recursion_guard.rb

require 'set'

module SuperDiff
  module RecursionGuard
    RECURSION_GUARD_KEY = "super_diff_recursion_guard_key".freeze
    PLACEHOLDER = "∙∙∙".freeze

    def self.guarding_recursion_of(*objects, &block)
      already_seen_objects, first_seen_objects = objects.partition do |object|
        !SuperDiff.primitive?(object) && already_seen?(object)
      end

      first_seen_objects.each do |object|
        already_seen_object_ids.add(object.object_id)
      end

      result =
        if block.arity > 0
          block.call(already_seen_objects.any?)
        else
          block.call
        end

      first_seen_objects.each do |object|
        already_seen_object_ids.delete(object.object_id)
      end

      result
    end

    def self.substituting_recursion_of(*objects)
      guarding_recursion_of(*objects) do |already_seen|
        if already_seen
          PLACEHOLDER
        else
          yield
        end
      end
    end

    def self.already_seen?(object)
      already_seen_object_ids.include?(object.object_id)
    end

    def self.already_seen_object_ids
      Thread.current[RECURSION_GUARD_KEY] ||= Set.new
    end
  end
end