max-comparing-poor-mans-benchmark.clj

(ns whatever)

; loop + recur
(defn max-comparing
  ([f xs]
     (when-let [[x & more] (seq xs)]
       (loop [v x, k (f x), xs more]
         (if-let [[x & xs] xs]
           (let [k' (f x)]
             (if (pos? (compare k k'))
               (recur v k  xs)
               (recur x k' xs)))
           v)))))

; map + reduce
(defn max-comparing-2 [f col]
  (when-let [xs (map #(vector (f %) %) col)]
    (second (reduce #(if (pos? (compare (first %1) (first %2))) %1 %2)
                    (first xs)
                    (rest xs)))))

; transducer
(defn max-comparing-3 [f xs]
  (if (seq xs)
    (second (reduce ((map #(vector (f %) %))
                          #(if (pos? (compare (first %1) (first %2))) %1 %2))
            [(f (first xs)) (first xs)]
            (rest xs)))))

(let [f       #(do (Thread/sleep 1) (- %))
      n       100
      xs      (repeatedly n #(rand-int Integer/MAX_VALUE))
      _       (count xs)
      samples 10]
  (assert (= (max-comparing   f xs)
             (max-comparing-2 f xs)
             (max-comparing-3 f xs)))
  (time (doseq [_ (range samples)] (max-comparing   f xs)))
  (time (doseq [_ (range samples)] (max-comparing-2 f xs)))
  (time (doseq [_ (range samples)] (max-comparing-3 f xs))))
;  => nil
; "Elapsed time: 1358.381828 msecs"
; "Elapsed time: 1347.90348 msecs"
; "Elapsed time: 1357.485049 msecs"