Solution for Project Euler's problem 34 (Clojure)

problem34.clj

(defn fact
"Return n!."
[^Integer n]
(if (< n 2)
  1
  (apply * (range 2 (+ n 1)))))

(def fast-fact
; This is a faster version of "fact", using a dictionary to return the
; factorial for any number between 0 and 9 (the only kind of number for which
; we want the factorial here).
(reduce (fn [dict n] (assoc dict n (fact n))) {} (range 0 10)))

(defn digits
"Return a list of the digits of n."
[^Integer n]
(map #(- (int %) 48) (.toString n)))

(defn test-number
"Check if n is equal to the factorials of its digits."
[^Integer n]
(let [factorials fast-fact]
  (= n (apply + (map fast-fact (digits n))))))

(defn solution
"Return the solution for the problem."
[^Integer max-n]
(apply + (filter test-number (range 10 max-n))))