[gnugo-devel] reverse followup valuation

[Paul Pogonyshev]

i have recently spotted quite a few positions where reverse
followup is completely inadequate.  here are two very common
cases:

        XXXX
        XO.O
        XO.O
        XXXX

in this position the moves at '.' are avarded large reverse
followups because they prevent an atari.  i can see no way to
prevent such positions from matching in `Sente13' pattern.
i'd propose the following solution: start a pattern database
named say `invalidate.db' which will remove inappropriate
followups/reverse followups/anything else.  the reason to
have a separate database is that we need to be sure that
invalidation patterns match after all other patterns.  first
pattern in this database would look like

Pattern Invalidate1

O*O
O.O

:+,O

A*B
AaB

; lib(A) == 2 || lib(B) == 2

> remove_reverse_followup(*);
> remove_reverse_followup(a);


second common position is like this:

OXXXXX
.OO*..
------

the move at '*' is avarded a reverse followup because it
"prevents" atari.  actually, it only prevents atari from
happening at '*' and moves it one/more vertices to the right.
the proper solution would be to improve defend_against_atari()
in `helpers.c'.  i'd suggest something like this:

IF move is adjacent to the string we are "saving" from atari
AND after the move the string still has two liberties
AND opponent has a safe atari on the string after the move
THEN no reverse followup should be added

maybe the first condition is not necessary because of not so
common third position:

O*XX
O.OX
O.OX
OXXX

here move at '*' receives a reverse followup because it prevents
atari on the second white string.  but actually, it doesn't -
it only prevents one of two possible safe ataries.

i'd like to hear comments on this.  reverse followup valuation is
important, especially in endgame.

Paul