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Subject: Re: negascout and PVS?

Author: Fabien Letouzey

Date: 02:02:52 07/29/04

Go up one level in this thread

On July 28, 2004 at 19:54:25, Russell Reagan wrote:

>Okay, I have thought about it some more and looked at more code. I think I
>understand now. The difference is when each version starts to search with a
>minimal window, right?


>Normal PVS:  Always searches with minimal window after the first move
>Bruce's PVS: Only searches with minimal window after finding a move that
>improves alpha


>So at a node where alpha == -infinity, they behave the same at that node since
>alpha is guarenteed to be improved after searching the first move. Do I
>understand correctly now?

Yes.  Bruce's idea makes sense when combining PVS with aspiration.

>Okay, the next question is which version is better?

That I can't answer.

What's interesting is not the code, but the hidden assumptions.

PVS assumes that the first move is likely to be the best one (move-based
heuristic).  Aspiration assumes that the root score will be in the aspiration
window (value-based heuristic).

Now assume an aspiration window is used.  If the first root move fails low, the
two assumptions conflict.  The programmer has to choose which assumption is most
likely to be true :)  Choosing one of them leads to PVS or Bruce's idea.

It seems more complex though, since Bruce's flag is also used inside of the
search tree, not just at the root.


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