clojure collatz for HN

(defn collatz-next
  "Computes the next number of the Collatz conjecture series."
  [a]
  (/ (if (even? a)
       a
       (inc (* 3 a )))
     2))

(defn collatz-length
  "Computes how long is Collatz conjecture series for particular n."
  [n]
  (loop [number n length 0]
      (if (= number 1)
      length
      (recur (collatz-next number) (inc length)))))

(defn longest-collatz
  [n]
  (loop [i 1 best '(1 0)]
    (let [length (collatz-length i)]
      (if (< i n)
        (if (> length (second best))
          (recur (inc i) (list i length))
          (recur (inc i) best))
      best))))


(time (longest-collatz 1000000))