[computer-go] How to cache local tactical searches?

[Chrilly]

The eval of Suzie has a ladder-searcher which I plan now to extend to Geta's 
and simple 3-Liberty Problems. Although the ladder-code is reasonable fast, 
in more complicated positions the speed of the eval goes down considerable 
due to the local tacitical search. So far I use the most primitive cache 
method. A hash-table with the global hash-code. Due to iterative deeping 
there are some cache-hits, but the percentage is much lower than with a more 
clever cache-scheme.
One idea is that the local search marks a Dirty-Bitmap. All the intersection 
which have been visited (and probably also the neighbours of the visited 
stones, because e.g. the status of a ladder-breaker depends also on its 
liberties). The dirty Bitmap and the current Board-Bitmaps are stored. When 
the local search is called again, the old result is used, if the new board 
matches the stored one on the intersections of the dirty bitmap.
I do not know if the method works in case of ladders at all, e.g. the 
liberties of a ladder breaker can depend on a stone in a completly other 
part of the board. Its not my intention to have a perfect ladder code. The 
question is, if the idea is flawed at all and not only in some tricky 
special cases.
A also do not know if the BitMap Key is the best way. I am not really 
satiesfied with this idea. One has to store 3 Bitmaps and also the 
comparision is relative slow (but of course much faster than doing the 
search again). Any other suggestions?

Chrilly