a) it must be a static evaluation with a small granularity;
>>>>     if pawn=32 it will work great, if pawn=1000 like in DIEP it won't
>>>>     work at all, other things aren't interesting then.
>>>
>>>I have found this to be true also with experiments, and the reason is obvious.
>>>
>>>The MTD(f) algorithm basically does a binary search between two bounds (on
>>>average -- it might be faster or slower in individual cases).  If the interval
>>>varies from 3276700 to -327600 you have between 22 and 23 searches to resolve
>>>it.  If the interval varies from  3276 to -3276 you have between 12 and 13
>>>searches to resolve it.
>>
>>perhaps it is good to clarify one thing. As soon as a MTD search
>>needs to use a binary form of search to get from value A to B, then
>>mtd will fail of course.
>>
>>the advantage of mtd is that if you have a bound search at 0.01 which says:
>>"it's more than 0.01" then you try 0.02. In this way you need only a
>>few minimal windows to get from 0.01 to 0.03
>>
>>The statement of Rudolf is very clearly: "such minimal windows are
>>very cheap", and looking to SOS i can only agree with him.
>>
>>A major problem is like bob says if you have a 0.3 difference.
>>even with pawn = 100 and 0.3 being 30, that means you need 30 researches.
>>
>>If you jump binary, then the only advantage which MTD offers is gone directly.
>>
>>So going from 1000 to 1300 in DIEP that's 300 researches.
>
>I think even worst case it won't do more than log2(interval) searches.  I
>suppose it is possible for each search to refine the interval by only 1 unit,
>but it seems incredibly improbable.
>
>I would expect a window of 300 to take about 8 searches to resolve, on average.
>Perhaps you tested with some pathological positions?  I have not seen behavior
>that bad, but maybe I have not looked at the worst cases.

The problem is you don't know in advance that it's going to get up 0.300
pawns.

And doing a search at 0.001 first then 0.002 that's way cheaper
than doing 0.001 first then 0.250. The problem of doing 2 searches
of 0.001 and then 0.250 or something, is that this second search
is just as expensive as a research of PVS is doing, whereas a search
of 0.002 after 0.001 is very cheap.

I made myself clear?

Best regards,
Vincent



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