troessner/reek

# frozen_string_literal: true

require_relative '../ast/reference_collector'
require_relative 'base_detector'

module Reek
  module SmellDetectors
    #
    # A Utility Function is any instance method that has no
    # dependency on the state of the instance.
    #
    # Currently +UtilityFunction+ will warn about any method that:
    #
    # * is non-empty, and
    # * does not override an inherited method, and
    # * calls at least one method on another object, and
    # * doesn't use any of self's instance variables, and
    # * doesn't use any of self's methods
    #
    # A Utility Function often arises because it must manipulate
    # other objects (usually its arguments) to get them into a
    # useful form; one force preventing them (the
    # arguments) doing this themselves is that the common
    # knowledge lives outside the arguments, or the arguments
    # are of too basic a type to justify extending that type.
    # Therefore there must be something which 'knows' about the contents
    # or purposes of the arguments.  That thing would have to
    # be more than just a basic type, because the basic types
    # are either containers which don't know about their
    # contents, or they are single objects which can't capture
    # their relationship with their fellows of the same type.
    # So, this thing with the extra knowledge should be
    # reified into a class, and the utility method will most
    # likely belong there.
    #
    # If the method does refer to self, but refers to some other object more,
    # +FeatureEnvy+ is reported instead.
    #
    # See {file:docs/Utility-Function.md} for details.
    class UtilityFunction < BaseDetector
      PUBLIC_METHODS_ONLY_KEY     = 'public_methods_only'
      PUBLIC_METHODS_ONLY_DEFAULT = false

      def self.default_config
        super.merge(PUBLIC_METHODS_ONLY_KEY => PUBLIC_METHODS_ONLY_DEFAULT)
      end

      class << self
        def contexts # :nodoc:
          [:def]
        end
      end

      #
      # Checks whether the given +method+ is a utility function.
      #
      # @return [Array<SmellWarning>]
      #
      def sniff
        return [] if context.singleton_method? || context.module_function?
        return [] if context.references_self?
        return [] if num_helper_methods.zero?
        return [] if ignore_method?

        [smell_warning(
          lines: [source_line],
          message: "doesn't depend on instance state (maybe move it to another class?)")]
      end

      private

      def num_helper_methods
        context.local_nodes(:send).to_a.length
      end

      def ignore_method?
        context.non_public_visibility? &&
          value(PUBLIC_METHODS_ONLY_KEY, context)
      end
    end
  end
end