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Re: [Bug-gnubg] Errors in evaluation of races with backgammons
From: |
Joseph Heled |
Subject: |
Re: [Bug-gnubg] Errors in evaluation of races with backgammons |
Date: |
Mon, 27 Feb 2012 13:09:08 +1300 |
Hi,
This position just falls under the special code I wrote ages ago.
There might be errors (bugs) there, but please remember that the
"exact" is *not* a '8 vs 2 1 0 0 3 cubeless bearoff', since clearing
checkers from the back is not the same as bearoff - you *can* move any
checker you want. I think that was the point behind the special code
....
-Joseph
On 27 February 2012 12:23, Philippe Michel <address@hidden> wrote:
> It looks like there are some bugs in the evaluation of backgammon chances in
> races, in raceBGprob() in eval.c . For instance, this position :
>
> +24-23-22-21-20-19------18-17-16-15-14-13-+ O: gnubg (Cube: 2)
> | O O O O O | | O | 0 points
> | O | | |
> | O | | |
> | | | |
> | | | |
> | |BAR| |v
> X | 8 | | |
> X | X | | |
> X | X O | | |
> XX | X O O | | | On roll
> XX | X O O O | | O | 0 points
> +-1--2--3--4--5--6-------7--8--9-10-11-12-+ X: pm
>
> evaluates as :
>
> Position ID: pU4AWhz/AAAAAA Match ID: QQkAAAAAAAAA
>
> Evaluator: Race
>
> Win W(g) W(bg) L(g) L(bg) Equity Cubeful static:
> 1.000 1.000 0.678 0.000 0.000 +2.678 +2.678
>
> The backgammon ratio is wrong by about 0.1.
>
> It is supposed to be computed exactly (not by a net) as the winning chance
> of a 8 vs 2 1 0 0 3 cubeless bearoff, either by not-particularly-obvious
> internal functions or with the bearoff databases.
>
> In the example above, it is the internal functions that are used and there
> may be a bug in them or, perhaps more likely, they are not used properly.
>
> If I twist the code in eval.c to only use the databases, I get :
>
> Win W(g) W(bg) L(g) L(bg) Equity Cubeful static:
> 1.000 1.000 0.784 0.000 0.000 +2.784 +2.784
> 1 ply: 1.000 1.000 0.784 0.000 0.000 +2.784 +2.784
> 2 ply: 1.000 1.000 0.744 0.000 0.000 +2.744 +2.744
> 3 ply: 1.000 1.000 0.744 0.000 0.000 +2.744 +2.744
> 4 ply: 1.000 1.000 0.709 0.000 0.000 +2.709 +2.709
>
> Now the bg rate is accurate for 0 and 1 ply, but not for deeper searches.
> The diminished backgammons surely don't come from misplays by X so there
> must be another problem here, either in shorter races or in the collating
> process to get the result from higher plies.
>
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