I'm implementing a map data structure where most of the values are maps or
sets, and these values can be cross-indexed by the keys they contain. I don't
know if it already has a name, but I'm calling it a cross-map. It's similar to
a two-way map, but they're not the same thing.
For instance, a common operation would be something like "give me all values of
this map that contain the key :a."
In order to do this efficiently, I'm maintaining a second map that maps keys in
the values of the main map to keys of the main map whose values contain that
key.
If that sounds confusing, consider this:
main-map:
{:foo {:a 1 :b 2} :bar {:a 2 :c 4} :baz {:b 3 :c 5}}
Corresponding cross-indices:
{:a #{:foo :bar} :b #{:foo :baz} :c #{:bar :baz}}
As you can see, each key maintains references to those entries where it is
found.
When a nested update occurs that adds an entry to one of the main map's values,
the efficient thing to do would be to simply conj that new key onto its
corresponding cross-index set.
However, I am trying to implement this as a clojure IPersistentMap, and the
only method I can override is assoc, not assoc-in.
Using regular assoc, I would have to compare the old value's keys to the new
value's keys and find the set difference of the two, which is not an O(1)
operation.
Is there any way to override the behaviour of nested associations or updates?
Thanks
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