mapcat.clj

(ns scratch.mapcat
  (:use [clojure.set :only (union)]))

(defn divisors [n]
  (filter (comp zero? (partial mod n))
          (range 2 n)))

(defn mapcat2 [f & colls]
  (reduce concat (apply map f colls)))

(defn reduce-vs-apply []
  (println "apply:")
  (time (dorun (mapcat divisors (range 1000))))
  (println "reduce:")
  (time (dorun (mapcat2 divisors (range 1000)))))

;; apply:
;; "Elapsed time: 1723.707991 msecs"
;; reduce:
;; "Elapsed time: 6491.475802 msecs"

;; (defn mapunion [f & colls]
;;  (reduce into (cons #{} (apply map f colls))))

(defn mapunion [f & colls]
  (->> (apply map f colls)
       (cons (transient #{}))
       (reduce (partial reduce conj!))
       persistent!))

(defn mapcat-vs-mapunion []
  (println "mapcat:")
  (time (dorun (mapcat divisors (range 2000))))
  (println "mapunion:")
  (time (dorun (mapunion divisors (range 2000)))))

;; mapcat:
;; "Elapsed time: 6523.554967 msecs"
;; mapunion:
;; "Elapsed time: 6505.352707 msecs"