package main

import (
	"bytes"
	"encoding/binary"
	"golang.org/x/crypto/chacha20poly1305"
	"golang.org/x/net/ipv4"
	"golang.org/x/net/ipv6"
	"net"
	"sync"
	"sync/atomic"
	"time"
)

type QueueHandshakeElement struct {
	msgType uint32
	packet  []byte
	buffer  *[MaxMessageSize]byte
	source  *net.UDPAddr
}

type QueueInboundElement struct {
	dropped int32
	mutex   sync.Mutex
	buffer  *[MaxMessageSize]byte
	packet  []byte
	counter uint64
	keyPair *KeyPair
}

func (elem *QueueInboundElement) Drop() {
	atomic.StoreInt32(&elem.dropped, AtomicTrue)
}

func (elem *QueueInboundElement) IsDropped() bool {
	return atomic.LoadInt32(&elem.dropped) == AtomicTrue
}

func (device *Device) addToInboundQueue(
	queue chan *QueueInboundElement,
	element *QueueInboundElement,
) {
	for {
		select {
		case queue <- element:
			return
		default:
			select {
			case old := <-queue:
				old.Drop()
			default:
			}
		}
	}
}

func (device *Device) addToDecryptionQueue(
	queue chan *QueueInboundElement,
	element *QueueInboundElement,
) {
	for {
		select {
		case queue <- element:
			return
		default:
			select {
			case old := <-queue:
				// drop & release to potential consumer
				old.Drop()
				old.mutex.Unlock()
			default:
			}
		}
	}
}

func (device *Device) addToHandshakeQueue(
	queue chan QueueHandshakeElement,
	element QueueHandshakeElement,
) {
	for {
		select {
		case queue <- element:
			return
		default:
			select {
			case elem := <-queue:
				device.PutMessageBuffer(elem.buffer)
			default:
			}
		}
	}
}

func (device *Device) RoutineReceiveIncomming() {

	logDebug := device.log.Debug
	logDebug.Println("Routine, receive incomming, started")

	for {

		// wait for new conn

		logDebug.Println("Waiting for udp socket")

		select {
		case <-device.signal.stop:
			return

		case <-device.signal.newUDPConn:

			// fetch connection

			device.net.mutex.RLock()
			conn := device.net.conn
			device.net.mutex.RUnlock()
			if conn == nil {
				continue
			}

			logDebug.Println("Listening for inbound packets")

			// receive datagrams until conn is closed

			buffer := device.GetMessageBuffer()

			for {

				// read next datagram

				size, raddr, err := conn.ReadFromUDP(buffer[:]) // Blocks sometimes

				if err != nil {
					break
				}

				if size < MinMessageSize {
					continue
				}

				// check size of packet

				packet := buffer[:size]
				msgType := binary.LittleEndian.Uint32(packet[:4])

				var okay bool

				switch msgType {

				// check if transport

				case MessageTransportType:

					// check size

					if len(packet) < MessageTransportType {
						continue
					}

					// lookup key pair

					receiver := binary.LittleEndian.Uint32(
						packet[MessageTransportOffsetReceiver:MessageTransportOffsetCounter],
					)
					value := device.indices.Lookup(receiver)
					keyPair := value.keyPair
					if keyPair == nil {
						continue
					}

					// check key-pair expiry

					if keyPair.created.Add(RejectAfterTime).Before(time.Now()) {
						continue
					}

					// create work element

					peer := value.peer
					elem := &QueueInboundElement{
						packet:  packet,
						buffer:  buffer,
						keyPair: keyPair,
						dropped: AtomicFalse,
					}
					elem.mutex.Lock()

					// add to decryption queues

					device.addToDecryptionQueue(device.queue.decryption, elem)
					device.addToInboundQueue(peer.queue.inbound, elem)
					buffer = device.GetMessageBuffer()
					continue

				// otherwise it is a handshake related packet

				case MessageInitiationType:
					okay = len(packet) == MessageInitiationSize

				case MessageResponseType:
					okay = len(packet) == MessageResponseSize

				case MessageCookieReplyType:
					okay = len(packet) == MessageCookieReplySize
				}

				if okay {
					device.addToHandshakeQueue(
						device.queue.handshake,
						QueueHandshakeElement{
							msgType: msgType,
							buffer:  buffer,
							packet:  packet,
							source:  raddr,
						},
					)
					buffer = device.GetMessageBuffer()
				}
			}
		}
	}
}

func (device *Device) RoutineDecryption() {
	var elem *QueueInboundElement
	var nonce [chacha20poly1305.NonceSize]byte

	logDebug := device.log.Debug
	logDebug.Println("Routine, decryption, started for device")

	for {
		select {
		case elem = <-device.queue.decryption:
		case <-device.signal.stop:
			return
		}

		// check if dropped

		if elem.IsDropped() {
			continue
		}

		// split message into fields

		counter := elem.packet[MessageTransportOffsetCounter:MessageTransportOffsetContent]
		content := elem.packet[MessageTransportOffsetContent:]

		// decrypt with key-pair

		var err error
		copy(nonce[4:], counter)
		elem.counter = binary.LittleEndian.Uint64(counter)
		elem.packet, err = elem.keyPair.receive.Open(
			elem.buffer[:0],
			nonce[:],
			content,
			nil,
		)
		if err != nil {
			elem.Drop()
		}
		elem.mutex.Unlock()
	}
}

/* Handles incomming packets related to handshake
 *
 *
 */
func (device *Device) RoutineHandshake() {

	logInfo := device.log.Info
	logError := device.log.Error
	logDebug := device.log.Debug
	logDebug.Println("Routine, handshake routine, started for device")

	var temp [MessageHandshakeSize]byte
	var elem QueueHandshakeElement

	for {
		select {
		case elem = <-device.queue.handshake:
		case <-device.signal.stop:
			return
		}

		// handle cookie fields and ratelimiting

		switch elem.msgType {

		case MessageCookieReplyType:

			// unmarshal packet

			logDebug.Println("Process cookie reply from:", elem.source.String())

			var reply MessageCookieReply
			reader := bytes.NewReader(elem.packet)
			err := binary.Read(reader, binary.LittleEndian, &reply)
			if err != nil {
				logDebug.Println("Failed to decode cookie reply")
				return
			}

			// lookup peer and consume response

			entry := device.indices.Lookup(reply.Receiver)
			if entry.peer == nil {
				return
			}
			entry.peer.mac.ConsumeReply(&reply)
			continue

		case MessageInitiationType, MessageResponseType:

			// check mac fields and ratelimit

			if !device.mac.CheckMAC1(elem.packet) {
				logDebug.Println("Received packet with invalid mac1")
				return
			}

			if device.IsUnderLoad() {
				if !device.mac.CheckMAC2(elem.packet, elem.source) {

					// construct cookie reply

					logDebug.Println("Sending cookie reply to:", elem.source.String())

					sender := binary.LittleEndian.Uint32(elem.packet[4:8]) // "sender" always follows "type"
					reply, err := device.mac.CreateReply(elem.packet, sender, elem.source)
					if err != nil {
						logError.Println("Failed to create cookie reply:", err)
						return
					}

					// marshal and send reply

					writer := bytes.NewBuffer(temp[:0])
					binary.Write(writer, binary.LittleEndian, reply)
					_, err = device.net.conn.WriteToUDP(
						writer.Bytes(),
						elem.source,
					)
					if err != nil {
						logDebug.Println("Failed to send cookie reply:", err)
					}
					continue
				}

				if !device.ratelimiter.Allow(elem.source.IP) {
					continue
				}
			}

		default:
			logError.Println("Invalid packet ended up in the handshake queue")
			continue
		}

		// handle handshake initation/response content

		switch elem.msgType {
		case MessageInitiationType:

			// unmarshal

			var msg MessageInitiation
			reader := bytes.NewReader(elem.packet)
			err := binary.Read(reader, binary.LittleEndian, &msg)
			if err != nil {
				logError.Println("Failed to decode initiation message")
				continue
			}

			// consume initiation

			peer := device.ConsumeMessageInitiation(&msg)
			if peer == nil {
				logInfo.Println(
					"Recieved invalid initiation message from",
					elem.source.IP.String(),
					elem.source.Port,
				)
				continue
			}

			// update timers

			peer.TimerAnyAuthenticatedPacketTraversal()
			peer.TimerAnyAuthenticatedPacketReceived()

			// update endpoint
			// TODO: Discover destination address also, only update on change

			peer.mutex.Lock()
			peer.endpoint = elem.source
			peer.mutex.Unlock()

			// create response

			response, err := device.CreateMessageResponse(peer)
			if err != nil {
				logError.Println("Failed to create response message:", err)
				continue
			}

			peer.TimerEphemeralKeyCreated()
			peer.NewKeyPair()

			logDebug.Println("Creating response message for", peer.String())

			writer := bytes.NewBuffer(temp[:0])
			binary.Write(writer, binary.LittleEndian, response)
			packet := writer.Bytes()
			peer.mac.AddMacs(packet)

			// send response

			_, err = peer.SendBuffer(packet)
			if err == nil {
				peer.TimerAnyAuthenticatedPacketTraversal()
			}

		case MessageResponseType:

			logDebug.Println("Process response")

			// unmarshal

			var msg MessageResponse
			reader := bytes.NewReader(elem.packet)
			err := binary.Read(reader, binary.LittleEndian, &msg)
			if err != nil {
				logError.Println("Failed to decode response message")
				continue
			}

			// consume response

			peer := device.ConsumeMessageResponse(&msg)
			if peer == nil {
				logInfo.Println(
					"Recieved invalid response message from",
					elem.source.IP.String(),
					elem.source.Port,
				)
				continue
			}

			peer.TimerEphemeralKeyCreated()

			// update timers

			peer.TimerAnyAuthenticatedPacketTraversal()
			peer.TimerAnyAuthenticatedPacketReceived()
			peer.TimerHandshakeComplete()

			// derive key-pair

			peer.NewKeyPair()
			peer.SendKeepAlive()
		}
	}
}

func (peer *Peer) RoutineSequentialReceiver() {
	var elem *QueueInboundElement

	device := peer.device

	logInfo := device.log.Info
	logError := device.log.Error
	logDebug := device.log.Debug
	logDebug.Println("Routine, sequential receiver, started for peer", peer.id)

	for {
		// wait for decryption

		select {
		case <-peer.signal.stop:
			return
		case elem = <-peer.queue.inbound:
		}
		elem.mutex.Lock()

		// process packet

		if elem.IsDropped() {
			continue
		}

		// check for replay

		if !elem.keyPair.replayFilter.ValidateCounter(elem.counter) {
			continue
		}

		peer.TimerAnyAuthenticatedPacketTraversal()
		peer.TimerAnyAuthenticatedPacketReceived()
		peer.KeepKeyFreshReceiving()

		// check if using new key-pair

		kp := &peer.keyPairs
		kp.mutex.Lock()
		if kp.next == elem.keyPair {
			peer.TimerHandshakeComplete()
			kp.previous = kp.current
			kp.current = kp.next
			kp.next = nil
		}
		kp.mutex.Unlock()

		// check for keep-alive

		if len(elem.packet) == 0 {
			logDebug.Println("Received keep-alive from", peer.String())
			continue
		}
		peer.TimerDataReceived()

		// verify source and strip padding

		switch elem.packet[0] >> 4 {
		case ipv4.Version:

			// strip padding

			if len(elem.packet) < ipv4.HeaderLen {
				continue
			}

			field := elem.packet[IPv4offsetTotalLength : IPv4offsetTotalLength+2]
			length := binary.BigEndian.Uint16(field)
			if int(length) > len(elem.packet) || int(length) < ipv4.HeaderLen {
				continue
			}

			elem.packet = elem.packet[:length]

			// verify IPv4 source

			src := elem.packet[IPv4offsetSrc : IPv4offsetSrc+net.IPv4len]
			if device.routingTable.LookupIPv4(src) != peer {
				logInfo.Println("Packet with unallowed source IP from", peer.String())
				continue
			}

		case ipv6.Version:

			// strip padding

			if len(elem.packet) < ipv6.HeaderLen {
				continue
			}

			field := elem.packet[IPv6offsetPayloadLength : IPv6offsetPayloadLength+2]
			length := binary.BigEndian.Uint16(field)
			length += ipv6.HeaderLen
			if int(length) > len(elem.packet) {
				continue
			}

			elem.packet = elem.packet[:length]

			// verify IPv6 source

			src := elem.packet[IPv6offsetSrc : IPv6offsetSrc+net.IPv6len]
			if device.routingTable.LookupIPv6(src) != peer {
				logInfo.Println("Packet with unallowed source IP from", peer.String())
				continue
			}

		default:
			logInfo.Println("Packet with invalid IP version from", peer.String())
			continue
		}

		// write to tun

		atomic.AddUint64(&peer.stats.rxBytes, uint64(len(elem.packet)))
		_, err := device.tun.device.Write(elem.packet)
		device.PutMessageBuffer(elem.buffer)
		if err != nil {
			logError.Println("Failed to write packet to TUN device:", err)
		}
	}
}