Re: [computer-go] Groups, liberties, and such

[Jason House <jhouse@xxxxxxxxx>]

I tried to explain why a localized liberty correction would not workin 
my original (long) post.  Consider stones in the shape of an uppercase U 
where there one stone in the bottom is the newly added stone, and the 
sides of the U are one stone apart.  The U can be made arbitrarily tall 
to invalidate local liberty corrections.  See the picture below.  The 
X's are pre-existing stones and the + is the new stone.

X X
X X
X+X

PS: In theory, the stones could separate from each other local to the 
connection but then come close to each other further on in each of the 
chains.

drd@xxxxxxxxxxxxxxxxx wrote:
> Jason,
>
> When a move connects 2 strings,  isn't it easy and fast to calculate 
> the liberty count of the newly combined string? 
>  
>    new_count = old_string_A_count + old_string_B_count - 1 + LNC.
>
> LNC means Liberties Not in Common.  For the newly placed stone (that does
> the connecting), we look at the 4 neighbor points.   If they are empty,
> we tally 1 if they are not liberties of EITHER of the OLD strings.
> This idea is easily extended for connections of 3 or 4 groups.
>
> When connecting 2 groups, you would have to look at 6 points at most,
> the 2 possible new liberties,  and the 3 possible points around these new 
> liberties to check if they touch the old group.
>
> Compared to other more complicated schemes, this is fast.   What little
> extra time this takes is very tiny when amortized over all the moves
> your program will be making.
>
> Am I understanding the problem correctly or am I confused about your
> requirements?
>
> If the new connection is also a capture,  this doesn't work, but that's
> a problem for any of the incremental liberty calculations, not just the
> connecting ones.
>
> - Don
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