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Subject: Re: Couple of chess programming questions: another MTD drawback

Author: Robert Hyatt

Date: 09:31:44 09/11/02

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On September 10, 2002 at 20:45:43, Vincent Diepeveen wrote:

>On September 10, 2002 at 18:06:01, Gian-Carlo Pascutto wrote:
>
>>On September 10, 2002 at 17:51:11, Vincent Diepeveen wrote:
>>
>>>On September 10, 2002 at 17:43:15, martin fierz wrote:
>>>
>>>>On September 10, 2002 at 17:18:24, Vincent Diepeveen wrote:
>>>>
>>>>>On September 10, 2002 at 17:10:38, martin fierz wrote:
>>>>>
>>>>>>On September 10, 2002 at 09:26:14, Eli Liang wrote:
>>>>>>
>>>>>>>(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?
>>>>>
>>>>>losing 1 bit is a problem for you?
>>>>
>>>>nope. losing 2 bytes is more like it...
>>>
>>>who stores a bound in 2 bytes?
>>>
>>>Why not in 1 bit?
>>
>>You want to store two actual values, not flags that indicate what
>>kind of bound it is.
>
>did i implement it smarter then or what?
>i used 2 bits in total. 'upperbound, lowerbound, truebound'.
>the search result is based upon a single bound. So it IS the same,
>it IS higher or it IS lower.
>
>What am i missing here?
>


A _lot_.

This is a known issue with mtd(f) and fail-soft.  If you bounce over the
true score, to the "other side" then suddenly where you were storing
upper bounds you are now storing lower bounds, and vice-versa.  Now if
you bounce back over the true score again, you have no useful upper bounds
where you need them, you only have lower bounds.  And you have no useful lower
bounds where you need them you have only upper bounds.

if you store _both_ then you don't take as big a performance hit when you
"bounce over" the true score.

This is what I mentioned when I said "doing mtd(f) is _not_ just a few lines
of code changes, it requires fundamental changes in places like hashing."

>>--
>>GCP



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