Collatz fun

collatz.clj

(ns collatz.core)

(defn collatz
  "Given n, it returns n/2 if it's even or n*3+1 if it's odd."
	[n]
	(if (even? n)
		(/ n 2)
		(+ (* 3 n) 1)))

(declare count-steps-of-collatz)

(def count-small-steps-of-collatz
  (memoize
   (fn [n]
     (if (= n 1) 0
         (inc (count-small-steps-of-collatz (collatz n)))))))

(def count-large-steps-of-collatz
   (fn [n]
     (if (= n 1) 0
         (inc (count-steps-of-collatz (collatz n))))))

(defn count-steps-of-collatz [n]
  (if (< n 100000)
    (count-small-steps-of-collatz n)
    (count-large-steps-of-collatz n)))

(defn integer-below-n-with-most-steps [n]
  (->> (pmap (juxt identity count-steps-of-collatz) (range 1 n))
       (apply (partial max-key second))))

(defn -main []
  (doseq [i (take 7 (iterate (partial * 10) 10))]
    (time (println (integer-below-n-with-most-steps i))))
  (System/exit 0))