Re: [gnugo-devel] Wrong liberty counts in worms?

[Gunnar Farneback]

Inge wrote:
> E14 : (dragon E14) white string of size 6 (6.62), genus 0: (3,5,6,3) - is a 
> cutting stone
> F15 : (dragon J14) black string of size 2 (2.67), genus 0: (4,3,5,1)
>
> The numbers in the parenthesis are the liberties, second order
> liberties, and so on.  The liberties seem to be correct, but second
> order and up seem to be totally bogus.
> 
> Is this just me or is something seriously wrong here?

It's just you who have forgot GNU Go's definition of higher order
liberties, maybe? From dragon.texi:

  @dfn{a liberty of order n} is an empty
  vertex which may be connected to the string by placing n
  stones of the same color on the board, but no fewer. The
  path of connection may pass through an intervening group
  of the same color. The stones placed at distance >1 may
  not touch a group of the opposite color.

For E14 in this position I interpret this as

   A B C D E F G H J K L M N O P Q R S T
19 4 3 . . . . . . . . . . . . . . . . . 19
18 3 O X X X . . . . . X . X . . . . . . 18
17 3 O O X . X . O . O . O . X . . . . . 17
16 4 3 2 O X . X O . + . . . . . X . . . 16
15 4 3 2 O 1 X . X O O . . . . X . . . . 15
14 3 2 2 1 O X . . X O . O . . . . X . . 14
13 2 O O 1 O O X X X O . . . O . O X . . 13
12 . X O O X O O O X O X . . . . . . . . 12
11 . . X O X X X X . O X . . . . . . . . 11
10 . . X X X O . . X + . . . O . X . . . 10
 9 . O . . . O . . . . . X . . . . . . . 9
 8 . . O . . O . . X . . . . O . . X . . 8
 7 . . . . . . . . . . . . . . . . . . . 7
 6 . . X . . O . . . . . O . O . O X . . 6
 5 . . . . . . . . . . . . . . . . X . . 5
 4 . . O O . . . . O O O . . X . X . . . 4
 3 . . . . . O O . X . X X . O X . . . . 3
 2 . . O . X X . X . . . . . . . . . . . 2
 1 . . . . . . . . . . . . . . . . . . . 1
   A B C D E F G H J K L M N O P Q R S T

which perfectly agrees with the reported numbers (3,5,6,3).

On the other hand higher order liberties are fairly crude measures
which are hardly used at all in the engine today. (If I remember
correctly second order liberties are used a few times and higher not
at all.)

/Gunnar