package heartbeatconn

import (
	"fmt"
	"net"
	"sync/atomic"
	"time"

	"github.com/zrepl/zrepl/rpc/dataconn/frameconn"
	"github.com/zrepl/zrepl/rpc/dataconn/timeoutconn"
)

type Conn struct {
	state                 state
	fc                    *frameconn.Conn
	sendInterval, timeout time.Duration
	stopSend              chan struct{}
	lastFrameSent         atomic.Value // time.Time
}

type HeartbeatTimeout struct{}

func (e HeartbeatTimeout) Error() string {
	return "heartbeat timeout"
}

func (e HeartbeatTimeout) Temporary() bool { return true }

func (e HeartbeatTimeout) Timeout() bool { return true }

var _ net.Error = HeartbeatTimeout{}

type state = int32

const (
	stateInitial state = 0
	stateClosed  state = 2
)

const (
	heartbeat uint32 = 1 << 24
)

// The 4 MSBs of ft are reserved for frameconn, we reserve the next 4 MSB for us.
func IsPublicFrameType(ft uint32) bool {
	return frameconn.IsPublicFrameType(ft) && (0xf<<24)&ft == 0
}

func assertPublicFrameType(frameType uint32) {
	if !IsPublicFrameType(frameType) {
		panic(fmt.Sprintf("heartbeatconn: frame type %v cannot be used by consumers of this package", frameType))
	}
}

func Wrap(nc timeoutconn.Wire, sendInterval, timeout time.Duration) *Conn {
	c := &Conn{
		fc:           frameconn.Wrap(timeoutconn.Wrap(nc, timeout)),
		stopSend:     make(chan struct{}),
		sendInterval: sendInterval,
		timeout:      timeout,
	}
	c.lastFrameSent.Store(time.Now())
	go c.sendHeartbeats()
	return c
}

func (c *Conn) Shutdown() error {
	normalClose := atomic.CompareAndSwapInt32(&c.state, stateInitial, stateClosed)
	if normalClose {
		close(c.stopSend)
	}
	return c.fc.Shutdown(time.Now().Add(c.timeout))
}

// started as a goroutine in constructor
func (c *Conn) sendHeartbeats() {
	sleepTime := func(now time.Time) time.Duration {
		lastSend := c.lastFrameSent.Load().(time.Time)
		return lastSend.Add(c.sendInterval).Sub(now)
	}
	timer := time.NewTimer(sleepTime(time.Now()))
	defer timer.Stop()
	for {
		select {
		case <-c.stopSend:
			return
		case now := <-timer.C:
			func() {
				defer func() {
					timer.Reset(sleepTime(time.Now()))
				}()
				if sleepTime(now) > 0 {
					return
				}
				debug("send heartbeat")
				// if the connection is in zombie mode (aka iptables DROP inbetween peers)
				// this call or one of its successors will block after filling up the kernel tx buffer
				err := c.fc.WriteFrame([]byte{}, heartbeat)
				if err != nil {
					debug("send heartbeat error: %s", err)
				}
				// ignore errors from WriteFrame to rate-limit SendHeartbeat retries
				c.lastFrameSent.Store(time.Now())
			}()
		}
	}
}

func (c *Conn) ReadFrame() (frameconn.Frame, error) {
	return c.readFrameFiltered()
}

func (c *Conn) readFrameFiltered() (frameconn.Frame, error) {
	for {
		f, err := c.fc.ReadFrame()
		if err != nil {
			return frameconn.Frame{}, err
		}
		if IsPublicFrameType(f.Header.Type) {
			return f, nil
		}
		if f.Header.Type != heartbeat {
			return frameconn.Frame{}, fmt.Errorf("unknown frame type %x", f.Header.Type)
		}
		// drop heartbeat frame
		debug("received heartbeat, resetting write timeout")
		// the peer's heartbeat proves to us that the peer is still live
		// => trust the peer at this point (DoS risks are ignored ATM)
		// => we assume that the connection is symmetric duplex, i.e., if receiving works for us,
		//    sending works for us, too.
		// So, let's grant the peer another write timeout.
		err = c.fc.ResetWriteTimeout()
		debug("renew frameconn write timeout returned errT=%T err=%s", err, err)
		if err != nil {
			return frameconn.Frame{}, err
		}
		continue
	}
}

func (c *Conn) WriteFrame(payload []byte, frameType uint32) error {
	assertPublicFrameType(frameType)
	err := c.fc.WriteFrame(payload, frameType)
	if err == nil {
		c.lastFrameSent.Store(time.Now())
	}
	return err
}