    # File lib/rubygems/validator.rb, line 79
        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)

            if(!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 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