roman_to_int.rb

class RomanToInt
  def initialize(roman)
    @roman = roman.downcase
  end

  def to_int
    result = 0
    roman = @roman

    exceptions.each do |exception, exception_value|
      count = roman.scan(exception).length
      result += exception_value * count
      roman.gsub! exception, ''
    end

    roman.chars.each do |roman_char|
      result += values[roman_char]
    end

    result
  end

  private

  def exceptions
    { 'iv' => 4,
      'ix' => 9,
      'xl' => 40,
      'xc' => 90,
      'cd' => 400,
      'cm' => 900 }
  end

  def values
    { 'i' => 1,
      'v' => 5,
      'x' => 10,
      'l' => 50,
      'c' => 100,
      'd' => 500,
      'm' => 1_000 }
  end
end

describe RomanToInt do
  describe '#to_int' do
    subject { described_class.new(roman).to_int }

    describe 'I' do
      let(:roman) { 'I' }
      it { should eq 1 }
    end

    describe 'II' do
      let(:roman) { 'II' }
      it { should eq 2 }
    end

    describe 'III' do
      let(:roman) { 'III' }
      it { should eq 3 }
    end

    describe 'IV' do
      let(:roman) { 'IV' }
      it { should eq 4 }
    end

    describe 'V' do
      let(:roman) { 'V' }
      it { should eq 5 }
    end

    describe 'VI' do
      let(:roman) { 'VI' }
      it { should eq 6 }
    end

    describe 'VII' do
      let(:roman) { 'VII' }
      it { should eq 7 }
    end

    describe 'VIII' do
      let(:roman) { 'VIII' }
      it { should eq 8 }
    end

    describe 'IX' do
      let(:roman) { 'IX' }
      it { should eq 9 }
    end

    describe 'X' do
      let(:roman) { 'X' }
      it { should eq 10 }
    end

    describe 'XI' do
      let(:roman) { 'XI' }
      it { should eq 11 }
    end

    describe 'MCMIV' do
      let(:roman) { 'MCMIV' }
      it { should eq 1904 }
    end

    describe 'MCMLIV' do
      let(:roman) { 'MCMLIV' }
      it { should eq 1954 }
    end

    describe 'MCMXC' do
      let(:roman) { 'MCMXC' }
      it { should eq 1990 }
    end

    describe 'MMXIV' do
      let(:roman) { 'MMXIV' }
      it { should eq 2014 }
    end
  end
end