Re: [gnugo-devel] regress endgame:603

[Evan Berggren Daniel]

On Fri, 11 Apr 2003 address@hidden wrote:

>
> > By my count, G6 is 2.5 miai and C1 is 3 miai, but tedomari concerns mean
> > that G6 is the correct play for black, and C1 is the correct play for w.
> > Is this incorrect somehow?  I'm not terribly confident in my endgame
> > counting, but that's how I read it...
>
> G6 is the best point for either player in this position. However
> there may to be a (sort-of) valid reason for having the engine
> prefer C1, as I will explain.
>
> G6 is 5 points gote. C1 is 3 points reverse sente.
>
> Whether 5 points gote or 3 points reverse sente is preferrable
> depends on the whole board position.
>
> It's sente for W to hane at D1. The difference on the board between
> W:D1 B:E1 W:C1 B:F2  and  B:C1 W:B1 B:D1 is 3 points. The points
> at issue are at B1, E1 and F2.
>
> The difference between B:G6 and W:G6 is 5 points.

I'm agreed here.  My reference for miai counting:
http://senseis.xmp.net/?MiaiCounting

The idea is that since there are two net plays between B:G6 and W:G6, each
of those plays is worth half of the 5 point difference, or 2.5 points.

Because the D1 hane is sente for w, there are 0 net stones if w makes the
hane, and 1 net stone played if b plays the revers sente.  So there is one
stone difference, and the reverse sente is valued at three points.

The problem here is that if b takes C1, then w gets tedomari (the last
play) with G6.  If b G6, then b gets the last play when w plays the D1
sente hane.

For a good explanation:
http://senseis.xmp.net/?tedomari

It looks to me like black should make a sacrifice of half a point in order
to get tedomari in this position.

Adding a 3 point gote to the board would definitely be a good solution.
Perhaps also leaving this problem but marking it as a difficult problem.

>
> If there is another yose point on the board it is conceivable
> that a 3 point reverse sente move would be preferrable to a 5 point
> gote position. Suppose that it's like this:
>
> [3 point reverse sente]  [5 points gote] [3 points gote].
>
> If B takes the 3 point reverse sente, then W takes the 5 gote,
> B takes the 3 gote, then B gets 6 points and W gets 5. If
> B takes the 5 gote then W gets both the other two by taking
> his sente first.
>
> But in this position, there is no third yose point. Each
> player will get one of the two plays and it's better to
> chose the big one.
>
> I think the point of the problem is that it is supposed to
> be policy to prefer a 3 point reverse sente to a 5 point gote.
> The engine does endgame analysis locally without taking into
> account what other local games (in the sense of combinatorial
> game theory) exist on the board. A real combinatorial endgame
> engine would definitely prefer G6 in this position, and the
> problem could be revised by placing another 3 point gote
> position on the board.
>
> The test was submitted by Gunnar in this archive link:
>
> http://mail.gnu.org/archive/html/gnugo-devel/2002-09/msg00390.html
>
> Dan
>
>
>
>
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