Re: [Bug-gnubg] Bug in analysis - position enclosed - more details

[Ned Cross]

Looking at it one way, it makes sense.  The "forced no-play" when opp is
closed out  (or primed and fully crunched) is a "ply" in terms of the
program definition.

Looking at it the other way, one might expect the program to recognize a
"forced no-play" situation and advance it's look-ahead the extra "ply" so
that it will "see" the right answer.  I've experienced this situation in
other positions, where a cube decision or move decision is "reversed" by the
extra ply.  It's notable because there are many positions where 1-ply does
not seem to get it right, yet 2-ply does.

Ned


----- Original Message -----
From: "Joern Thyssen" <address@hidden>
To: "Ned Cross" <address@hidden>
Cc: "Joseph Heled" <address@hidden>; "gnubg" <address@hidden>
Sent: Wednesday, October 30, 2002 11:32 AM
Subject: Re: [Bug-gnubg] Bug in analysis - position enclosed - more details


> On Mon, Oct 28, 2002 at 11:26:07PM -0800, Ned Cross wrote
> > > > I'm wondering if there are two separate issues here:
> > > >
> > > > 1) That GNU doesn't understand this position.
> > > >
> > > > 2) That the reported 2-ply win %  for the candidate move ("after the
roll")
> > > > does NOT equal the reported 2-ply win %  for the position resulting
from the
> > > > candidate move ("before the next roll"). Right or wrong, shouldn't
those two
> > > > numbers be the same?
> > > >
> > >
> > > Should be. Perhaps one is cubelss and the other cubefull? Are you sure
the
> > > parameters are the same?
> >
> > (NC) The output in my original post was generated from a GNU 2-ply
> > analysis (cubeful checker play) with 2-ply cube. The analysis
> > parameters were the same.  After digging deeper, I have found the
> > cause of the discrepancy.  The value for 2-ply analysis AFTER the move
> > is the same as the value for 1-ply analysis BEFORE the next move.
> > (Similarly, a 3-ply move analysis of the 6/1* 5/1 move predicts the
> > same 82.2% wins as the 2-ply cube analysis before the next roll).
> > This does not seem correct to me.  Shouldn't 2-ply = 2-ply if there is
> > no move in between?
>
> There is actually a move in between: O fans!!! If check the cube
> decision for O as he fans you see that he has exactly 14.2% chance of
> winning.
>
> The 2-ply chequer play decision will work as follows:
>
> X moves 6/1 5/1. Evaluation the resulting position at 2-ply:
>
> equity = 0
> loop over all rolls for O
>    find best move for O
>    equity += evaluate resulting position at 1-ply   [a]
>
> final equity = equity/36
>
> However, since there are no legal moves for O all positions evaluated
> at [a] are the same, thus "equity" is the same, hence the resulting "final
> equity" is identical to the 1-ply equity (which is what you've
> observed).
>
> I hope this makes sence!
>
> Jørn
>
> --
> Joern Thyssen, PhD
> Vendsysselgade 3, 3., DK-9000 Aalborg, Denmark
> +45 9813 2791 (private) / +45 2818 0183 (mobile) / +45 9633 7036 (work)
> Note: new mobile number!
>