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 = Gem.cache_gem(gem_spec.file_name, install_dir)
      spec_path = File.join install_dir, "specifications", gem_spec.spec_name
      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 Digest::MD5.hexdigest(f.read).to_s ==
                    Digest::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
#
Checks the gem directory for the following potential
inconsistencies/problems:

* Checksum gem itself
* For each file in each gem, check consistency of installed versions
* Check for files that aren't part of the gem but are in the gems directory
* 1 cache - 1 spec - 1 directory.

returns a hash of ErrorData objects, keyed on the problem gem's name.