module Digest::UUID

def self.pack_uuid_namespace(namespace)
  if [DNS_NAMESPACE, OID_NAMESPACE, URL_NAMESPACE, X500_NAMESPACE].include?(namespace)
    namespace
  else
    match_data = namespace.match(/\A(\h{8})-(\h{4})-(\h{4})-(\h{4})-(\h{4})(\h{8})\z/)
    raise ArgumentError, "Only UUIDs are valid namespace identifiers" unless match_data.present?
    match_data.captures.map { |s| s.to_i(16) }.pack("NnnnnN")
  end
end