Solving the Roman Numeral Kata in Clojure

core_test.clj

(ns roman-numerals.core-test
  (:require [clojure.test :refer :all]))

(def mapping {
              0 ""
              1 "I"
              4 "IV"
              5 "V"
              9 "IX"
              10 "X"
             })

(defn highest-reducer [x]
  (let [x (->> mapping
               keys
               (filter #(<= % x))
               (apply max))]
    [x (mapping x)]))

(defn convert
  ([x] (convert x ""))
  ([x result-string]
    (let [[arabic roman] (highest-reducer x)]
      (if (= x 0)
        result-string
        (recur (- x arabic)
               (str result-string roman))))))


(deftest convert-test
  (testing "converts arabic numbers to roman numerals"
    (are [x y] (= (convert x) y)
         1 "I"
         2 "II"
         4 "IV"
         5 "V"
         6 "VI"
         7 "VII"
         8 "VIII"
         9 "IX"
         10 "X"
         11 "XI"
         13 "XIII"
         14 "XIV"
         )))