it-tools/src/tools/roman-numeral-converter/roman-numeral-converter.service.ts

export const MIN_ARABIC_TO_ROMAN = 1;
export const MAX_ARABIC_TO_ROMAN = 3999;
export function arabicToRoman(num: number) {
  if (num < MIN_ARABIC_TO_ROMAN || num > MAX_ARABIC_TO_ROMAN) {
    return '';
  }

  const lookup: { [key: string]: number } = {
    M: 1000,
    CM: 900,
    D: 500,
    CD: 400,
    C: 100,
    XC: 90,
    L: 50,
    XL: 40,
    X: 10,
    IX: 9,
    V: 5,
    IV: 4,
    I: 1,
  };
  let roman = '';
  for (const i in lookup) {
    while (num >= lookup[i]) {
      roman += i;
      num -= lookup[i];
    }
  }
  return roman;
}

const ROMAN_NUMBER_REGEX = /^M{0,3}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$/;

export function isValidRomanNumber(romanNumber: string) {
  return ROMAN_NUMBER_REGEX.test(romanNumber);
}

export function romanToArabic(s: string) {
  if (!isValidRomanNumber(s)) {
    return null;
  }
  const map: { [key: string]: number } = { I: 1, V: 5, X: 10, L: 50, C: 100, D: 500, M: 1000 };
  return [...s].reduce((r, c, i, s) => (map[s[i + 1]] > map[c] ? r - map[c] : r + map[c]), 0);
}