return int(uint(runtime_rand()) >> 1) // clear sign bit
}

func randIntn(n int) int {
	return randInt() % n
}

// reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP
// address addr suitable for rDNS (PTR) record lookup or an error if it fails
// to parse the IP address.
func reverseaddr(addr string) (arpa string, err error) {
	ip := ParseIP(addr)
	if ip == nil {

	if order, conf := systemConf().addrLookupOrder(r, addr); order != hostLookupCgo {
		return r.goLookupPTR(ctx, addr, order, conf)
	}
	arpa, err := reverseaddr(addr)
	if err != nil {
		return
	}
	lines, err := queryDNS(ctx, arpa, "ptr")
	if err != nil {
		return nil, handlePlan9DNSError(err, addr)
	}
	for _, line := range lines {
		f := getFields(line)

		return nil, &DNSError{
			Name:      addr,
			Err:       mapErr(err).Error(),
			IsTimeout: ctx.Err() == context.DeadlineExceeded,
		}
	}
	defer releaseThread()
	arpa, err := reverseaddr(addr)
	if err != nil {
		return nil, err
	}
	var rec *syscall.DNSRecord
	e := syscall.DnsQuery(arpa, syscall.DNS_TYPE_PTR, 0, nil, &rec, nil)
	if e != nil {

		names := lookupStaticAddr(addr)
		if len(names) > 0 {
			return names, nil
		}

		if order == hostLookupFiles {
			return nil, newDNSError(errNoSuchHost, addr, "")
		}
	}

	arpa, err := reverseaddr(addr)
	if err != nil {
		return nil, err
	}
	p, server, err := r.lookup(ctx, arpa, dnsmessage.TypePTR, conf)
	if err != nil {
		var dnsErr *DNSError
		if errors.As(err, &dnsErr) && dnsErr.IsNotFound {

	{"1234:567::bcbc:adad::89a:bcde", "", "unrecognized address"},
}

func TestReverseAddress(t *testing.T) {
	defer dnsWaitGroup.Wait()
	for i, tt := range revAddrTests {
		a, err := reverseaddr(tt.Addr)
		if len(tt.ErrPrefix) > 0 && err == nil {
			t.Errorf("#%d: expected %q, got <nil> (error)", i, tt.ErrPrefix)
			continue
		}
		if len(tt.ErrPrefix) == 0 && err != nil {