computer-go: What makes a ladder

["Njoroge" <Njoroge@xxxxxxxxxxx>]

I have been trying to gather rules about what makes a ladder and have
determined the following. (This can be in 1 of 8 orientations - rotated,
flipped and mirrored)

 . L1.
 # a L2
 . O .

 # - A stone in atari
 a - The only liberty of @
 O - White stone or a position off the side of the board.
 L1 and L2 - liberties should @ play at 'a'
 . - don't care

When O is off the side of the board the correct attacking move is L2,
otherwise  it is L1.

The ladder fails if:
    - L2 is found to be a black stone
    - O has less than 2 liberties

The ladder is successful if:
    - L2 is either outside the board or is a white stone
    - L2 is a black stone with only 2 or less liberties
    - L2 is a black stone on the edge with only 3 or less liberties
    - L2 is a black stone in the corner with only 4 or less liberties

In the last of 2 cases I believe that the success of the ladder depends on
another ladder that will occur when L2 is contended.

Example 1 - L2 is on the edge and has 3 liberties

# to play
. . . . . . .
. . O . . . .
--O-#---#----


. . . . . . .
. . O . O . .
--O-#-#-#----


. . . O . . .
. . O # O . .
--O-#-#-#----


Example 2 - L2 is in the corner and has 4 liberties

# to play

. . . . |
. . . . O
. . . . #
. . . . #
. . O . #
--O-#---#

. . . . |
. . . . O
. . . . #
. . . O #
. . O . #
--O-#-#-#

. . . . |
. . . . O
. . O # #
. . . O #
. . O . #
--O-#-#-#


Is this always true?
Does it matter where L2's liberties are - edge/middle?
Does this algorithm detect all ladders and determine whether they succeed of
fail?

Sadly I'm no Go player - probably 22kyu - but I've been very seriously
studying the features that occur on the board for my Go program - Repligo.
I've spent a lot of time on ladders because of the *huge* human look ahead
when these situations occur.

I've been creating rules like this one for other board features aswell, but
I've mamnaged to figure most things out for myself.  For instance an eye is
a ponnuki shale that has 3 safe corners.  A safe corner can either be a
stone of the same colour or the center of another eye. Each edge that the
ponnuki overlaps counts for 1 safe corner.

Ponnuki with 1 safe corner
 . . . . .
 . O O . .
 . O . O .
 . . O . .
 . . . . .

Ponnuki with 1 safe corner
 . . . . .
 . O O . .
 --O---O--

2 ponnuki, each with 3 safe corners
 . . . . |
 . . O O |
 . . O . O
 ----O-O-'

3 ponnuki starting from left working right - 2, 0, 2 safe corners

| . . . .
| O . . .
O . O O .
'-O---O--

Not that any of that's important - I just need help with my ladders!

Regards,

Njoroge