Clojure - Count occurences of nested key in nested map?

Question

I have a nested map like so:

{:A {:B {:A {:B {:A {:B 0}}}}}}

I want to count the occurrences of the pair [:A :B] so that in the case above, the result is 3.

My initial idea was to use clojure.walk/postwalk to traverse the map and increment a counter.

Is there a more optimal way of achieving this?

Answer 1

tree-seq works out nice in this case:

(defn count-ab [data]
  (->> data
       (tree-seq coll? seq)
       (keep #(get-in % [:A :B]))
       count))

user> (def data {:A {:B {:A {:B {:A {:B 0}}}}}})
#'user/data

user> (count-ab data)
3

user> (def data-1 {:A {:B {:C {:A {:B {:A {:B 0}}}}}}})
#'user/data-1

user> (count-ab data-1)
3

user> (def data-2 {:A {:X {:A {:B 100}}
                       :B {:C {:A {:B {:A {:B 0}}}}}}})
#'user/data-2

user> (count-ab data-2)
4