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Subject: Re: calculating the PV using MTD

Author: Fabien Letouzey

Date: 23:16:45 09/11/02

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On September 11, 2002 at 20:22:42, martin fierz wrote:

>my whole interest in getting a pv out of the search rather than from the
>hashtable was that i wanted a guarantee that i got to the end of my PV, for
>debugging purposes, like if i want to find out why my program makes a certain
>bad move with a high score - what the hell was it thinking there?
>retrieving from the HT is ok most of the time, but obviously for this purpose,
>it is not so good because sometimes it will not give me the full PV because
>something was overwritten in the HT. in my implementation, i will also miss the
>entire qsearch because i don't store that in the HT. so your way of doing this
>really helps :-)

Now that I read this, semi-PV is perhaps exactly what you want in this case.
The key is that it shows you where the evaluation is from.  However you need
to be careful when interpreting a single semi-PV (not combined), see below.

>and if i understand you correctly, then i can just get your "semiPV" for my last
>MTD test, and that actually IS what my program was thinking when it made the
>move, right?

Now it's tougher.  I'd rather say that it is what your program thinks in case
of a fail-high, and what its opponent thinks in case of a fail-low.  Because
MTD(f) search "direction" depends on the first estimate, I would interpret the
last semi-PV differently in both cases.  I have never used MTD, but if I had
access to only a single semi-PV, I would prefer to see the last fail-high
semi-PV (I might be wrong here, programmers with MTD experience are welcome).

Let's take an example.  If I understand well, you use strict MTD(f) (no
accelerator).  Let's pick an "upward" MTD search (you underestimated the true
value).  All the searches except the last one are fail-highs.  They are proofs
that your "best" move is good (and it selects a new move from time to time).
So in your debug example, if you know this move is bad, a semi-PV could show
you why Cake thinks it is good (I think this is what you want).  Now comes the
last search.  It fails low.  Interpretation is completely different here.  It
first proves the value of your first move, then shows the other moves are not
better.  Think about the semi-PV content in this case.  It starts with your
best move (making the semi-PV *look* like a PV), then contains good opponent
moves showing why you cannot improve its value.  I believe this is not what you
want (only you can tell me).

I can only repeat.  Semi-PVs might or might not be what you want.  Just make
sure you interpret them carefully, they are different from PVs.  We can talk
further about the non optimal moves they contain in the general case (it is
move-ordering and fail-soft dependant).

Fabien.




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