gistfile1.clj

(ns euler.p8)

(defn ones-digit-of
  [number]
  (mod number 10))

(defn shift-digit-from
  [number]
  (/ (- number (ones-digit-of number)) 10))

(defn sample-digits-from
  ([number to-sample] (sample-digits-from number to-sample []))
  ([number to-sample sampled]
     (if (= (count sampled) to-sample)
       sampled
       (recur (shift-digit-from number)
                           to-sample
                           (cons (ones-digit-of number) sampled)))))

(defn greatest-prod
  ([number digits] (greatest-prod number digits 0))
  ([number digits largest]
     (if (= number 0)
       largest
       (let [seq-prod (reduce * (sample-digits-from number digits))
             new-largest (apply max [seq-prod largest])]
         (recur (shift-digit-from number) digits new-largest)))))