Valkyria correctly behaves as if this is seki. It actually does not
now it is seki explicitely, but it prunes all moves that would break
the seki.
The principle to do this as simple as possible is thus not actually to
identify seki in general but to find simple rules that prunes bad
moves. This means one has to have many rules, but each rule becomes
simple.
Pruning J3 for X is a very neat example which is easy to do with the
rich boardrepresentation of Valkyria.
First the program at som point identifies J3 as a false eye. Some
false eyes must be played and some most not be played. In this case
XJ2 and XH3 both have two liberties, then V. looks at the corners of
J3 and finds OH2 which also has two liberties. Now the liberties of
XJ2 and XH3 that are not the false eye are matched to the liberties of
OH2. Also these liberties are at the moment suicidal for both players.
This is enough to safely conclude that filling in the false eye cannot
be a good move becuase it just connect two liberties that cannot be
played anyway.
But! There are always exceptions. In this case one exceptions is when
the false eye is on the 1-1 point and only two stones are connect and
that the resulting shape is a precursor to bent four in the corner. In
that case filling in the false eye is essential.
I cannot exclude that are no more exceptions to this rule, but so far
it works nicely.
Oh, by the way I find on the sight of it hareder to prune O J1. I know
Valkyria prunes this but I do not remember exactly how it is done, but
I guess the code that handle sacrfices has to check for false eyes
becoming real eyes because of the sacrfice or something like that.
-Magnus
Quoting Brian Sheppard <sheppar...@aol.com>:
There is a seki in the lower left of the position below:
1 2 3 4 5 6 7 8 9
A - O X X - X - - -
B X O X X X X X - X
C - O O X X - X X -
D O O O O X X X X X
E X X O - O X - X O
F - X X O O O X X O
G O X X X O X X O -
H O O X X O X O - O
J - X - X O O O O -
It is obvious that X cannot play F1. O cannot play F1 because that would
sacrifice a "straight four".
Pebbles has followed Magnus's advice and added a rule that prevents the
"two-for-one" trade that would occur when X plays J1, so that is also not a
problem. And for O, playing J1 is ruled out because X has another eye on J3.
What isn't obvious is that X cannot play J3. If X plays J3 then O follows
with J1 atari, and X loses because of the nakade shape of O's stones.
The bottom line is that Pebbles rates this position as hugely favorable for
O, because X stumbles into J3 in the playouts.
How does your program handle this situation?
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--
Magnus Persson
Berlin, Germany
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