Results from Roman Numeral Calculator Kata at @cincinnatirb

roman_numeral_calculator.rb

class RomanNumeralCalculator
  def calculate(numeral)
    if numeral =~ /^\d+$/
      arabic_to_roman(numeral)
    else
      roman_to_arabic(numeral)
    end
  end

  private

  ROMAN_REDUCTIONS = [
    ["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]]

  ROMAN_VALUES = Hash[*ROMAN_REDUCTIONS.flatten]

  def arabic_to_roman(numeral)
    n = numeral.to_i
    result = ""
    ROMAN_REDUCTIONS.each do |roman_digit, val|
      while n >= val
        result << roman_digit
        n -= val
      end
    end
    result
  end

  def roman_to_arabic(numeral)
    result = 0
    numerals = numeral.split(//)
    while !numerals.empty?
      a, b = numerals
      if value_of(a) >= value_of(b)
        result += value_of(a)
      else
        result += value_of(b) - value_of(a)
        numerals.shift
      end
      numerals.shift
    end
    result.to_s
  end

  def value_of(roman_digit)
    ROMAN_VALUES[roman_digit] || 0
  end
end

roman_numeral_calculator_spec.rb

require 'rspec/given'
require 'roman_numeral_calculator'

describe RomanNumeralCalculator do
  Given(:calculator) { RomanNumeralCalculator.new }

  def calculate(n)
    calculator.calculate(n)
  end

  describe "converting Roman to Arabic" do
    describe "with single digits" do
      Then { calculate("I").should == "1" }
      Then { calculate("V").should == "5" }
      Then { calculate("X").should == "10" }
      Then { calculate("L").should == "50" }
      Then { calculate("C").should == "100" }
      Then { calculate("D").should == "500" }
      Then { calculate("M").should == "1000" }
    end

    describe "with multiple digits" do
      Then { calculate("II").should == "2" }
      Then { calculate("VIII").should == "8" }
      Then { calculate("MMMCCCXXXIII").should == "3333" }
    end

    describe "with inverted ordered digits" do
      Then { calculate("IV").should == "4" }
      Then { calculate("IX").should == "9" }
      Then { calculate("XL").should == "40" }
      Then { calculate("XC").should == "90" }
      Then { calculate("CD").should == "400" }
      Then { calculate("CM").should == "900" }
    end

    describe "with the largest number" do
      Then { calculate("MMMCMXCIX").should == "3999" }
    end
  end

  describe "converting arabic to roman" do
    Then { calculate("1").should == "I" }
    Then { calculate("2").should == "II" }
    Then { calculate("3").should == "III" }
    Then { calculate("4").should == "IV" }
    Then { calculate("5").should == "V" }
    Then { calculate("6").should == "VI" }
    Then { calculate("9").should == "IX" }
    Then { calculate("10").should == "X" }
    Then { calculate("30").should == "XXX" }
    Then { calculate("40").should == "XL" }
    Then { calculate("50").should == "L" }
    Then { calculate("90").should == "XC" }
    Then { calculate("100").should == "C" }
    Then { calculate("400").should == "CD" }
    Then { calculate("900").should == "CM" }
    Then { calculate("1000").should == "M" }
    Then { calculate("3999").should == "MMMCMXCIX" }
  end
end