numerals_test.rb

gem 'minitest'
require 'minitest/autorun'
require 'minitest/pride'

class Numeral
  def self.convert(number)
    accumulator, number = reduce_by("", number, 20, "XX")
    accumulator, number = reduce_by(accumulator, number, 10, "X")
    accumulator, number = reduce_by(accumulator, number, 5, "V")
    (1..number).inject(accumulator) { |product| product << "I" }
  end

  def self.reduce_by(accumulator, number, reducer, numeral)
    if number == reducer-1
      accumulator += ("I" + numeral)
      number -= reducer
    end

    if number >= reducer
      accumulator += numeral
      number -= reducer
    end

    [accumulator, number]
  end
end

describe Numeral do

  it("converts 1 to I") { Numeral.convert(1).must_equal("I") }
  it("converts 2 to II") { Numeral.convert(2).must_equal("II") }
  it("converts 3 to III") { Numeral.convert(3).must_equal("III") }
  it("converts 4 to IV") { Numeral.convert(4).must_equal("IV") }
  it("converts 5 to V") { Numeral.convert(5).must_equal("V") }
  it("converts 6 to VI") { Numeral.convert(6).must_equal("VI") }
  it("converts 7 to VII") { Numeral.convert(7).must_equal("VII") }
  it("converts 8 to VIII") { Numeral.convert(8).must_equal("VIII") }
  it("converts 9 to IX") { Numeral.convert(9).must_equal("IX") }
  it("converts 10 to X") { Numeral.convert(10).must_equal("X") }
  it("converts 11 to XI") { Numeral.convert(11).must_equal("XI") }
  it("converts 13 to XIII") { Numeral.convert(13).must_equal("XIII") }
  it("converts 15 to XV") { Numeral.convert(15).must_equal("XV") }
  it("converts 16 to XVI") { Numeral.convert(16).must_equal("XVI") }
  it("converts 18 to XVIII") { Numeral.convert(18).must_equal("XVIII") }
  it("converts 20 to XX") { Numeral.convert(20).must_equal("XX") }
  it("converts 21 to XXI") { Numeral.convert(21).must_equal("XXI") }

end