Updating a java.util.HashMap in parallel with reducers

parallel_java_map_reducers.clj

;; main idea is to create a vector out of the keys
;; and use already existent foldvec to reduce-combine
;; on the key set. Each parallel thread takes an
;; unique set of keys and update the same HashMap
;; instance. "identity" is used as transforming function "f".
;; Just replace with the transformation you need.

(require '[clojure.core.reducers :as r])
(import 'java.util.HashMap)

(extend-protocol r/CollFold
  java.util.HashMap
  (coll-fold
    [m n combinef reducef]
    (#'r/foldvec (into [] (keys m)) n combinef reducef)))

(defn large-map   [i]
  (into {}
    (map vector (range i) (range i))))

(def res
  (let [m (HashMap. (large-map 1000000))
        f identity]
    (r/fold (constantly m) (fn [m k] (.put m k (f (.get m k))) m) m)))