graph-invert.clj

(defn invert
  "inverts a graph represented as a map from source to target nodes"
  [graph]
  (reduce-kv (fn [acc src trgs]
               (if (seq trgs)
                 (reduce (fn [acc- trg]
                           (update acc- trg (fnil conj #{}) src))
                         acc
                         trgs)
                 ;; to preserve nodes without edges
                 (assoc acc src #{})))
             {}
             graph))