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Subject: Re: Couple of chess programming questions

Author: martin fierz

Date: 11:30:56 09/10/02

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On September 10, 2002 at 09:26:14, Eli Liang wrote:

>A couple of chess programming questions:
hmm, i only wrote a checkers program, but here's my take:

>(1) Are there any uses for ProbCut and/or Multi-ProbCut in chess positions where
>the variance of leaf-nodes is low?

i've tried multi-probcut and it works well in checkers. i never tuned it as much
as my own pruning algorithm, and it doesn't perform quite as well - but it is BY
FAR better than no pruning. i'll be trying to tune it in the near future. for
games where the eval doesnt swing wildly, MPC is a fantastic algorithm.

>(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?

i'm using MTD. i tried windowed search, PVS and MTD. in my tests, in long engine
matches, MTD performed marginally (no statistical significance...) better than
PVS. it typically searched a low 1-digit % less nodes for a given depth than
PVS.
i don't know how to get a PV out of MTD. in normal searches, a pv node is where
the value is > alpha but < beta. in MTD, you never get this condition.
retrieving a PV from the hashtable is possible, but in all probability, you will
not get the full PV. which is real bad for debugging if you want to know what
the program was thinking at the time... i once asked here how to get a pv from
MTD but got no answer - and if you can't get the pv, then that is a major
drawback.

>(6) Has anyone found any real "practical" benefits to fractional-ply extensions?

yes. i tried recapture extensions of different depth, and half a ply gave the
best result. don't ask me why, it's just an observation.

aloha
  martin



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