assoc-in is defined in terms of assoc:
https://github.com/clojure/clojure/blob/clojure-1.7.0/src/clj/clojure/core.clj#L5901
And this structure is fairly common these days, it's basically a index of
tuples [e a v], and you're creating a eav index and then an av index.
Datomic does this same sort of thing, for the datom [e a v t] it creates
indices for :eavt :avet and a few others that escape my memory at the
moment.
On Mon, Apr 18, 2016 at 5:08 PM, JvJ <kfjwhee...@gmail.com> wrote:
> 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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