Roman Numeral Calculator

roman_numeral_calculator.rb

require 'rspec/autorun'

class RomanNumeralCalculator
  SYMBOL_MAPPING = {
    1 => 'I',
    4 => 'IV',
    5 => 'V',
    9 => 'IX',
    10 => 'X',
    40 => 'XL',
    50 => 'L',
    90 => 'XC',
    100 => 'C',
    400 => 'CD',
    500 => 'D',
    900 => 'CM',
    1_000 => 'M'
  }

  def initialize(number)
    @number = number
  end

  attr_reader :number

  def convert_to_roman
    symbols = ''
    accumulator = 0

    until accumulator == number
      remainder = number - accumulator
      value, symbol = tuple(accumulator.zero? ? number : remainder)
      accumulator += value
      symbols << symbol
    end

    symbols
  end

  private

  def tuple(value)
    value = SYMBOL_MAPPING.keys.select {|key| key <= value }.last
    SYMBOL_MAPPING.find.with_index do |_, index|
      index == SYMBOL_MAPPING.keys.index(value)
    end
  end
end

describe 'RomanNumeralCalculator' do
  [
    [ 1, 'I' ],
    [ 2, 'II' ],
    [ 3, 'III' ],
    [ 4, 'IV' ],
    [ 5, 'V' ],
    [ 6, 'VI' ],
    [ 8, 'VIII' ],
    [ 15, 'XV' ],
    [ 21, 'XXI' ],
    [ 35, 'XXXV' ],
    [ 44, 'XLIV' ],
    [ 98, 'XCVIII' ]
  ].each do |arabic_numeral, roman_numeral|
    it "returns #{roman_numeral} for #{arabic_numeral}" do
      numeral = RomanNumeralCalculator.new(arabic_numeral)
      expect(numeral.convert_to_roman).to eq(roman_numeral)
    end
  end
end