I updated the repository for the item stacker. I changed it a bit to better facilitate filtering by category. Namely I've changed it to stack each item in advance in its stackable subtypes, rather than bunch all items of the same type, then later figure out the stackable subtypes. This makes filtering out a specific subtype easier later on. (actual filtering not included as it's probably better to add that to a different branch?)
Also noticed a getItem function which (worst case) scanned the entire item inventory for each tested location of the stacker. With the other changes I've done to the stacker it tries to find room for stacks rather than individual items. So with the stacker no longer needing to know which item is present, just that something is there, I changed it to a small occupancy lookup table instead. The new version is about 10 times faster on the previous test save you gave me (and scales better).