roman-numerals-helper.js

// Write two functions that convert a roman numeral to and from an integer value. Multiple roman numeral values will be tested for each function.

// Modern Roman numerals are written by expressing each digit separately starting with the left most digit and skipping any digit with a value of zero. In Roman numerals:

// 1990 is rendered: 1000=M, 900=CM, 90=XC; resulting in MCMXC
// 2008 is written as 2000=MM, 8=VIII; or MMVIII
// 1666 uses each Roman symbol in descending order: MDCLXVI.
// Input range : 1 <= n < 4000

// In this kata 4 should be represented as IV, NOT as IIII (the "watchmaker's four").

// Examples
// to roman:
// 2000 -> "MM"
// 1666 -> "MDCLXVI"
//   86 -> "LXXXVI"
//    1 -> "I"

// from roman:
// "MM"      -> 2000
// "MDCLXVI" -> 1666
// "LXXXVI"  ->   86
// "I"       ->    1
// Help
// +--------+-------+
// | Symbol | Value |
// +--------+-------+
// |    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 |
// +--------+-------+

class RomanNumerals {
  static toRoman(num) {
    const romanNumerals = {
      1000: 'M', 900: 'CM', 500: 'D', 400: 'CD',
      100: 'C', 90: 'XC', 50: 'L', 40: 'XL',
      10: 'X', 9: 'IX', 5: 'V', 4: 'IV', 1: 'I'
    };
    let roman = '';
    for (let value of Object.keys(romanNumerals).sort((a, b) => b - a)) {
      while (num >= value) {
        roman += romanNumerals[value];
        num -= value;
      }
    }
    return roman;
  }
  
  static fromRoman(str) {
        const romanNumerals = {
      '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 result = 0;
    let prevValue = 0;
    for (let i = str.length - 1; i >= 0; i--) {
      let value = romanNumerals[str[i]];
      if (value < prevValue) {
        result -= value;
      } else {
        result += value;
      }
      prevValue = value;
    }
    return result;
  }  
}