module Memoizable
  # Usage:
  #
  # class Foo
  #   extend Memoizable
  #
  #   memoize def bar
  #     'bar'
  #   end
  # end

  def memoize(method_name)
    if interceptor.instance_variable_defined?(:"@memoized_#{method_name}")
      raise "The method :#{method_name} has already been memoized."
    end

    interceptor.send(:define_method, method_name) do |*args, &block|
      if instance_variable_defined?(:"@memoized_#{method_name}")
        return instance_variable_get(:"@memoized_#{method_name}")
      end

      instance_variable_set(:"@memoized_#{method_name}", super(*args, &block))
    end
  end

  private

  def interceptor
    existing_interceptor || create_interceptor
  end

  def interceptor_name
    'MemoizeInterceptor'
  end

  def existing_interceptor
    const_get(interceptor_name) if const_defined?(interceptor_name)
  end

  def create_interceptor
    prepend(const_set(interceptor_name, Module.new))
  end
end