alien (Gem::Validator)

    # File lib/rubygems/validator.rb, line 89
      def alien(gems=[])
        errors = Hash.new { |h,k| h[k] = {} }

        Gem::SourceIndex.from_installed_gems.each do |gem_name, gem_spec|
          next unless gems.include? gem_spec.name unless gems.empty?

          install_dir = gem_spec.installation_path
          gem_path = File.join(install_dir, "cache", gem_spec.full_name) + ".gem"
          spec_path = File.join(install_dir, "specifications",
                                gem_spec.full_name) + ".gemspec"
          gem_directory = gem_spec.full_gem_path

          unless File.directory? gem_directory then
            errors[gem_name][gem_spec.full_name] =
              "Gem registered but doesn't exist at #{gem_directory}"
            next
          end

          unless File.exist? spec_path then
            errors[gem_name][spec_path] = "Spec file missing for installed gem"
          end

          begin
            verify_gem_file(gem_path)

            good, gone, unreadable = nil, nil, nil, nil

            open gem_path, Gem.binary_mode do |file|
              format = Gem::Format.from_file_by_path(gem_path)

              good, gone = format.file_entries.partition { |entry, _|
                File.exist? File.join(gem_directory, entry['path'])
              }

              gone.map! { |entry, _| entry['path'] }
              gone.sort.each do |path|
                errors[gem_name][path] = "Missing file"
              end

              good, unreadable = good.partition { |entry, _|
                File.readable? File.join(gem_directory, entry['path'])
              }

              unreadable.map! { |entry, _| entry['path'] }
              unreadable.sort.each do |path|
                errors[gem_name][path] = "Unreadable file"
              end

              good.each do |entry, data|
                begin
                  next unless data # HACK `gem check -a mkrf`

                  open File.join(gem_directory, entry['path']), Gem.binary_mode do |f|
                    unless Gem::MD5.hexdigest(f.read).to_s ==
                        Gem::MD5.hexdigest(data).to_s then
                      errors[gem_name][entry['path']] = "Modified from original"
                    end
                  end
                end
              end
            end

            installed_files = find_files_for_gem(gem_directory)
            good.map! { |entry, _| entry['path'] }
            extras = installed_files - good - unreadable

            extras.each do |extra|
              errors[gem_name][extra] = "Extra file"
            end
          rescue Gem::VerificationError => e
            errors[gem_name][gem_path] = e.message
          end
        end

        errors.each do |name, subhash|
          errors[name] = subhash.map { |path, msg| ErrorData.new(path, msg) }
        end

        errors
      end