p3.clj

(defn <sqrt [n]
  (range 3 (+ (int (Math/sqrt n)) 1)))

(defn prime-inner
  [n]
  (nil? (some #(= (mod n %) 0) (<sqrt n))))

(defn is-prime? [n]
  (cond
    (< n 2) false
    (= n 2) true
    (= n 3) true
    (= (mod n 2) 0) false
    :else (prime-inner n)))

(defn next-prime [n]
  (first (drop-while #(not (is-prime? %)) (iterate inc (inc n)))))

(defn factors-impl [n curr xs]
  (cond
    (= n 1) xs
    (= (mod n curr) 0)
      (cons curr (factors-impl (/ n curr) curr xs))
    :else
      (factors-impl n (next-prime curr) xs)))

(defn factors [n]
  (distinct (factors-impl n 2 [])))