gistfile1.clj

(defn rindex-by-fn
  "Uses core.reducers/fold to return a map indexed by the value of key-fn                                                                                                                                                                                                                 
   applied to each element in coll.                                                                                                                                                                                                                                                       
   Note: key-fn can return a collection of multiple keys."
  ([key-fn coll ] (rindex-by-fn key-fn identity coll))
  ([key-fn value-fn coll] (doall (clojure.core.reducers/fold
                                  ;; combinef                                                                                                                                                                                                                                             
	                          (clojure.core.reducers/monoid (partial merge-with into)
                                   				hash-map)
                                  ;; reducef                                                                                                                                                                                                                                              
                                  (fn [ndx elem] (let [key (key-fn elem)]
                                                   (if (coll? key)
                                                     (reduce (fn [sndx selem] (assoc sndx selem (conj (get sndx selem [])
                                                                                                      (value-fn elem))))
                                                             ndx key)
                                                     (assoc ndx key (conj (get ndx key [])
	                                                                  (value-fn elem))))))
	                          coll))))