bigsum.clj

;; Re: http://twoguysarguing.wordpress.com/2010/08/09/sum-to-a-million/
;; Note: (n*(n+1))/2 is obviously faster

(defn triangle-num [s e]
  (let [n (long e)]
    (loop [sum (long 0)
           i (long s)]
      (if (> i n)
        sum
        (recur (unchecked-add sum i)
               (unchecked-inc i))))))

(defn par-triangle-num [n]
  (let [batches (+ 2 (.availableProcessors (Runtime/getRuntime)))
        batch-size (/ n batches)
        futures (doall
                 (for [i (range batches)]
                   (future (triangle-num (inc (* i batch-size))
                                         (* (inc i) batch-size)))))]
    (reduce + (map deref futures))))

(comment

;; 1,000,000

(dotimes [_ 10] (time (triangle-num 1 1000000)))
"Elapsed time: 0.931 msecs"
"Elapsed time: 0.963 msecs"
"Elapsed time: 0.941 msecs"
"Elapsed time: 0.957 msecs"
"Elapsed time: 0.857 msecs"
"Elapsed time: 0.861 msecs"
"Elapsed time: 0.988 msecs"
"Elapsed time: 0.828 msecs"
"Elapsed time: 0.895 msecs"
"Elapsed time: 0.869 msecs"
nil

(dotimes [_ 10] (time (par-triangle-num 1000000)))
"Elapsed time: 0.842 msecs"
"Elapsed time: 0.857 msecs"
"Elapsed time: 0.913 msecs"
"Elapsed time: 0.814 msecs"
"Elapsed time: 0.834 msecs"
"Elapsed time: 0.726 msecs"
"Elapsed time: 0.62 msecs"
"Elapsed time: 0.61 msecs"
"Elapsed time: 0.853 msecs"
"Elapsed time: 0.879 msecs"
nil

;; 100,000,000

(dotimes [_ 10] (time (triangle-num 1 100000000)))
"Elapsed time: 86.021 msecs"
"Elapsed time: 74.434 msecs"
"Elapsed time: 79.351 msecs"
"Elapsed time: 85.847 msecs"
"Elapsed time: 75.201 msecs"
"Elapsed time: 87.021 msecs"
"Elapsed time: 78.764 msecs"
"Elapsed time: 90.479 msecs"
"Elapsed time: 83.884 msecs"
"Elapsed time: 87.911 msecs"
nil

user> (dotimes [_ 10] (time (par-triangle-num 100000000)))
"Elapsed time: 45.023 msecs"
"Elapsed time: 46.927 msecs"
"Elapsed time: 44.417 msecs"
"Elapsed time: 49.23 msecs"
"Elapsed time: 44.339 msecs"
"Elapsed time: 46.003 msecs"
"Elapsed time: 45.256 msecs"
"Elapsed time: 45.341 msecs"
"Elapsed time: 45.255 msecs"
"Elapsed time: 46.893 msecs"
nil

)