String Calculator Kata

string_calculator_specs.rb

require "test_notifier/runner/rspec"
TestNotifier.default_notifier = :notify_send

RSpec::Matchers.define :add_to do |expected|
  match do |string|
    string.add == expected
  end
end

module AddableString
  def add
    numbers.inject(0) { |sum, number| sum + number }
  end

  private
    def numbers
      split(any_delimiter).map(&:to_i)
    end

    def any_delimiter
      Regexp.union delimiters
    end

    def delimiters
      return default_delimiters + singlechar_delimiter + bracketed_delimiters
    end

    def default_delimiters
      [",", "\n"]
    end

    def singlechar_delimiter
      has_custom_delimiters?? [self[2]] : []
    end

    def bracketed_delimiters
      scan(/\[(.+?)\]/).map{ |m| m[0] }
    end

    def has_custom_delimiters?
      start_with?("//")
    end
end

class String
  include AddableString
end

describe "String Calculator" do
  it "should return 0 for ''" do
    "".should add_to 0
  end

  context "can add one number" do
    it "should return 1 for 1" do
      "1".should add_to 1
    end

    it "should return 2 for 2" do
      "2".should add_to 2
    end

    it "should return 42 for 42" do
      "42".should add_to 42
    end
  end

  context "can add two numbers" do
    it "should return 2 for 1,1" do
      "1,1".should add_to 2
    end

    it "should return 3 for 1,2" do
      "1,2".should add_to 3
    end

    it "should return 12 for 1,11" do
      "1,11".should add_to 12
    end
  end

  context "can add any amount of numbers" do
    it "should return 6 for 1,2,3" do
      "1,2,3".should add_to 6
    end

    it "should return 200 for one-hundred twos" do
      Array.new(2,100).join(',').should add_to 200
    end
  end

  context "delimiters" do
    it "supports \n as delimiter" do
      "1\n2".should add_to 3
    end

    it "supports ; as delimiter"  do
      "//;\n1;2".should add_to 3
    end

    it "supports % as delimiter" do
      "//%\n1%2".should add_to 3
    end

    context "delimiters of any length" do
      it "supports ;: as delimiter" do
        "//[;:]\n1;:2".should add_to 3
      end

      it "supports ::: as delimiter" do
        "//[:::]\n1:::2".should add_to 3
      end
    end

    context "multiple delimiters" do
      it "supports %, & as delimiters" do
        "//[%][&]\n1%2&3".should add_to 6
      end
    end
  end
end

@xaviuzz el add que proponías devuelve nil en vez de 0 cuando summands == []