Semeai reading

With the help of STS-RV it's easier to see where GNU Go goes
wrong in
semeai reading. The analysis below is based on STS-RV_1.tst,
which
mostly features semeai problems where both colors have one
big eye
each. What varies is the shape and size of the big eyes, the
number of
common liberties, and the number of outer liberties. All of
these are
fairly straighforward to analyze. First fill the outer
liberties, then
fill up the opponent's eye. If you're ahead, continue with
the common
liberties, followed by repeated filling of the eyespace
after each
capture. Of course also capture opponent stones in your own
eye when
your stones are in atari. There's no reason why GNU Go
shouldn't be
able to read this right.

Current CVS is failing 54 out of 208 tests in STS-RV_1. So
what goes
wrong? Here are a number of recurring problems, some with
examples:

1. Tactical reading mistake of a three liberty string causes
an
    overamalgamation which the semeai reading can't recover
from.
    STS-RV_1:23

    9 O O O . . .
    8 X X O . . .
    7 . X O O . .
    6 X X X O . .
    5 . . X O . .
    4 O O O X X .
    3 . O . X . X
    2 O O . X X X
    1 X X X X . X
      A B C D E F

    A8 is considered tactically dead.

2. The same thing but for a two liberty string. STS-RV_1:58

       A B C D E F G H
    19 O . X X O X . X
    18 O . X O O X X X
    17 O O O O O X . X
    16 X X X O X X X X
    15 X O X X O O O O
    14 . O O X O . O .
    13 X . X X O O O X
    12 X X X O . O . X
    11 O O O O O O . X

    A19 is considered tactically dead.

3. Common liberties are played before eyefilling.
STS-RV_1:56

       D E F G H J K L M N O P Q
    16 X . . O O O O . . . . O X 16
    15 O O O O O O O X X X X O O 15
    14 O O O X X X X O O O X X X 14
    13 O O X X . X . O . O O X . 13
    12 O O X . O X X O X X O X X 12
    11 . O X X . X X O . O O X X 11
    10 O O O X X X O O O O X X X 10
     9 . O O O O X O O X X X X . 9
     8 O X X . O O X X X X . X X 8
       D E F G H J K L M N O P Q

    White plays K13 instead of G12. (Second move choice, see
also item
    6.)

4. Opponent stones in maximally filled eyespace are captured
while
    there is still an open common liberty, so own stones are
not in
    atari.

5. In low liberty situations, stones inside big eyes look
like lunches
    and not eyespace, causing the eye evaluation to go
wrong. As a
    consequence the other player may look ahead on eyes (one
eye beats
    no eye) and incorrectly be declared winner.

6. As above, but instead of a premature termination of
reading a move
    to capture lunch inside own eyespace is generated, which
tends to
    be fatal. STS-RV_1:56

    Same position as in 3. First move choice for white is to
capture
    lunch at M11.

7. Outer liberty detection mistakenly finds a move inside
the
    opponent's big eye. STS-RV_1:68

       A B C D E F G H J K L
    19 O O O . X X O . X O O
    18 . X O O X . O . X O O
    17 . X . O X X . X X O O
    16 O . O O X X X X O O .
    15 O O O O O X O O O . O
    14 O X X X X O O O O O O
    13 X X X X . . . . . . .

    Black finds A18 as a supposed outer liberty, when it is
in fact an
    incorrect eyefilling move (white responds at A17).

8. Common liberties are played although the opponent is
ahead. After
    failing pass should be considered, aiming for seki.

9. Unjustified branching causes the semeai nodes to be
exhausted,
    leading to a mostly random result being returned.

The real killer is item 6. It causes much of the semeai
reading in
these tests to become nonsensical, also when the right
result is
returned.

/Gunnar

_______________________________________________
gnugo-devel mailing list
gnugo-develgnu.org
htt
p://lists.gnu.org/mailman/listinfo/gnugo-devel

Semeai reading

Thread: Semeai reading