Roman Numerals (Clojure)

roman.clj

(ns roman-numerals.roman)

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

(defn accumulate [acc [a,b]]
  (if (>= a b) (+ acc a) (- acc a)))

(defn decode [s]
  "Return the integer value of a string of Roman Numerals,
   or nil if the string contains invalid characters."
  (let [ds (map (partial get digits) (seq s))]
    (when-not (some nil? ds)
      (->> ds
        (partition 2 1 [0])
        (reduce accumulate 0)))))


(decode "DCCCLXXXIV")
; 884