Coding Challenge: Turn Roman Numerals into their Numeric Equivalents

romanNumeralConverter.js

// a mapping of the numerals and their values
const NUMERALS = {
  I: 1,
  V: 5,
  X: 10,
  L: 50,
  C: 100,
  D: 500,
  M: 1000
};

/**
 * Given a Roman Numeral, convert it to a number
 * @param {string} numeral - a valid Roman Numeral
 * @returns {number}
 */
function splitNumerals(numeral) {
  const reversedArray = numeral.split("").reverse();
  const length = reversedArray.length;
  const result = reversedArray.reduce(
    (acc, char, i) => {
      const val = NUMERALS[char]; // the numeric value of the character
      const { buffer, value } = acc;

      // final item requires collapsing the buffer
      if (i === length - 1) {
        if (buffer && val < buffer) {
          // handle a subtraction in the initial place
          return {
            buffer: 0,
            value: value + buffer - val
          };
        } else {
          // collapse all the values
          return {
            buffer: 0,
            value: value + val + buffer
          };
        }
      }

      // if there's no buffer, then store this value to ensure
      // it's not part of a subtraction
      if (!buffer) {
        return {
          buffer: val,
          value
        };
      }

      // if it is a subtraction, collapse the value
      // substractions by rule can only be a single number
      // i.e. IIX is invalid, it would be VIII
      if (val < buffer) {
        return {
          buffer: 0,
          value: value + (buffer - val)
        };
      }

      // regular process, add the buffer to the value and
      // store the current number
      return {
        buffer: val,
        value: value + buffer
      };
    },
    { buffer: 0, value: 0 }
  );

  return result.value + result.buffer;
}

const tests = [
  {
    expected: 1,
    test: "I"
  },
  {
    expected: 5,
    test: "V"
  },
  {
    expected: 9,
    test: "IX"
  },
  {
    expected: 9,
    test: "VIIII"
  },
  {
    expected: 10,
    test: "X"
  },
  {
    expected: 14,
    test: "XIV"
  },
  {
    expected: 19,
    test: "XIX"
  },
  {
    expected: 50,
    test: "L"
  },
  {
    expected: 89,
    test: "LXXXIX"
  },
  {
    expected: 90,
    test: "XC"
  },
  {
    expected: 100,
    test: "C"
  },
  {
    expected: 499,
    test: "ID"
  },
  {
    expected: 500,
    test: "D"
  },
  {
    expected: 1000,
    test: "M"
  },
  {
    expected: 2,
    test: "II"
  },
  {
    expected: 7,
    test: "VII"
  },
  {
    expected: 9,
    test: "IX"
  },
  {
    expected: 20,
    test: "XX"
  },
  {
    expected: 505,
    test: "DV"
  },
  {
    expected: 919,
    test: "CMXIX"
  }
];

const results = tests.map(t => {
  const result = splitNumerals(t.test);
  return {
    passes: result === t.expected,
    expected: t.expected,
    result,
    test: t.test
  };
});

const testResults = results.filter(t => {
  return !t.passes;
});

results.forEach(tr => {
  console.log(tr);
});