// Package ping is a simple but powerful ICMP echo (ping) library.
//
// Here is a very simple example that sends and receives three packets:
//
//	pinger, err := ping.NewPinger("www.google.com")
//	if err != nil {
//		panic(err)
//	}
//	pinger.Count = 3
//	err = pinger.Run() // blocks until finished
//	if err != nil {
//		panic(err)
//	}
//	stats := pinger.Statistics() // get send/receive/rtt stats
//
// Here is an example that emulates the traditional UNIX ping command:
//
//	pinger, err := ping.NewPinger("www.google.com")
//	if err != nil {
//		panic(err)
//	}
//	// Listen for Ctrl-C.
//	c := make(chan os.Signal, 1)
//	signal.Notify(c, os.Interrupt)
//	go func() {
//		for _ = range c {
//			pinger.Stop()
//		}
//	}()
//	pinger.OnRecv = func(pkt *ping.Packet) {
//		fmt.Printf("%d bytes from %s: icmp_seq=%d time=%v\n",
//			pkt.Nbytes, pkt.IPAddr, pkt.Seq, pkt.Rtt)
//	}
//	pinger.OnFinish = func(stats *ping.Statistics) {
//		fmt.Printf("\n--- %s ping statistics ---\n", stats.Addr)
//		fmt.Printf("%d packets transmitted, %d packets received, %v%% packet loss\n",
//			stats.PacketsSent, stats.PacketsRecv, stats.PacketLoss)
//		fmt.Printf("round-trip min/avg/max/stddev = %v/%v/%v/%v\n",
//			stats.MinRtt, stats.AvgRtt, stats.MaxRtt, stats.StdDevRtt)
//	}
//	fmt.Printf("PING %s (%s):\n", pinger.Addr(), pinger.IPAddr())
//	err = pinger.Run()
//	if err != nil {
//		panic(err)
//	}
//
// It sends ICMP Echo Request packet(s) and waits for an Echo Reply in response.
// If it receives a response, it calls the OnRecv callback. When it's finished,
// it calls the OnFinish callback.
//
// For a full ping example, see "cmd/ping/ping.go".
//
package ping

import (
	"bytes"
	"encoding/binary"
	"errors"
	"fmt"
	"math"
	"math/rand"
	"net"
	"runtime"
	"sync"
	"sync/atomic"
	"syscall"
	"time"

	"golang.org/x/net/icmp"
	"golang.org/x/net/ipv4"
	"golang.org/x/net/ipv6"
)

const (
	timeSliceLength  = 8
	trackerLength    = 8
	protocolICMP     = 1
	protocolIPv6ICMP = 58
)

var (
	ipv4Proto = map[string]string{"icmp": "ip4:icmp", "udp": "udp4"}
	ipv6Proto = map[string]string{"icmp": "ip6:ipv6-icmp", "udp": "udp6"}
)

// New returns a new Pinger struct pointer.
func New(addr string) *Pinger {
	r := rand.New(rand.NewSource(getSeed()))
	return &Pinger{
		Count:      -1,
		Interval:   time.Second,
		RecordRtts: true,
		Size:       timeSliceLength + trackerLength,
		Timeout:    time.Duration(math.MaxInt64),
		Tracker:    r.Uint64(),

		addr:              addr,
		done:              make(chan bool),
		id:                r.Intn(math.MaxUint16),
		ipaddr:            nil,
		ipv4:              false,
		network:           "ip",
		protocol:          "udp",
		awaitingSequences: map[int]struct{}{},
	}
}

// NewPinger returns a new Pinger and resolves the address.
func NewPinger(addr string) (*Pinger, error) {
	p := New(addr)
	return p, p.Resolve()
}

// Pinger represents a packet sender/receiver.
type Pinger struct {
	// Interval is the wait time between each packet send. Default is 1s.
	Interval time.Duration

	// Timeout specifies a timeout before ping exits, regardless of how many
	// packets have been received.
	Timeout time.Duration

	// Count tells pinger to stop after sending (and receiving) Count echo
	// packets. If this option is not specified, pinger will operate until
	// interrupted.
	Count int

	// Debug runs in debug mode
	Debug bool

	// Number of packets sent
	PacketsSent int

	// Number of packets received
	PacketsRecv int

	// Number of duplicate packets received
	PacketsRecvDuplicates int

	// Round trip time statistics
	minRtt    time.Duration
	maxRtt    time.Duration
	avgRtt    time.Duration
	stdDevRtt time.Duration
	stddevm2  time.Duration
	statsMu   sync.RWMutex

	// If true, keep a record of rtts of all received packets.
	// Set to false to avoid memory bloat for long running pings.
	RecordRtts bool

	// rtts is all of the Rtts
	rtts []time.Duration

	// OnSend is called when Pinger sends a packet
	OnSend func(*Packet)

	// OnRecv is called when Pinger receives and processes a packet
	OnRecv func(*Packet)

	// OnFinish is called when Pinger exits
	OnFinish func(*Statistics)

	// OnDuplicateRecv is called when a packet is received that has already been received.
	OnDuplicateRecv func(*Packet)

	// Size of packet being sent
	Size int

	// Tracker: Used to uniquely identify packets
	Tracker uint64

	// Source is the source IP address
	Source string

	// stop chan bool
	done chan bool

	ipaddr *net.IPAddr
	addr   string

	ipv4     bool
	id       int
	sequence int
	// awaitingSequences are in-flight sequence numbers we keep track of to help remove duplicate receipts
	awaitingSequences map[int]struct{}
	// network is one of "ip", "ip4", or "ip6".
	network string
	// protocol is "icmp" or "udp".
	protocol string
}

type packet struct {
	bytes  []byte
	nbytes int
	ttl    int
}

// Packet represents a received and processed ICMP echo packet.
type Packet struct {
	// Rtt is the round-trip time it took to ping.
	Rtt time.Duration

	// IPAddr is the address of the host being pinged.
	IPAddr *net.IPAddr

	// Addr is the string address of the host being pinged.
	Addr string

	// NBytes is the number of bytes in the message.
	Nbytes int

	// Seq is the ICMP sequence number.
	Seq int

	// TTL is the Time To Live on the packet.
	Ttl int
}

// Statistics represent the stats of a currently running or finished
// pinger operation.
type Statistics struct {
	// PacketsRecv is the number of packets received.
	PacketsRecv int

	// PacketsSent is the number of packets sent.
	PacketsSent int

	// PacketsRecvDuplicates is the number of duplicate responses there were to a sent packet.
	PacketsRecvDuplicates int

	// PacketLoss is the percentage of packets lost.
	PacketLoss float64

	// IPAddr is the address of the host being pinged.
	IPAddr *net.IPAddr

	// Addr is the string address of the host being pinged.
	Addr string

	// Rtts is all of the round-trip times sent via this pinger.
	Rtts []time.Duration

	// MinRtt is the minimum round-trip time sent via this pinger.
	MinRtt time.Duration

	// MaxRtt is the maximum round-trip time sent via this pinger.
	MaxRtt time.Duration

	// AvgRtt is the average round-trip time sent via this pinger.
	AvgRtt time.Duration

	// StdDevRtt is the standard deviation of the round-trip times sent via
	// this pinger.
	StdDevRtt time.Duration
}

func (p *Pinger) updateStatistics(pkt *Packet) {
	p.statsMu.Lock()
	defer p.statsMu.Unlock()

	p.PacketsRecv++
	if p.RecordRtts {
		p.rtts = append(p.rtts, pkt.Rtt)
	}

	if p.PacketsRecv == 1 || pkt.Rtt < p.minRtt {
		p.minRtt = pkt.Rtt
	}

	if pkt.Rtt > p.maxRtt {
		p.maxRtt = pkt.Rtt
	}

	pktCount := time.Duration(p.PacketsRecv)
	// welford's online method for stddev
	// https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm
	delta := pkt.Rtt - p.avgRtt
	p.avgRtt += delta / pktCount
	delta2 := pkt.Rtt - p.avgRtt
	p.stddevm2 += delta * delta2

	p.stdDevRtt = time.Duration(math.Sqrt(float64(p.stddevm2 / pktCount)))
}

// SetIPAddr sets the ip address of the target host.
func (p *Pinger) SetIPAddr(ipaddr *net.IPAddr) {
	p.ipv4 = isIPv4(ipaddr.IP)

	p.ipaddr = ipaddr
	p.addr = ipaddr.String()
}

// IPAddr returns the ip address of the target host.
func (p *Pinger) IPAddr() *net.IPAddr {
	return p.ipaddr
}

// Resolve does the DNS lookup for the Pinger address and sets IP protocol.
func (p *Pinger) Resolve() error {
	if len(p.addr) == 0 {
		return errors.New("addr cannot be empty")
	}
	ipaddr, err := net.ResolveIPAddr(p.network, p.addr)
	if err != nil {
		return err
	}

	p.ipv4 = isIPv4(ipaddr.IP)

	p.ipaddr = ipaddr

	return nil
}

// SetAddr resolves and sets the ip address of the target host, addr can be a
// DNS name like "www.google.com" or IP like "127.0.0.1".
func (p *Pinger) SetAddr(addr string) error {
	oldAddr := p.addr
	p.addr = addr
	err := p.Resolve()
	if err != nil {
		p.addr = oldAddr
		return err
	}
	return nil
}

// Addr returns the string ip address of the target host.
func (p *Pinger) Addr() string {
	return p.addr
}

// SetNetwork allows configuration of DNS resolution.
// * "ip" will automatically select IPv4 or IPv6.
// * "ip4" will select IPv4.
// * "ip6" will select IPv6.
func (p *Pinger) SetNetwork(n string) {
	switch n {
	case "ip4":
		p.network = "ip4"
	case "ip6":
		p.network = "ip6"
	default:
		p.network = "ip"
	}
}

// SetPrivileged sets the type of ping pinger will send.
// false means pinger will send an "unprivileged" UDP ping.
// true means pinger will send a "privileged" raw ICMP ping.
// NOTE: setting to true requires that it be run with super-user privileges.
func (p *Pinger) SetPrivileged(privileged bool) {
	if privileged {
		p.protocol = "icmp"
	} else {
		p.protocol = "udp"
	}
}

// Privileged returns whether pinger is running in privileged mode.
func (p *Pinger) Privileged() bool {
	return p.protocol == "icmp"
}

// Run runs the pinger. This is a blocking function that will exit when it's
// done. If Count or Interval are not specified, it will run continuously until
// it is interrupted.
func (p *Pinger) Run() error {
	var conn *icmp.PacketConn
	var err error
	if p.ipaddr == nil {
		err = p.Resolve()
	}
	if err != nil {
		return err
	}
	if p.ipv4 {
		if conn, err = p.listen(ipv4Proto[p.protocol]); err != nil {
			return err
		}
		if err = conn.IPv4PacketConn().SetControlMessage(ipv4.FlagTTL, true); runtime.GOOS != "windows" && err != nil {
			return err
		}
	} else {
		if conn, err = p.listen(ipv6Proto[p.protocol]); err != nil {
			return err
		}
		if err = conn.IPv6PacketConn().SetControlMessage(ipv6.FlagHopLimit, true); runtime.GOOS != "windows" && err != nil {
			return err
		}
	}
	defer conn.Close()
	defer p.finish()

	var wg sync.WaitGroup
	recv := make(chan *packet, 5)
	defer close(recv)
	wg.Add(1)
	//nolint:errcheck
	go p.recvICMP(conn, recv, &wg)

	err = p.sendICMP(conn)
	if err != nil {
		return err
	}

	timeout := time.NewTicker(p.Timeout)
	defer timeout.Stop()
	interval := time.NewTicker(p.Interval)
	defer interval.Stop()

	for {
		select {
		case <-p.done:
			wg.Wait()
			return nil
		case <-timeout.C:
			close(p.done)
			wg.Wait()
			return nil
		case <-interval.C:
			if p.Count > 0 && p.PacketsSent >= p.Count {
				continue
			}
			err = p.sendICMP(conn)
			if err != nil {
				// FIXME: this logs as FATAL but continues
				fmt.Println("FATAL: ", err.Error())
			}
		case r := <-recv:
			err := p.processPacket(r)
			if err != nil {
				// FIXME: this logs as FATAL but continues
				fmt.Println("FATAL: ", err.Error())
			}
		}
		if p.Count > 0 && p.PacketsRecv >= p.Count {
			close(p.done)
			wg.Wait()
			return nil
		}
	}
}

func (p *Pinger) Stop() {
	close(p.done)
}

func (p *Pinger) finish() {
	handler := p.OnFinish
	if handler != nil {
		s := p.Statistics()
		handler(s)
	}
}

// Statistics returns the statistics of the pinger. This can be run while the
// pinger is running or after it is finished. OnFinish calls this function to
// get it's finished statistics.
func (p *Pinger) Statistics() *Statistics {
	p.statsMu.RLock()
	defer p.statsMu.RUnlock()
	sent := p.PacketsSent
	loss := float64(sent-p.PacketsRecv) / float64(sent) * 100
	s := Statistics{
		PacketsSent:           sent,
		PacketsRecv:           p.PacketsRecv,
		PacketsRecvDuplicates: p.PacketsRecvDuplicates,
		PacketLoss:            loss,
		Rtts:                  p.rtts,
		Addr:                  p.addr,
		IPAddr:                p.ipaddr,
		MaxRtt:                p.maxRtt,
		MinRtt:                p.minRtt,
		AvgRtt:                p.avgRtt,
		StdDevRtt:             p.stdDevRtt,
	}
	return &s
}

func (p *Pinger) recvICMP(
	conn *icmp.PacketConn,
	recv chan<- *packet,
	wg *sync.WaitGroup,
) error {
	defer wg.Done()
	for {
		select {
		case <-p.done:
			return nil
		default:
			bytes := make([]byte, p.getMessageLength())
			if err := conn.SetReadDeadline(time.Now().Add(time.Millisecond * 100)); err != nil {
				return err
			}
			var n, ttl int
			var err error
			if p.ipv4 {
				var cm *ipv4.ControlMessage
				n, cm, _, err = conn.IPv4PacketConn().ReadFrom(bytes)
				if cm != nil {
					ttl = cm.TTL
				}
			} else {
				var cm *ipv6.ControlMessage
				n, cm, _, err = conn.IPv6PacketConn().ReadFrom(bytes)
				if cm != nil {
					ttl = cm.HopLimit
				}
			}
			if err != nil {
				if neterr, ok := err.(*net.OpError); ok {
					if neterr.Timeout() {
						// Read timeout
						continue
					} else {
						close(p.done)
						return err
					}
				}
			}

			select {
			case <-p.done:
				return nil
			case recv <- &packet{bytes: bytes, nbytes: n, ttl: ttl}:
			}
		}
	}
}

func (p *Pinger) processPacket(recv *packet) error {
	receivedAt := time.Now()
	var proto int
	if p.ipv4 {
		proto = protocolICMP
	} else {
		proto = protocolIPv6ICMP
	}

	var m *icmp.Message
	var err error
	if m, err = icmp.ParseMessage(proto, recv.bytes); err != nil {
		return fmt.Errorf("error parsing icmp message: %s", err.Error())
	}

	if m.Type != ipv4.ICMPTypeEchoReply && m.Type != ipv6.ICMPTypeEchoReply {
		// Not an echo reply, ignore it
		return nil
	}

	inPkt := &Packet{
		Nbytes: recv.nbytes,
		IPAddr: p.ipaddr,
		Addr:   p.addr,
		Ttl:    recv.ttl,
	}

	switch pkt := m.Body.(type) {
	case *icmp.Echo:
		if !p.matchID(pkt.ID) {
			return nil
		}

		if len(pkt.Data) < timeSliceLength+trackerLength {
			return fmt.Errorf("insufficient data received; got: %d %v",
				len(pkt.Data), pkt.Data)
		}

		tracker := bytesToUint(pkt.Data[timeSliceLength:])
		timestamp := bytesToTime(pkt.Data[:timeSliceLength])

		if tracker != p.Tracker {
			return nil
		}

		inPkt.Rtt = receivedAt.Sub(timestamp)
		inPkt.Seq = pkt.Seq
		// If we've already received this sequence, ignore it.
		if _, inflight := p.awaitingSequences[pkt.Seq]; !inflight {
			p.PacketsRecvDuplicates++
			if p.OnDuplicateRecv != nil {
				p.OnDuplicateRecv(inPkt)
			}
			return nil
		}
		// remove it from the list of sequences we're waiting for so we don't get duplicates.
		delete(p.awaitingSequences, pkt.Seq)
		p.updateStatistics(inPkt)
	default:
		// Very bad, not sure how this can happen
		return fmt.Errorf("invalid ICMP echo reply; type: '%T', '%v'", pkt, pkt)
	}

	handler := p.OnRecv
	if handler != nil {
		handler(inPkt)
	}

	return nil
}

func (p *Pinger) sendICMP(conn *icmp.PacketConn) error {
	var typ icmp.Type
	if p.ipv4 {
		typ = ipv4.ICMPTypeEcho
	} else {
		typ = ipv6.ICMPTypeEchoRequest
	}

	var dst net.Addr = p.ipaddr
	if p.protocol == "udp" {
		dst = &net.UDPAddr{IP: p.ipaddr.IP, Zone: p.ipaddr.Zone}
	}

	t := append(timeToBytes(time.Now()), uintToBytes(p.Tracker)...)
	if remainSize := p.Size - timeSliceLength - trackerLength; remainSize > 0 {
		t = append(t, bytes.Repeat([]byte{1}, remainSize)...)
	}

	body := &icmp.Echo{
		ID:   p.id,
		Seq:  p.sequence,
		Data: t,
	}

	msg := &icmp.Message{
		Type: typ,
		Code: 0,
		Body: body,
	}

	msgBytes, err := msg.Marshal(nil)
	if err != nil {
		return err
	}

	for {
		if _, err := conn.WriteTo(msgBytes, dst); err != nil {
			if neterr, ok := err.(*net.OpError); ok {
				if neterr.Err == syscall.ENOBUFS {
					continue
				}
			}
		}
		handler := p.OnSend
		if handler != nil {
			outPkt := &Packet{
				Nbytes: len(msgBytes),
				IPAddr: p.ipaddr,
				Addr:   p.addr,
				Seq:    p.sequence,
			}
			handler(outPkt)
		}
		// mark this sequence as in-flight
		p.awaitingSequences[p.sequence] = struct{}{}
		p.PacketsSent++
		p.sequence++
		break
	}

	return nil
}

func (p *Pinger) listen(netProto string) (*icmp.PacketConn, error) {
	conn, err := icmp.ListenPacket(netProto, p.Source)
	if err != nil {
		close(p.done)
		return nil, err
	}
	return conn, nil
}

func bytesToTime(b []byte) time.Time {
	var nsec int64
	for i := uint8(0); i < 8; i++ {
		nsec += int64(b[i]) << ((7 - i) * 8)
	}
	return time.Unix(nsec/1000000000, nsec%1000000000)
}

func isIPv4(ip net.IP) bool {
	return len(ip.To4()) == net.IPv4len
}

func timeToBytes(t time.Time) []byte {
	nsec := t.UnixNano()
	b := make([]byte, 8)
	for i := uint8(0); i < 8; i++ {
		b[i] = byte((nsec >> ((7 - i) * 8)) & 0xff)
	}
	return b
}

func bytesToUint(b []byte) uint64 {
	return uint64(binary.BigEndian.Uint64(b))
}

func uintToBytes(tracker uint64) []byte {
	b := make([]byte, 8)
	binary.BigEndian.PutUint64(b, tracker)
	return b
}

var seed int64 = time.Now().UnixNano()

// getSeed returns a goroutine-safe unique seed
func getSeed() int64 {
	return atomic.AddInt64(&seed, 1)
}
