Project Eularの Problem 3 のプログラム (Clojure版)

eularProblem3.clj

;; Answer of Eular Problem 3 "Largest prime factor"

(defn primes []
  (letfn
    [(primes [p ns]
      (let [ns (filter #(not= (rem % p) 0) ns)]
        (lazy-seq (cons p (primes (first ns) ns)))))]
    (primes 2 (iterate inc 2))))

(defn largePrimeFactorOf [x]
  (loop [primes (primes) x x]
    (let [p (first primes)]
      (if (= (rem x p) 0)
        (if (= (quot x p) 1)
          p
          (recur primes (quot x p)))
        (recur (rest primes) x)))))

(defn answer [] (largePrimeFactorOf 600851475143))