Read Roman Numerals (4clojure problem 92)

flengyel-4clojure-solution92.clj

;; flengyel's solution to Read Roman numerals
;; https://4clojure.com/problem/92
;; 4clojure rates this as "hard" but I found it easy. 
;; Uses a reduce with destructuring to keep track of the "accumulated state".
(fn  [numeral] 
  (-> (let [roman {\I 1, \V 5, \X 10, \L 50, \C 100, \D 500, \M 1000} 
                   digit (roman (first (vec numeral)))]
        (reduce
	    (fn [[acc prv] cur] 
                (let [nacc (+ acc cur)] 
	            [(if (< prv cur) (- nacc (* 2 prv)) nacc) cur]))
	    [digit digit]  (map roman (rest (vec numeral))))) (first)))