Budapest ClojureDojo (2014.07.22) - Get the Roman numeral representation of a number

gistfile1.clj

(def roman-digits ["I" "V" "X" "L" "C" "D" "M"])

(defn get-digit-set [exponent]
  (let [base-index (* 2 exponent)]
    [
     ["" 
      (roman-digits base-index)
      (roman-digits (+ base-index 1))]
     [
      (roman-digits (+ base-index 1))
      (roman-digits base-index)
      (roman-digits (+ base-index 2))]
    ]))

(def digit-patterns { 0 '()
                      1 '(0 1)
                      2 '(0 1 1)
                      3 '(0 1 1 1)
                      4 '(1 2)
                      5 '(2)})

(defn single-to-roman [n digit-set]
  (let [i (if (<= n 5) n (- n 5))
        m (digit-set (if (<= n 5) 0 1))
        s (digit-patterns i)]
    (apply str (map m s))
  )
)

(defn to-roman [n]
  (loop [n              n
         exp            0
         acc-roman-str  ""]
    (let [digit-set (get-digit-set exp)
          single-digit (unchecked-remainder-int n 10)
          next-roman-str (str (single-to-roman single-digit digit-set) acc-roman-str)]
      (if (= exp 2)
        next-roman-str
        (recur (unchecked-divide-int n 10)
               (inc exp)
               next-roman-str)))))

(to-roman 637)