class Gem::Validator

def alien(gems=[])
  errors = Hash.new { |h,k| h[k] = {} }
  Gem::Specification.each do |spec|
    next unless gems.include? spec.name unless gems.empty?
    next if spec.default_gem?
    gem_name      = spec.file_name
    gem_path      = spec.cache_file
    spec_path     = spec.spec_file
    gem_directory = spec.full_gem_path
    unless File.directory? gem_directory then
      errors[gem_name][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|
        package = Gem::Package.new gem_path
        good, gone = package.contents.partition { |file_name|
          File.exist? File.join(gem_directory, file_name)
        }
        gone.sort.each do |path|
          errors[gem_name][path] = "Missing file"
        end
        good, unreadable = good.partition { |file_name|
          File.readable? File.join(gem_directory, file_name)
        }
        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`
            source = File.join gem_directory, entry['path']
            open source, Gem.binary_mode do |f|
              unless f.read == data then
                errors[gem_name][entry['path']] = "Modified from original"
              end
            end
          end
        end
      end
      installed_files = find_files_for_gem(gem_directory)
      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 do |path, msg|
      ErrorData.new path, msg
    end.sort
  end
  errors
end