Very simple Mutator for the concept of Mutation-Testing in Ruby! Just for fun :)

mutator.rb

class Mutator
  def initialize(ruby_file)
    @ruby_file = ruby_file
  end

  def run_test
    spec_file_name = @ruby_file.split(/\./)[0]
    system("rspec #{spec_file_name}_spec.rb")
  end

  def mutate_code
    mutated = mutate_bigger_than_comparisons
    File.open(@ruby_file, "w") { |f| f.print(mutated) }
  end

  def unmutate_code
    unmutated = unmutate_bigger_than_comparisons
    File.open(@ruby_file, "w") { |f| f.print(unmutated) }
  end

  private
    def mutate_bigger_than_comparisons
      ruby_code = get_content
      ruby_code.gsub(">", "<")
    end

    def unmutate_bigger_than_comparisons
      ruby_code = get_content
      ruby_code.gsub("<", ">")
    end

    def get_content
      File.open(@ruby_file, "r") { |f| f.read }
    end
end

mutator = Mutator.new("number_descriptor.rb")
mutator.mutate_code
mutator.run_test
mutator.unmutate_code
mutator.run_test

number_descriptor.rb

def describe_number(num)
  return "Big" if (num > 1000)
  "Small"
end

describe "Number Description" do
  it "says Big for number bigger than 1000" do
    describe_number(2000).should == "Big"
  end

  it "says Small for number smaller than 1000" do
    describe_number(500).should == "Small"
  end
end

If you're interested in mutation testing you should check out https://github.com/mbj/mutant/