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@zmaril
Created February 1, 2013 02:26
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Collatz fun
(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))
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