core.clj

(ns roman-nums.core
  (:require
   [clojure.test :refer [testing are]]))

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

(def ^:private roman-partitions
  (partition 3 2 roman-digits))

(defn ^:private num-to-roman-one-digit [c low middle high]
  (apply str
         (case c
           0 (list "")
           1 (list low)
           2 (list low low)
           3 (list low low low)
           4 (list low middle)
           5 (list middle)
           6 (list middle low)
           7 (list middle low low)
           8 (list middle low low low)
           9 (list low high))))

(defn num-to-roman
  "Convert a number to roman"
  [n]
  (assert (integer? n))
  (assert (not (neg? n)))
  (loop [buf (), n n, partitions roman-partitions]
    (if (zero? n)
      (apply str buf)
      (let [[[low middle high] & tail] partitions c (rem n 10)]
        (recur
           (conj buf
            (num-to-roman-one-digit c low middle high))
           (quot n 10) tail)))))

(testing
  (are [i s] (= s (num-to-roman i))
       1 "I", 2 "II", 3 "III", 4 "IV"
       5 "V", 6 "VI", 7 "VII", 41 "XLI"
       971 "CMLXXI",  871 "DCCCLXXI"
       900 "CM",      91 "XCI"
       9 "IX"))