pmapn.clj

(require '[clojure.contrib.seq-utils :refer [indexed]])
(import (java.util.concurrent Executors ConcurrentHashMap))

(defn pmapn [n f coll]
  (let [executor (Executors/newFixedThreadPool n)
        total (count coll)
        results (ConcurrentHashMap. total 0.75 n)]
    ;; enqueue work
    (doseq [[i x] (indexed coll)]
      (.execute executor #(.put results i (f x))))
    ;; wait for threads to complete
    (loop []
      (if (< (.size results) total)
        (do (Thread/sleep 100) (recur))
        (map #(.get results %) (range total))))))