roman numeral kata in Clojure

romannumeralkata.clj

;; in this kata we want to be able to convert a string to its roman numeral value
;;Roman numerals are written in descending order so we should be able to parse
;;the string and add up the values

;;the only issue is the subtraction rule
;; iv = 4

;;  we'll have to split the string into a vector of characters
;;  convert each of those characters into a value
;;  reduce the all those values into one final value

(def roman-nums {\I 1
                 \V 5
                 \X 10
                 \L 50
                 \C 100
                 \D 500
                 \M 1000})

(defn roman-rules [[b a]] (if (< a b) (- a) a))

(defn roman-numeral-value [s]
  (reduce + (map roman-rules (partition 2 1 (cons 0 (map roman-nums (reverse s)))))))

;;tests for roman-rules
(and
 (= -1 (roman-rules [5 1]))
 (= 1 (roman-rules [1 1])))

;;simple cases
(and
 (= 1 (roman-numeral-value "I"))
 (= 2 (roman-numeral-value "II"))
 (= 10 (roman-numeral-value "X"))
 (= 20 (roman-numeral-value "XX"))
 (= 1001 (roman-numeral-value "MI")))

;;tests for strings with subtraction rules
(and
 (= 4 (roman-numeral-value "IV"))
 (= 14 (roman-numeral-value "XIV"))
 (= 40 (roman-numeral-value "XL")))