FedP2P/dht/dht.go

package dht

import (
	"crypto"
	_ "crypto/sha1"
	"encoding/binary"
	"errors"
	"fmt"
	"io"
	"log"
	"math/big"
	"net"
	"os"
	"sync"
	"time"

	"bitbucket.org/anacrolix/go.torrent/logonce"
	"bitbucket.org/anacrolix/go.torrent/util"
	"github.com/anacrolix/libtorgo/bencode"
)

type Server struct {
	id               string
	socket           net.PacketConn
	transactions     []*transaction
	transactionIDInt uint64
	nodes            map[string]*Node // Keyed by dHTAddr.String().
	mu               sync.Mutex
	closed           chan struct{}

	NumConfirmedAnnounces int
}

type dHTAddr interface {
	net.Addr
}

func newDHTAddr(addr *net.UDPAddr) (ret dHTAddr) {
	ret = addr
	return
}

type ServerConfig struct {
	Addr string
	Conn net.PacketConn
}

func (s *Server) LocalAddr() net.Addr {
	return s.socket.LocalAddr()
}

func makeSocket(addr string) (socket *net.UDPConn, err error) {
	addr_, err := net.ResolveUDPAddr("", addr)
	if err != nil {
		return
	}
	socket, err = net.ListenUDP("udp", addr_)
	return
}

func NewServer(c *ServerConfig) (s *Server, err error) {
	if c == nil {
		c = &ServerConfig{}
	}
	s = &Server{}
	if c.Conn != nil {
		s.socket = c.Conn
	} else {
		s.socket, err = makeSocket(c.Addr)
		if err != nil {
			return
		}
	}
	err = s.init()
	if err != nil {
		return
	}
	go func() {
		err := s.serve()
		select {
		case <-s.closed:
			return
		default:
		}
		if err != nil {
			panic(err)
		}
	}()
	go func() {
		err := s.bootstrap()
		if err != nil {
			panic(err)
		}
	}()
	return
}

func (s *Server) String() string {
	return fmt.Sprintf("dht server on %s", s.socket.LocalAddr())
}

type Node struct {
	addr          dHTAddr
	id            string
	lastHeardFrom time.Time
	lastSentTo    time.Time
	announceToken string
}

func (n *Node) NodeInfo() (ret NodeInfo) {
	ret.Addr = n.addr
	if n := copy(ret.ID[:], n.id); n != 20 {
		panic(n)
	}
	return
}

func (n *Node) Good() bool {
	if len(n.id) != 20 {
		return false
	}
	if n.lastSentTo.IsZero() {
		return true
	}
	if n.lastSentTo.Before(n.lastHeardFrom) {
		return true
	}
	if time.Now().Sub(n.lastHeardFrom) >= 1*time.Minute {
		return false
	}
	return true
}

type Msg map[string]interface{}

var _ fmt.Stringer = Msg{}

func (m Msg) String() string {
	return fmt.Sprintf("%#v", m)
}

func (m Msg) T() (t string) {
	tif, ok := m["t"]
	if !ok {
		return
	}
	t, _ = tif.(string)
	return
}

func (m Msg) Nodes() []NodeInfo {
	var r findNodeResponse
	if err := r.UnmarshalKRPCMsg(m); err != nil {
		return nil
	}
	return r.Nodes
}

type KRPCError struct {
	Code int
	Msg  string
}

func (me KRPCError) Error() string {
	return fmt.Sprintf("KRPC error %d: %s", me.Code, me.Msg)
}

var _ error = KRPCError{}

func (m Msg) Error() (ret *KRPCError) {
	if m["y"] != "e" {
		return
	}
	ret = &KRPCError{}
	switch e := m["e"].(type) {
	case []interface{}:
		ret.Code = int(e[0].(int64))
		ret.Msg = e[1].(string)
	case string:
		ret.Msg = e
	default:
		logonce.Stderr.Printf(`KRPC error "e" value has unexpected type: %T`, e)
	}
	return
}

// Returns the token given in response to a get_peers request for future
// announce_peer requests to that node.
func (m Msg) AnnounceToken() string {
	defer func() { recover() }()
	return m["r"].(map[string]interface{})["token"].(string)
}

type transaction struct {
	mu         sync.Mutex
	remoteAddr dHTAddr
	t          string
	Response   chan Msg
	onResponse func(Msg)
	done       chan struct{}
}

func (t *transaction) timeout() {
	t.Close()
}

func (t *transaction) closing() bool {
	select {
	case <-t.done:
		return true
	default:
		return false
	}
}

func (t *transaction) Close() {
	t.mu.Lock()
	defer t.mu.Unlock()
	if t.closing() {
		return
	}
	close(t.Response)
	close(t.done)
}

func (t *transaction) handleResponse(m Msg) {
	t.mu.Lock()
	if t.closing() {
		t.mu.Unlock()
		return
	}
	close(t.done)
	t.mu.Unlock()
	if t.onResponse != nil {
		t.onResponse(m)
	}
	select {
	case t.Response <- m:
	default:
		panic("blocked handling response")
	}
	close(t.Response)
}

func (s *Server) setDefaults() (err error) {
	if s.id == "" {
		var id [20]byte
		h := crypto.SHA1.New()
		ss, err := os.Hostname()
		if err != nil {
			log.Print(err)
		}
		ss += s.socket.LocalAddr().String()
		h.Write([]byte(ss))
		if b := h.Sum(id[:0:20]); len(b) != 20 {
			panic(len(b))
		}
		if len(id) != 20 {
			panic(len(id))
		}
		s.id = string(id[:])
	}
	s.nodes = make(map[string]*Node, 10000)
	return
}

func (s *Server) init() (err error) {
	err = s.setDefaults()
	if err != nil {
		return
	}
	s.closed = make(chan struct{})
	return
}

func (s *Server) processPacket(b []byte, addr dHTAddr) {
	var d Msg
	err := bencode.Unmarshal(b, &d)
	if err != nil {
		if se, ok := err.(*bencode.SyntaxError); !ok || se.Offset != 0 {
			log.Printf("%s: received bad krpc message: %s: %q", s, err, b)
		}
		return
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	if d["y"] == "q" {
		s.handleQuery(addr, d)
		return
	}
	t := s.findResponseTransaction(d.T(), addr)
	if t == nil {
		//log.Printf("unexpected message: %#v", d)
		return
	}
	t.handleResponse(d)
	s.removeTransaction(t)
	id := ""
	if d["y"] == "r" {
		id = d["r"].(map[string]interface{})["id"].(string)
	}
	s.heardFromNode(addr, id)
}

func (s *Server) serve() error {
	for {
		var b [0x10000]byte
		n, addr, err := s.socket.ReadFrom(b[:])
		if err != nil {
			return err
		}
		s.processPacket(b[:n], newDHTAddr(addr.(*net.UDPAddr)))
	}
}

func (s *Server) AddNode(ni NodeInfo) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.nodes == nil {
		s.nodes = make(map[string]*Node)
	}
	n := s.getNode(ni.Addr)
	if n.id == "" {
		n.id = string(ni.ID[:])
	}
}

func (s *Server) nodeByID(id string) *Node {
	for _, node := range s.nodes {
		if node.id == id {
			return node
		}
	}
	return nil
}

func (s *Server) handleQuery(source dHTAddr, m Msg) {
	args := m["a"].(map[string]interface{})
	s.heardFromNode(source, args["id"].(string))
	switch m["q"] {
	case "ping":
		s.reply(source, m["t"].(string), nil)
	case "get_peers":
		targetID := args["info_hash"].(string)
		var rNodes []NodeInfo
		// TODO: Reply with "values" list if we have peers instead.
		for _, node := range s.closestGoodNodes(8, targetID) {
			rNodes = append(rNodes, node.NodeInfo())
		}
		nodesBytes := make([]byte, CompactNodeInfoLen*len(rNodes))
		for i, ni := range rNodes {
			err := ni.PutCompact(nodesBytes[i*CompactNodeInfoLen : (i+1)*CompactNodeInfoLen])
			if err != nil {
				panic(err)
			}
		}
		s.reply(source, m["t"].(string), map[string]interface{}{
			"nodes": string(nodesBytes),
			"token": "hi",
		})
	case "find_node":
		targetID := args["target"].(string)
		if len(targetID) != 20 {
			log.Printf("bad DHT query: %v", m)
			return
		}
		var rNodes []NodeInfo
		if node := s.nodeByID(targetID); node != nil {
			rNodes = append(rNodes, node.NodeInfo())
		} else {
			for _, node := range s.closestGoodNodes(8, targetID) {
				rNodes = append(rNodes, node.NodeInfo())
			}
		}
		nodesBytes := make([]byte, CompactNodeInfoLen*len(rNodes))
		for i, ni := range rNodes {
			err := ni.PutCompact(nodesBytes[i*CompactNodeInfoLen : (i+1)*CompactNodeInfoLen])
			if err != nil {
				panic(err)
			}
		}
		s.reply(source, m["t"].(string), map[string]interface{}{
			"nodes": string(nodesBytes),
		})
	case "announce_peer":
		// TODO(anacrolix): Implement this lolz.
		// log.Print(m)
	case "vote":
		// TODO(anacrolix): Or reject, I don't think I want this.
	default:
		log.Printf("%s: not handling received query: q=%s", s, m["q"])
		return
	}
}

func (s *Server) reply(addr dHTAddr, t string, r map[string]interface{}) {
	if r == nil {
		r = make(map[string]interface{}, 1)
	}
	r["id"] = s.IDString()
	m := map[string]interface{}{
		"t": t,
		"y": "r",
		"r": r,
	}
	b, err := bencode.Marshal(m)
	if err != nil {
		panic(err)
	}
	err = s.writeToNode(b, addr)
	if err != nil {
		panic(err)
	}
}

func (s *Server) heardFromNode(addr dHTAddr, id string) {
	n := s.getNode(addr)
	if len(id) == 20 {
		n.id = id
	}
	n.lastHeardFrom = time.Now()
}

func (s *Server) getNode(addr dHTAddr) (n *Node) {
	if util.AddrPort(addr) == 0 {
		panic(addr)
	}
	n = s.nodes[addr.String()]
	if n == nil {
		n = &Node{
			addr: addr,
		}
		s.nodes[addr.String()] = n
	}
	return
}

func (s *Server) writeToNode(b []byte, node dHTAddr) (err error) {
	n, err := s.socket.WriteTo(b, node)
	if err != nil {
		err = fmt.Errorf("error writing %d bytes to %s: %s", len(b), node, err)
		return
	}
	if n != len(b) {
		err = io.ErrShortWrite
		return
	}
	s.sentToNode(node)
	return
}

func (s *Server) sentToNode(addr dHTAddr) {
	n := s.getNode(addr)
	n.lastSentTo = time.Now()
}

func (s *Server) findResponseTransaction(transactionID string, sourceNode dHTAddr) *transaction {
	for _, t := range s.transactions {
		if t.t == transactionID && t.remoteAddr.String() == sourceNode.String() {
			return t
		}
	}
	return nil
}

func (s *Server) nextTransactionID() string {
	var b [binary.MaxVarintLen64]byte
	n := binary.PutUvarint(b[:], s.transactionIDInt)
	s.transactionIDInt++
	return string(b[:n])
}

func (s *Server) removeTransaction(t *transaction) {
	for i, tt := range s.transactions {
		if t == tt {
			last := len(s.transactions) - 1
			s.transactions[i] = s.transactions[last]
			s.transactions = s.transactions[:last]
			return
		}
	}
	panic("transaction not found")
}

func (s *Server) addTransaction(t *transaction) {
	s.transactions = append(s.transactions, t)
}

func (s *Server) IDString() string {
	if len(s.id) != 20 {
		panic("bad node id")
	}
	return s.id
}

func (s *Server) timeoutTransaction(t *transaction) {
	select {
	case <-t.done:
		return
	case <-time.After(time.Minute):
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	select {
	case <-t.done:
		return
	default:
	}
	t.timeout()
	s.removeTransaction(t)
}

func (s *Server) query(node dHTAddr, q string, a map[string]interface{}) (t *transaction, err error) {
	tid := s.nextTransactionID()
	if a == nil {
		a = make(map[string]interface{}, 1)
	}
	a["id"] = s.IDString()
	d := map[string]interface{}{
		"t": tid,
		"y": "q",
		"q": q,
		"a": a,
	}
	b, err := bencode.Marshal(d)
	if err != nil {
		return
	}
	t = &transaction{
		remoteAddr: node,
		t:          tid,
		Response:   make(chan Msg, 1),
		done:       make(chan struct{}),
	}
	s.addTransaction(t)
	err = s.writeToNode(b, node)
	if err != nil {
		s.removeTransaction(t)
		return
	}
	go s.timeoutTransaction(t)
	return
}

const CompactNodeInfoLen = 26

type NodeInfo struct {
	ID   [20]byte
	Addr dHTAddr
}

func (ni *NodeInfo) PutCompact(b []byte) error {
	if n := copy(b[:], ni.ID[:]); n != 20 {
		panic(n)
	}
	ip := util.AddrIP(ni.Addr).To4()
	if len(ip) != 4 {
		panic(ip)
	}
	if n := copy(b[20:], ip); n != 4 {
		panic(n)
	}
	binary.BigEndian.PutUint16(b[24:], uint16(util.AddrPort(ni.Addr)))
	return nil
}

func (cni *NodeInfo) UnmarshalCompact(b []byte) error {
	if len(b) != 26 {
		return errors.New("expected 26 bytes")
	}
	if 20 != copy(cni.ID[:], b[:20]) {
		panic("impossibru!")
	}
	cni.Addr = newDHTAddr(&net.UDPAddr{
		IP:   net.IPv4(b[20], b[21], b[22], b[23]),
		Port: int(binary.BigEndian.Uint16(b[24:26])),
	})
	return nil
}

func (s *Server) Ping(node *net.UDPAddr) (*transaction, error) {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.query(newDHTAddr(node), "ping", nil)
}

// Announce a local peer. This can only be done to nodes that gave us an
// announce token, which is received in responses during GetPeers. It's
// recommended then that GetPeers is called before this method.
func (s *Server) AnnouncePeer(port int, impliedPort bool, infoHash string) (err error) {
	s.mu.Lock()
	defer s.mu.Unlock()
	for _, node := range s.closestNodes(160, infoHash, func(n *Node) bool {
		return n.Good() && n.announceToken != ""
	}) {
		err = s.announcePeer(node.addr, infoHash, port, node.announceToken, impliedPort)
		if err != nil {
			break
		}
	}
	return
}

func (s *Server) announcePeer(node dHTAddr, infoHash string, port int, token string, impliedPort bool) error {
	t, err := s.query(node, "announce_peer", map[string]interface{}{
		"implied_port": func() int {
			if impliedPort {
				return 1
			} else {
				return 0
			}
		}(),
		"info_hash": infoHash,
		"port":      port,
		"token":     token,
	})
	t.setOnResponse(func(m Msg) {
		if err := m.Error(); err != nil {
			logonce.Stderr.Printf("announce_peer response: %s", err)
			return
		}
		s.NumConfirmedAnnounces++
	})
	return err
}

type findNodeResponse struct {
	Nodes []NodeInfo
}

func getResponseNodes(m Msg) (s string, err error) {
	defer func() {
		r := recover()
		if r == nil {
			return
		}
		err = fmt.Errorf("couldn't get response nodes: %s: %#v", r, m)
	}()
	s = m["r"].(map[string]interface{})["nodes"].(string)
	return
}

func (me *findNodeResponse) UnmarshalKRPCMsg(m Msg) error {
	b, err := getResponseNodes(m)
	if err != nil {
		return err
	}
	for i := 0; i < len(b); i += 26 {
		var n NodeInfo
		err := n.UnmarshalCompact([]byte(b[i : i+26]))
		if err != nil {
			return err
		}
		me.Nodes = append(me.Nodes, n)
	}
	return nil
}

func (t *transaction) setOnResponse(f func(m Msg)) {
	if t.onResponse != nil {
		panic(t.onResponse)
	}
	t.onResponse = f
}

func unmarshalNodeInfoBinary(b []byte) (ret []NodeInfo, err error) {
	if len(b)%26 != 0 {
		err = errors.New("bad buffer length")
		return
	}
	ret = make([]NodeInfo, 0, len(b)/26)
	for i := 0; i < len(b); i += 26 {
		var ni NodeInfo
		err = ni.UnmarshalCompact(b[i : i+26])
		if err != nil {
			return
		}
		ret = append(ret, ni)
	}
	return
}

func extractNodes(d Msg) (nodes []NodeInfo, err error) {
	if d["y"] != "r" {
		return
	}
	r, ok := d["r"]
	if !ok {
		err = errors.New("missing r dict")
		return
	}
	rd, ok := r.(map[string]interface{})
	if !ok {
		err = errors.New("bad r value type")
		return
	}
	n, ok := rd["nodes"]
	if !ok {
		return
	}
	ns, ok := n.(string)
	if !ok {
		err = errors.New("bad nodes value type")
		return
	}
	return unmarshalNodeInfoBinary([]byte(ns))
}

func (s *Server) liftNodes(d Msg) {
	if d["y"] != "r" {
		return
	}
	var r findNodeResponse
	err := r.UnmarshalKRPCMsg(d)
	if err != nil {
		// log.Print(err)
	} else {
		for _, cni := range r.Nodes {
			if util.AddrPort(cni.Addr) == 0 {
				// TODO: Why would people even do this?
				continue
			}
			n := s.getNode(cni.Addr)
			n.id = string(cni.ID[:])
		}
		// log.Printf("lifted %d nodes", len(r.Nodes))
	}
}

// Sends a find_node query to addr. targetID is the node we're looking for.
func (s *Server) findNode(addr dHTAddr, targetID string) (t *transaction, err error) {
	t, err = s.query(addr, "find_node", map[string]interface{}{"target": targetID})
	if err != nil {
		return
	}
	// Scrape peers from the response to put in the server's table before
	// handing the response back to the caller.
	t.setOnResponse(func(d Msg) {
		s.liftNodes(d)
	})
	return
}

func extractValues(m Msg) (vs []util.CompactPeer) {
	r, ok := m["r"]
	if !ok {
		return
	}
	rd, ok := r.(map[string]interface{})
	if !ok {
		return
	}
	v, ok := rd["values"]
	if !ok {
		return
	}
	vl, ok := v.([]interface{})
	if !ok {
		panic(v)
	}
	vs = make([]util.CompactPeer, 0, len(vl))
	for _, i := range vl {
		s, ok := i.(string)
		if !ok {
			panic(i)
		}
		var cp util.CompactPeer
		err := cp.UnmarshalBinary([]byte(s))
		if err != nil {
			log.Printf("error decoding values list element: %s", err)
			continue
		}
		vs = append(vs, cp)
	}
	return
}

func (s *Server) getPeers(addr dHTAddr, infoHash string) (t *transaction, err error) {
	if len(infoHash) != 20 {
		err = fmt.Errorf("infohash has bad length")
		return
	}
	t, err = s.query(addr, "get_peers", map[string]interface{}{"info_hash": infoHash})
	if err != nil {
		return
	}
	t.setOnResponse(func(m Msg) {
		s.liftNodes(m)
		s.getNode(addr).announceToken = m.AnnounceToken()
	})
	return
}

func bootstrapAddr() (net.Addr, error) {
	return net.ResolveUDPAddr("udp4", "router.bittorrent.com:6881")
}

func (s *Server) addRootNode() error {
	addr, err := net.ResolveUDPAddr("udp4", "router.bittorrent.com:6881")
	if err != nil {
		return err
	}
	s.nodes[addr.String()] = &Node{
		addr: newDHTAddr(addr),
	}
	return nil
}

// Populates the node table.
func (s *Server) bootstrap() (err error) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if len(s.nodes) == 0 {
		err = s.addRootNode()
	}
	if err != nil {
		return
	}
	for {
		var outstanding sync.WaitGroup
		for _, node := range s.nodes {
			var t *transaction
			t, err = s.findNode(node.addr, s.id)
			if err != nil {
				return
			}
			outstanding.Add(1)
			go func() {
				<-t.Response
				outstanding.Done()
			}()
		}
		noOutstanding := make(chan struct{})
		go func() {
			outstanding.Wait()
			close(noOutstanding)
		}()
		s.mu.Unlock()
		select {
		case <-s.closed:
			s.mu.Lock()
			return
		case <-time.After(15 * time.Second):
		case <-noOutstanding:
		}
		s.mu.Lock()
		// log.Printf("now have %d nodes", len(s.nodes))
		if s.numGoodNodes() >= 160 {
			break
		}
	}
	return
}

func (s *Server) numGoodNodes() (num int) {
	for _, n := range s.nodes {
		if n.Good() {
			num++
		}
	}
	return
}

func (s *Server) NumNodes() int {
	s.mu.Lock()
	defer s.mu.Unlock()
	return len(s.nodes)
}

func (s *Server) Nodes() (nis []NodeInfo) {
	s.mu.Lock()
	defer s.mu.Unlock()
	for _, node := range s.nodes {
		// if !node.Good() {
		// 	continue
		// }
		ni := NodeInfo{
			Addr: node.addr,
		}
		if n := copy(ni.ID[:], node.id); n != 20 && n != 0 {
			panic(n)
		}
		nis = append(nis, ni)
	}
	return
}

func (s *Server) Close() {
	s.mu.Lock()
	select {
	case <-s.closed:
	default:
		close(s.closed)
		s.socket.Close()
	}
	s.mu.Unlock()
}

type distance interface {
	Cmp(distance) int
	BitCount() int
	IsZero() bool
}

type bigIntDistance struct {
	*big.Int
}

// How many bits?
func bitCount(n *big.Int) int {
	var count int = 0
	for _, b := range n.Bytes() {
		count += int(bitCounts[b])
	}
	return count
}

// The bit counts for each byte value (0 - 255).
var bitCounts = []int8{
	// Generated by Java BitCount of all values from 0 to 255
	0, 1, 1, 2, 1, 2, 2, 3,
	1, 2, 2, 3, 2, 3, 3, 4,
	1, 2, 2, 3, 2, 3, 3, 4,
	2, 3, 3, 4, 3, 4, 4, 5,
	1, 2, 2, 3, 2, 3, 3, 4,
	2, 3, 3, 4, 3, 4, 4, 5,
	2, 3, 3, 4, 3, 4, 4, 5,
	3, 4, 4, 5, 4, 5, 5, 6,
	1, 2, 2, 3, 2, 3, 3, 4,
	2, 3, 3, 4, 3, 4, 4, 5,
	2, 3, 3, 4, 3, 4, 4, 5,
	3, 4, 4, 5, 4, 5, 5, 6,
	2, 3, 3, 4, 3, 4, 4, 5,
	3, 4, 4, 5, 4, 5, 5, 6,
	3, 4, 4, 5, 4, 5, 5, 6,
	4, 5, 5, 6, 5, 6, 6, 7,
	1, 2, 2, 3, 2, 3, 3, 4,
	2, 3, 3, 4, 3, 4, 4, 5,
	2, 3, 3, 4, 3, 4, 4, 5,
	3, 4, 4, 5, 4, 5, 5, 6,
	2, 3, 3, 4, 3, 4, 4, 5,
	3, 4, 4, 5, 4, 5, 5, 6,
	3, 4, 4, 5, 4, 5, 5, 6,
	4, 5, 5, 6, 5, 6, 6, 7,
	2, 3, 3, 4, 3, 4, 4, 5,
	3, 4, 4, 5, 4, 5, 5, 6,
	3, 4, 4, 5, 4, 5, 5, 6,
	4, 5, 5, 6, 5, 6, 6, 7,
	3, 4, 4, 5, 4, 5, 5, 6,
	4, 5, 5, 6, 5, 6, 6, 7,
	4, 5, 5, 6, 5, 6, 6, 7,
	5, 6, 6, 7, 6, 7, 7, 8,
}

func (me bigIntDistance) BitCount() int {
	return bitCount(me.Int)
}

func (me bigIntDistance) Cmp(d distance) int {
	return me.Int.Cmp(d.(bigIntDistance).Int)
}

func (me bigIntDistance) IsZero() bool {
	return me.Int.Cmp(big.NewInt(0)) == 0
}

type bitCountDistance int

func (me bitCountDistance) BitCount() int { return int(me) }

func (me bitCountDistance) Cmp(rhs distance) int {
	rhs_ := rhs.(bitCountDistance)
	if me < rhs_ {
		return -1
	} else if me == rhs_ {
		return 0
	} else {
		return 1
	}
}

func (me bitCountDistance) IsZero() bool {
	return me == 0
}

func idDistance(a, b string) distance {
	if true {
		if len(a) != 20 {
			panic(a)
		}
		if len(b) != 20 {
			panic(b)
		}
		x := new(big.Int)
		y := new(big.Int)
		x.SetBytes([]byte(a))
		y.SetBytes([]byte(b))
		dist := new(big.Int)
		return bigIntDistance{dist.Xor(x, y)}
	} else {
		ret := 0
		for i := 0; i < 20; i++ {
			for j := uint(0); j < 8; j++ {
				ret += int(a[i]>>j&1 ^ b[i]>>j&1)
			}
		}
		return bitCountDistance(ret)
	}
}

func (s *Server) closestGoodNodes(k int, targetID string) []*Node {
	return s.closestNodes(k, targetID, func(n *Node) bool { return n.Good() })
}

func (s *Server) closestNodes(k int, targetID string, filter func(*Node) bool) []*Node {
	sel := newKClosestNodesSelector(k, targetID)
	idNodes := make(map[string]*Node, len(s.nodes))
	for _, node := range s.nodes {
		if !filter(node) {
			continue
		}
		sel.Push(node.id)
		idNodes[node.id] = node
	}
	ids := sel.IDs()
	ret := make([]*Node, 0, len(ids))
	for _, id := range ids {
		ret = append(ret, idNodes[id])
	}
	return ret
}