interface_checker.rb

RSpec.shared_examples_for "interface checker" do
  def diff_for(array_1, array_2)
    (array_1 - array_2) + (array_2 - array_1)
  end

  def arities_for(object)
    object.public_methods.map do |m|
      arity = object.method(m).arity
      [m, arity == -1 ? anything : arity]
    end
  end

  it "all objects abide by the same interface" do
    objects.each_cons(2) do |left, right|
      left_methods = left.public_methods
      right_methods = right.public_methods

      diff = diff_for(left_methods, right_methods)

      expect(left_methods).to(
        match_array(right_methods), (<<~MESSAGE).split("\n").join(" ")
          Expected #{left.class} to have the same
          interface as #{right.class}, however the
          following extra methods were implemented: #{diff}
        MESSAGE
      )
    end
  end

  it "all methods have the same arity" do
    objects.each_cons(2) do |left, right|
      left_arities = arities_for(left)
      right_arities = arities_for(right)

      diff = diff_for(left_arities, right_arities)

      expect(left_arities).to(
        match_array(right_arities), (<<~MESSAGE).split("\n").join(" ")
          Expected #{left.class} to have the methods
          with the same arity as #{right.class}, however the
          following arities do not match: #{diff}
        MESSAGE
      )
    end
  end
end