module Virtus

  # A module that adds type lookup to a class
  module TypeLookup

    TYPE_FORMAT      = /\A[A-Z]\w*\z/.freeze
    EXTRA_CONST_ARGS = RUBY_VERSION < '1.9' || RUBY_ENGINE == 'rbx' ? [] : [ false ]

    # Returns a descendant based on a name or class
    #
    # @example
    #   MyClass.determine_type('String')  # => MyClass::String
    #
    # @param [Class, #to_s] class_or_name
    #   name of a class or a class itself
    #
    # @return [Class]
    #   a descendant
    #
    # @return [nil]
    #   nil if the type cannot be determined by the class_or_name
    #
    # @api public
    def determine_type(class_or_name)
      case class_or_name
      when singleton_class
        determine_type_from_descendant(class_or_name)
      when Class
        determine_type_from_primitive(class_or_name)
      else
        determine_type_from_string(class_or_name.to_s)
      end
    end

    # Return the default primitive supported
    #
    # @return [Class]
    #
    # @api private
    def primitive
      raise NotImplementedError, "#{self}.primitive must be implemented"
    end

  private

    # Return the class given a descendant
    #
    # @param [Class] descendant
    #
    # @return [Class]
    #
    # @api private
    def determine_type_from_descendant(descendant)
      descendant if descendant < self
    end

    # Return the class given a primitive
    #
    # @param [Class] primitive
    #
    # @return [Class]
    #
    # @return [nil]
    #   nil if the type cannot be determined by the primitive
    #
    # @api private
    def determine_type_from_primitive(primitive)
      type = nil
      descendants.reverse_each do |descendant|
        descendant_primitive = descendant.primitive
        next unless primitive <= descendant_primitive
        type = descendant if type.nil? || type.primitive > descendant_primitive
      end
      type
    end

    # Return the class given a string
    #
    # @param [String] string
    #
    # @return [Class]
    #
    # @return [nil]
    #   nil if the type cannot be determined by the string
    #
    # @api private
    def determine_type_from_string(string)
      if string =~ TYPE_FORMAT && const_defined?(string, *EXTRA_CONST_ARGS)
        const_get(string, *EXTRA_CONST_ARGS)
      end
    end

  end # module TypeLookup
end # module Virtus