module SubstAttr
  module Substitute
    extend self

    def build(interface=nil)
      if interface
        specialization = specialization(interface, :Substitute)
        return specialization if specialization
      end

      return NullObject.build interface
    end

    def call(attr_name, receiver)
      interface = receiver.send(attr_name).class
      substitute = build(interface)
      receiver.send :"#{attr_name}=", substitute
    end

    def specialization(interface, const_name)
      unless interface.const_defined?(const_name)
        return nil
      end

      mod = interface.const_get(const_name)

      unless mod.respond_to?(:build)
        return nil
      end

      mod.send :build
    end

    module NullObject
      extend self

      Weak = Naught.build do |config|
        config.define_explicit_conversions
        config.define_implicit_conversions
        config.predicates_return false
        config.singleton
      end

      def build(interface=nil)
        if interface
          specialization = Substitute.specialization(interface, :NullObject)

          if specialization
            return specialization
          else
            return strict interface
          end
        end

        weak
      end

      def weak
        Weak.get
      end

      def strict(interface)
        Naught.build do |config|
          config.singleton
          config.impersonate interface
        end.get
      end
    end
  end
end