alien (Gem::Validator)

    # File lib/rubygems/validator.rb, line 84
      def alien
        errors = {}

        Gem::SourceIndex.from_installed_gems.each do |gem_name, gem_spec|
          errors[gem_name] ||= []

          gem_path = File.join(Gem.dir, "cache", gem_spec.full_name) + ".gem"
          spec_path = File.join(Gem.dir, "specifications", gem_spec.full_name) + ".gemspec"
          gem_directory = File.join(Gem.dir, "gems", gem_spec.full_name)

          installed_files = find_files_for_gem(gem_directory)

          unless File.exist? spec_path then
            errors[gem_name] << ErrorData.new(spec_path, "Spec file doesn't exist for installed gem")
          end

          begin
            verify_gem_file(gem_path)

            open gem_path, Gem.binary_mode do |file|
              format = Gem::Format.from_file_by_path(gem_path)
              format.file_entries.each do |entry, data|
                # Found this file.  Delete it from list
                installed_files.delete remove_leading_dot_dir(entry['path'])

                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] << ErrorData.new(entry['path'], "installed file doesn't match original from gem")
                  end
                end
              end
            end
          rescue Gem::VerificationError => e
            errors[gem_name] << ErrorData.new(gem_path, e.message)
          end

          # Clean out directories that weren't explicitly included in the gemspec
          # FIXME: This still allows arbitrary incorrect directories.
          installed_files.delete_if {|potential_directory|
            File.directory?(File.join(gem_directory, potential_directory))
          }
          if(installed_files.size > 0) then
            errors[gem_name] << ErrorData.new(gem_path, "Unmanaged files in gem: #{installed_files.inspect}")
          end
        end

        errors
      end