roman.clj

(defn ->roman
  ; https://www.codewars.com/kata/51b62bf6a9c58071c600001b/train/clojure
  [n]
  (let [rn ["I" "V" "X" "L" "C" "D" "M"]
        rn# (partition 3 2 [nil nil] rn)
        ->rn (fn [x n]
               (let [[a b c] (nth rn# x)]
                 (get {9 [a c]
                       8 [b a a a]
                       7 [b a a]
                       6 [b a]
                       5 [b]
                       4 [a b]
                       3 [a a a]
                       2 [a a]
                       1 [a]} n)))]
    (loop [x 3
           n n
           res []]
      (let [x# (int (Math/pow 10 x))
            n# (mod n x#)]
        (cond
          (= x 0) (reduce str (concat res (->rn x n)))
          (= n# n) (recur (dec x) (mod n# x#) res)
          :else (recur (dec x) (mod n# x#) (concat res (->rn x (Math/floorDiv n x#)))))))))

(defn <-roman
  [n]
  (let [m {\I 1
           \V 5
           \X 10
           \L 50
           \C 100
           \D 500
           \M 1000}]
    (loop [[x & xs] n
           res 0]
      (if (nil? x)
        res
        (let [x# (get m x)
              nx (get m (first xs) 0)]
          (if (< x# nx)
            (recur (next xs) (+ res (- nx x#)))
            (recur xs (+ res x#))))))))

(def roman-prop
  (prop/for-all [n gen/nat]
    (= n (<-roman (->roman n)))))

#_(tc/quick-check 200000 roman-prop {:max-size 3999})