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Subject: Re: A Question on simple Alpha-Beta versus PVS/Negascout

Author: Bruce Moreland

Date: 00:19:53 03/22/00

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On March 22, 2000 at 03:04:26, Rudolf Posch wrote:

>One small comment: in the last line of your pseudo code you return alpha.
>Should this line not be "return score", because if the value of the actual
>position doesnt reach alfa you return a too high value?

The code is correct as written, unless the caller does something weird.

If you do "return score" it's wrong because if there are no legal moves you
return an undefined value.

And if there are legal moves, this simply returns the score for the last move

Some of these algorithms attempt to return valid scores that are outside the
window.  I've never figured out how to do this.  One of the systems that tries
to do this doesn't work with a variable depth search.

I think it makes as much sense to do ...

    if (score >= beta)
        return beta;

... as it does to do ...

    if (score >= beta)
        return score;

At the edge of the window is just as good as outside it, and in normal alpha
beta it's all you really "know" if your search non-trivially deep.


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