npmap.clj

(use 'clojure.contrib.seq-utils)

(defn npmap [n f coll]
  (apply concat
         (pmap #(doall (map f %))
              (partition-all (Math/ceil (/ (count coll)
                                           (float n)))
                             coll))))

(defn fac 
  ([n] (fac n 1))
  ([n x] 
     (if (= n 1)
       x
       (recur (- n 1) (* n x)))))


;(time (do (doall (npmap 1 fac (take 10 (repeat 50000)))) nil))
;"Elapsed time: 30053.218 msecs"

;(time (do (doall (npmap 2 fac (take 10 (repeat 50000)))) nil))
;"Elapsed time: 19077.705 msecs"

;(time (do (doall (npmap 3 fac (take 10 (repeat 50000)))) nil))
;"Elapsed time: 17598.7 msecs"

;(time (do (doall (npmap 4 fac (take 10 (repeat 50000)))) nil))
;"Elapsed time: 16788.853 msecs"

;By way of comparison, the built-ins:

;(time (do (doall (map fac (take 10 (repeat 50000)))) nil))
;"Elapsed time: 30073.203 msecs"

;(time (do (doall (pmap fac (take 10 (repeat 50000)))) nil))
;"Elapsed time: 18448.07 msecs"