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Great stuff as usual Charles. All your comments around the partitioning mirrored my thoughts it seems. My confusion over the compatibility issue was based purely around the need for it to be based on having a map which we agree on.
I guess the question is that memory vs performance tradeoff and whether to just go ahead and build dup group maps on the fly. That is not the only place the exemption map is used of course as I have for instance menu items enabled based on whether a book has exemptions, along with the show all exemptions stuff. So these would need to change to use the build on the fly stuff too.
Your answer to the issue of removing from an exemption group is a clever one. I was thinking of partitioning the exemption groups but yours is interesting and simpler. I honestly am sceptical about how frequently users would be removing subsets of a group, we gave them the ability but in reality I doubt few would use it and sparingly at best. As what it is saying is that you screwed up and have decided that some authors or books are actually duplicates of each other. Well if they are duplicates, you are going to end up renaming the author or merging the books, so they will disappear from the exemption lists when my house cleaning kicks in anyways.
So in other words I am not too concerned with ongoing fragmentation of the exemption sets from that particular action.
Sounds like the beginnings of a plan. I will try some testing I guess to see what the performance impact is if we build the map on the fly. I think 1000 members of a group is a reasonable limit anyways? To get that many members you must have either a really crappy set of book metadata like all the titles unknown, or be storing something like magazines where the majority of the name is identical. It is trivial for a user to exclude magazines or whatever by using a search restriction if they don't want them to appear in the results of a soundex or fuzzy search.
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