roman-numerals.el

(setq numerals
      '((ones "" "I" "II" "III" "IV" "V" "VI" "VII" "VIII" "IX")
        (tens "" "X" "XX" "XXX" "XL" "L" "LX" "LXX" "LXXX" "XC")
        (hundreds "" "C" "CC" "CCC" "CD" "D" "DC" "DCC" "DCCC" "CM")
        (thousands "" "M" "MM" "MMM")))

(defun divmod (x y)
  "Helper function to divide X by Y and return the quotient and remainder."
  (let
      ((quotient (/ x y))
       (remainder (% x y)))
    (cl-values quotient remainder)))

(defun to-roman-iterative (number)
  "Iterative version of the to-roman function. Takes an arabic NUMBER to convert to roman numberal."
  (let ((roman ()))
    (-map (lambda (place)
            (let*
                ((finalize (divmod number 10))
                 (quotient (elt finalize 0))
                 (remainder (elt finalize 1)))
              (setq roman (cons (elt (cdr (assoc (elt place 0) numerals)) remainder) roman))
              (setq number quotient)
              roman))
          numerals)
    (mapconcat 'identity roman "")))

(defun helper (numerals roman number)
  "Helper function for to-roman. Takes a NUMERALS alist, ROMAN nil, NUMBER to convert to Roman."
  (if (not numerals)
      roman
    (let*
        ((finalize (divmod number 10))
         (quotient (elt finalize 0))
         (remainder (elt finalize 1))
         (place (car (car numerals)))
         (new-roman (cons (elt (cdr (assoc place numerals)) remainder) roman)))
      (helper (cdr numerals) new-roman quotient))))

(defun to-roman (number)
  "Takes an arabic NUMBER to convert to roman numeral."
    (mapconcat 'identity (helper numerals () number) ""))