// Copyright 2015 The Chihaya Authors. All rights reserved.
// Use of this source code is governed by the BSD 2-Clause license,
// which can be found in the LICENSE file.

package udp

import (
	"bytes"
	"encoding/binary"
	"net"

	"github.com/chihaya/chihaya/stats"
	"github.com/chihaya/chihaya/tracker/models"
)

// initialConnectionID is the magic initial connection ID specified by BEP 15.
var initialConnectionID = []byte{0, 0, 0x04, 0x17, 0x27, 0x10, 0x19, 0x80}

// eventIDs maps IDs to event names.
var eventIDs = []string{"", "completed", "started", "stopped"}

var (
	errMalformedPacket = models.ProtocolError("malformed packet")
	errMalformedIP     = models.ProtocolError("malformed IP address")
	errMalformedEvent  = models.ProtocolError("malformed event ID")
	errBadConnectionID = models.ProtocolError("bad connection ID")
)

// handleTorrentError writes err to w if err is a models.ClientError.
func handleTorrentError(err error, w *Writer) {
	if err == nil {
		return
	}

	if _, ok := err.(models.ClientError); ok {
		w.WriteError(err)
		stats.RecordEvent(stats.ClientError)
	}
}

// handlePacket decodes and processes one UDP request, returning the response.
func (s *Server) handlePacket(packet []byte, addr *net.UDPAddr) (response []byte, actionName string) {
	if len(packet) < 16 {
		return // Malformed, no client packets are less than 16 bytes.
	}

	connID := packet[0:8]
	action := binary.BigEndian.Uint32(packet[8:12])
	transactionID := packet[12:16]
	generatedConnID := GenerateConnectionID(addr.IP)

	writer := &Writer{
		buf: new(bytes.Buffer),

		connectionID:  connID,
		transactionID: transactionID,
	}

	defer func() {
		if writer.buf.Len() > 0 {
			response = writer.buf.Bytes()
		}
	}()

	if action != 0 && !bytes.Equal(connID, generatedConnID) {
		writer.WriteError(errBadConnectionID)
		return
	}

	switch action {
	case 0:
		actionName = "connect"
		if !bytes.Equal(connID, initialConnectionID) {
			return // Malformed packet.
		}

		writer.writeHeader(0)
		writer.buf.Write(generatedConnID)

	case 1:
		actionName = "announce"
		ann, err := s.newAnnounce(packet, addr.IP)

		if err == nil {
			err = s.tracker.HandleAnnounce(ann, writer)
		}

		handleTorrentError(err, writer)

	case 2:
		actionName = "scrape"
		scrape, err := s.newScrape(packet)

		if err == nil {
			err = s.tracker.HandleScrape(scrape, writer)
		}

		handleTorrentError(err, writer)
	}

	return
}

// newAnnounce decodes one announce packet, returning a models.Announce.
func (s *Server) newAnnounce(packet []byte, ip net.IP) (*models.Announce, error) {
	if len(packet) < 98 {
		return nil, errMalformedPacket
	}

	infohash := packet[16:36]
	peerID := packet[36:56]

	downloaded := binary.BigEndian.Uint64(packet[56:64])
	left := binary.BigEndian.Uint64(packet[64:72])
	uploaded := binary.BigEndian.Uint64(packet[72:80])

	eventID := packet[83]
	if eventID > 3 {
		return nil, errMalformedEvent
	}

	ipbuf := packet[84:88]
	if !bytes.Equal(ipbuf, []byte{0, 0, 0, 0}) {
		ip = net.ParseIP(string(ipbuf))
	}
	if ip == nil {
		return nil, errMalformedIP
	}
	if ipv4 := ip.To4(); ipv4 != nil {
		ip = ipv4
	}

	// TODO(pushrax): what exactly is the key "key" used for?

	numWant := binary.BigEndian.Uint32(packet[92:96])
	port := binary.BigEndian.Uint16(packet[96:98])

	return &models.Announce{
		Config:     s.config,
		Downloaded: downloaded,
		Event:      eventIDs[eventID],
		IPv4:       ip,
		Infohash:   string(infohash),
		Left:       left,
		NumWant:    int(numWant),
		PeerID:     string(peerID),
		Port:       port,
		Uploaded:   uploaded,
	}, nil
}

// newScrape decodes one announce packet, returning a models.Scrape.
func (s *Server) newScrape(packet []byte) (*models.Scrape, error) {
	if len(packet) < 36 {
		return nil, errMalformedPacket
	}

	var infohashes []string
	packet = packet[16:]

	if len(packet)%20 != 0 {
		return nil, errMalformedPacket
	}

	for len(packet) >= 20 {
		infohash := packet[:20]
		infohashes = append(infohashes, string(infohash))
		packet = packet[20:]
	}

	return &models.Scrape{
		Config:     s.config,
		Infohashes: infohashes,
	}, nil
}