string_calculator.rb

require 'test/unit'

class StringCalculator
  def add(string)
    string.split(',').map(&:to_i).inject(:+) || 0
  end
end

class TestStringCalculator < Test::Unit::TestCase

  def add(string)
    StringCalculator.new.add(string)
  end

  def test_empty_string_returns_0
    assert_equal(0, add(""))
  end

  def test_one_number_returns_the_number
    assert_equal(5, add("5"))
  end

  def test_two_numbers_returns_sum
    assert_equal(8, add("5,3"))
  end

  def test_four_numbers_returns_sum
    assert_equal(100, add("5,3,12,80"))
  end
end