package dns

// Envelope is used when doing [IA]XFR with a remote server.
type Envelope struct {
	RR    []RR  // The set of RRs in the answer section of the AXFR reply message.
	Error error // If something went wrong, this contains the error.
}

// TransferIn performs a [AI]XFR request (depends on the message's Qtype). It returns
// a channel of *Envelope on which the replies from the server are sent. At the end of
// the transfer the channel is closed.
// The messages are TSIG checked if
// needed, no other post-processing is performed. The caller must dissect the returned
// messages.
//
// Basic use pattern for receiving an AXFR:
//
//	// m contains the AXFR request
//	t, e := c.TransferIn(m, "127.0.0.1:53")
//	for r := range t {
//		// ... deal with r.RR or r.Error
//	}
func (c *Client) TransferIn(q *Msg, a string) (chan *Envelope, error) {
	w := new(reply)
	w.client = c
	w.addr = a
	w.req = q
	if err := w.dial(); err != nil {
		return nil, err
	}
	if err := w.send(q); err != nil {
		return nil, err
	}
	e := make(chan *Envelope)
	switch q.Question[0].Qtype {
	case TypeAXFR:
		go w.axfrIn(q, e)
		return e, nil
	case TypeIXFR:
		go w.ixfrIn(q, e)
		return e, nil
	default:
		return nil, nil
	}
	panic("dns: not reached")
}

func (w *reply) axfrIn(q *Msg, c chan *Envelope) {
	first := true
	defer w.conn.Close()
	defer close(c)
	for {
		in, err := w.receive()
		if err != nil {
			c <- &Envelope{nil, err}
			return
		}
		if in.Id != q.Id {
			c <- &Envelope{in.Answer, ErrId}
			return
		}
		if first {
			if !checkXfrSOA(in, true) {
				c <- &Envelope{in.Answer, ErrSoa}
				return
			}
			first = !first
		}

		if !first {
			w.tsigTimersOnly = true // Subsequent envelopes use this.
			if checkXfrSOA(in, false) {
				c <- &Envelope{in.Answer, nil}
				return
			}
			c <- &Envelope{in.Answer, nil}
		}
	}
	panic("dns: not reached")
}

func (w *reply) ixfrIn(q *Msg, c chan *Envelope) {
	var serial uint32 // The first serial seen is the current server serial
	first := true
	defer w.conn.Close()
	defer close(c)
	for {
		in, err := w.receive()
		if err != nil {
			c <- &Envelope{in.Answer, err}
			return
		}
		if q.Id != in.Id {
			c <- &Envelope{in.Answer, ErrId}
			return
		}
		if first {
			// A single SOA RR signals "no changes"
			if len(in.Answer) == 1 && checkXfrSOA(in, true) {
				c <- &Envelope{in.Answer, nil}
				return
			}

			// Check if the returned answer is ok
			if !checkXfrSOA(in, true) {
				c <- &Envelope{in.Answer, ErrSoa}
				return
			}
			// This serial is important
			serial = in.Answer[0].(*SOA).Serial
			first = !first
		}

		// Now we need to check each message for SOA records, to see what we need to do
		if !first {
			w.tsigTimersOnly = true
			// If the last record in the IXFR contains the servers' SOA,  we should quit
			if v, ok := in.Answer[len(in.Answer)-1].(*SOA); ok {
				if v.Serial == serial {
					c <- &Envelope{in.Answer, nil}
					return
				}
			}
			c <- &Envelope{in.Answer, nil}
		}
	}
	panic("dns: not reached")
}

// Check if he SOA record exists in the Answer section of 
// the packet. If first is true the first RR must be a SOA
// if false, the last one should be a SOA.
func checkXfrSOA(in *Msg, first bool) bool {
	if len(in.Answer) > 0 {
		if first {
			return in.Answer[0].Header().Rrtype == TypeSOA
		} else {
			return in.Answer[len(in.Answer)-1].Header().Rrtype == TypeSOA
		}
	}
	return false
}

// TransferOut performs an outgoing [AI]XFR depending on the request message. The
// caller is responsible for sending the correct sequence of RR sets through
// the channel c. For reasons of symmetry Envelope is re-used.
// Errors are signaled via the error pointer, when an error occurs the function
// sets the error and returns (it does not close the channel).
// TSIG and enveloping is handled by TransferOut.
// 
// Basic use pattern for sending an AXFR:
//
//	// q contains the AXFR request
//	c := make(chan *Envelope)
//	var e *error
//	err := TransferOut(w, q, c, e)
//	w.Hijack()		// hijack the connection so that the package doesn't close it
//	for _, rrset := range rrsets {	// rrsets is a []RR
//		c <- &{Envelope{RR: rrset}
//		if e != nil {
//			close(c)
//			break
//		}
//	}
//	// w.Close() // Don't! Let the client close the connection
func TransferOut(w ResponseWriter, q *Msg, c chan *Envelope, e *error) error {
	switch q.Question[0].Qtype {
	case TypeAXFR, TypeIXFR:
		go xfrOut(w, q, c, e)
		return nil
	default:
		return nil
	}
	panic("dns: not reached")
}

// TODO(mg): count the RRs and the resulting size.
func xfrOut(w ResponseWriter, req *Msg, c chan *Envelope, e *error) {
	rep := new(Msg)
	rep.SetReply(req)
	rep.Authoritative = true

	for x := range c {
		// assume it fits
		rep.Answer = append(rep.Answer, x.RR...)
		if err := w.WriteMsg(rep); e != nil {
			*e = err
			return
		}
		w.TsigTimersOnly(true)
		rep.Answer = nil
	}
}