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Re: [Bug-gnubg] Is eval-4ply and 0-ply-rollout-truncated-4ply the same?

From: Massimiliano Maini
Subject: Re: [Bug-gnubg] Is eval-4ply and 0-ply-rollout-truncated-4ply the same?
Date: Wed, 28 Nov 2007 13:46:58 +0100

address@hidden wrote on 28/11/2007 11:07:57:

> On gammonu Michael Depreli asked,
> Is truncate at 4 the same as a 4-ply eval assuming you roll out at 0-ply ?
> I thought that the answer was an obvious yes, and Raccoon did some
> tests showing that the results where almost the same, but not quite.
> Jordan Lampe seems to think differently:
> >No.
> >
> >First of all, it makes a difference whether we are looking at a
> checker play or a cube decision,
> >but let's suppose it's a cube decision just to make things easier.
> Let's further make things
> >easier by assuming there are only two die rolls (1 and 2) and only
> two possible moves (A and B) at each step.
> >
> >A 2-ply evaluation looks at all of the following 16 positions: 1A1A,
> 1A1B, 1A2A,
> > 1A2B, 1B1A, 1B1B, 1B2A, 1B2B 2A1A, 2A1B, 2A2A, 2A2B, 2B1A, 2B1B, 2B2A, 2B2B
> >
> >A 0-ply, trucated at 2-rollout, on the roll sequence 1 followed by
> 2 does this:
> >first it looks at 1A and 1B and decides which is better. Assume 1A
> looks better,
> >then it looks at 1A2A and 1A2B and decides which is better.
> >
> >So, in particular, the truncated rollout NEVER looks at 1B1A, 1B1B,
> 1B2A or 1B2B, no matter what dice it rolls.
> This seems horrible to me. Anybody know what is actually going on or
> do I have to go look at the code? :)

Isn't this simply the fact that when doing a rollout gnubg plays optimally
against itself ? For a given roll, all the moves are evaluated (at the
specified ply), but only the best one is "played" (i.e. kept in the rollout process).
This is because the equity after a given roll is the equity of the position you
reach playing the move optimally. Hence whatever happens in other branches does
not affect that equity.

Still, a 0-ply rollout truncated to 4 is not the same thing of a 4-ply eval.
But if our neural net is good, the results shouldn't be far apart in most
cases, right ? I would expect the 4-ply eval to be more accurate.

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