package tunnel

import (
	"io"
	"sync"
	"time"

	"github.com/google/uuid"
	"github.com/micro/go-micro/transport"
)

type link struct {
	transport.Socket

	sync.RWMutex
	// stops the link
	closed chan bool
	// send queue
	sendQueue chan *packet
	// receive queue
	recvQueue chan *packet
	// unique id of this link e.g uuid
	// which we define for ourselves
	id string
	// whether its a loopback connection
	// this flag is used by the transport listener
	// which accepts inbound quic connections
	loopback bool
	// whether its actually connected
	// dialled side sets it to connected
	// after sending the message. the
	// listener waits for the connect
	connected bool
	// the last time we received a keepalive
	// on this link from the remote side
	lastKeepAlive time.Time
	// channels keeps a mapping of channels and last seen
	channels map[string]time.Time
	// the weighted moving average roundtrip
	length int64
	// weighted moving average of bits flowing
	rate float64
	// keep an error count on the link
	errCount int
}

// packet send over link
type packet struct {
	// message to send or received
	message *transport.Message

	// status returned when sent
	status chan error

	// receive related error
	err error
}

func newLink(s transport.Socket) *link {
	l := &link{
		Socket:        s,
		id:            uuid.New().String(),
		lastKeepAlive: time.Now(),
		closed:        make(chan bool),
		channels:      make(map[string]time.Time),
		sendQueue:     make(chan *packet, 128),
		recvQueue:     make(chan *packet, 128),
	}
	go l.process()
	go l.expiry()
	return l
}

func (l *link) setRTT(d time.Duration) {
	l.Lock()
	defer l.Unlock()

	if l.length <= 0 {
		l.length = d.Nanoseconds()
		return
	}

	// https://fishi.devtail.io/weblog/2015/04/12/measuring-bandwidth-and-round-trip-time-tcp-connection-inside-application-layer/
	length := 0.8*float64(l.length) + 0.2*float64(d.Nanoseconds())
	// set new length
	l.length = int64(length)
}

// process deals with the send queue
func (l *link) process() {
	// receive messages
	go func() {
		for {
			m := new(transport.Message)
			err := l.recv(m)
			if err != nil {
				l.Lock()
				l.errCount++
				l.Unlock()
			}

			// process new received message

			select {
			case l.recvQueue <- &packet{message: m, err: err}:
			case <-l.closed:
				return
			}
		}
	}()

	// send messages

	for {
		select {
		case pk := <-l.sendQueue:
			// send the message
			pk.status <- l.send(pk.message)
		case <-l.closed:
			return
		}
	}
}

// watches the channel expiry
func (l *link) expiry() {
	t := time.NewTicker(time.Minute)
	defer t.Stop()

	for {
		select {
		case <-l.closed:
			return
		case <-t.C:
			// drop any channel mappings older than 2 minutes
			var kill []string
			killTime := time.Minute * 2

			l.RLock()
			for ch, t := range l.channels {
				if d := time.Since(t); d > killTime {
					kill = append(kill, ch)
				}
			}
			l.RUnlock()

			// if nothing to kill don't bother with a wasted lock
			if len(kill) == 0 {
				continue
			}

			// kill the channels!
			l.Lock()
			for _, ch := range kill {
				delete(l.channels, ch)
			}
			l.Unlock()
		}
	}
}

func (l *link) send(m *transport.Message) error {
	dataSent := len(m.Body)

	// set header length
	for k, v := range m.Header {
		dataSent += (len(k) + len(v))
	}

	// get time now
	now := time.Now()

	// send the message
	err := l.Socket.Send(m)

	l.Lock()
	defer l.Unlock()

	// there's an error increment the counter and bail
	if err != nil {
		l.errCount++
		return err
	}

	// reset the counter
	l.errCount = 0

	// calculate based on data
	if dataSent > 0 {
		// measure time taken
		delta := time.Since(now)

		// bit sent
		bits := dataSent * 1024

		// rate of send in bits per nanosecond
		rate := float64(bits) / float64(delta.Nanoseconds())

		// default the rate if its zero
		if l.rate == 0 {
			// rate per second
			l.rate = rate * 1e9
		} else {
			// set new rate per second
			l.rate = 0.8*l.rate + 0.2*(rate*1e9)
		}
	}

	return err
}

// recv a message on the link
func (l *link) recv(m *transport.Message) error {
	if m.Header == nil {
		m.Header = make(map[string]string)
	}

	// TODO measure the receive latency if possible

	// receive the transport message
	return l.Socket.Recv(m)
}

// Delay is the current load on the link
func (l *link) Delay() int64 {
	return int64(len(l.sendQueue) + len(l.recvQueue))
}

// Current transfer rate as bits per second (lower is better)
func (l *link) Rate() float64 {
	l.RLock()
	defer l.RUnlock()

	return l.rate
}

// Length returns the roundtrip time as nanoseconds (lower is better).
// Returns 0 where no measurement has been taken.
func (l *link) Length() int64 {
	l.RLock()
	defer l.RUnlock()

	return l.length
}

func (l *link) Id() string {
	l.RLock()
	defer l.RUnlock()

	return l.id
}

func (l *link) Close() error {
	select {
	case <-l.closed:
		return nil
	default:
		close(l.closed)
	}

	return nil
}

// Send sencs a message on the link
func (l *link) Send(m *transport.Message) error {
	// create a new packet to send over the link
	p := &packet{
		message: m,
		status:  make(chan error, 1),
	}

	select {
	case <-l.closed:
		return io.EOF
	case l.sendQueue <- p:
		// in the send queue
	}

	// wait for response
	select {
	case <-l.closed:
		return io.EOF
	case err := <-p.status:
		return err
	}

	// never reached
	return nil
}

// Accept accepts a message on the socket
func (l *link) Recv(m *transport.Message) error {
	select {
	case <-l.closed:
		return io.EOF
	case pk := <-l.recvQueue:
		// check the packet receive error
		if pk.err != nil {
			return pk.err
		}
		*m = *pk.message
	}
	return nil
}

// Status can return connected, closed, error
func (l *link) Status() string {
	select {
	case <-l.closed:
		return "closed"
	default:
		l.RLock()
		defer l.RUnlock()
		if l.errCount > 3 {
			return "error"
		}
		return "connected"
	}
}