package vpn

import (
	"crypto/rand"
	"encoding/binary"
	"encoding/json"
	"fmt"
	"log"
	"net"
	"os"
	"os/exec"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"
	"syscall"
	"time"
)

// EoIPInstance represents a MikroTik Ethernet-over-IP tunnel instance
type EoIPInstance struct {
	ID            string `json:"id"`
	Name          string `json:"name"`           // e.g. "Goldix" or "eoip-tunnel1"
	TunnelID      int    `json:"tunnel_id"`      // 1 - 65535
	RemoteAddress string `json:"remote_address"` // Remote MikroTik IPv4
	LocalAddress  string `json:"local_address"`  // Local router IPv4 (optional)
	IPAddress     string `json:"ip_address,omitempty"` // Overlay IP/CIDR (e.g. "172.16.33.254/30")
	MACAddress    string `json:"mac_address"`    // e.g. "02:64:83:6b:46:44"
	MTU           int    `json:"mtu"`            // default 1500
	ClampMSS      bool   `json:"clamp_mss"`      // Clamp TCP MSS for PMTUD
	Comment       string `json:"comment,omitempty"`
	Disabled      bool   `json:"disabled"`
	Running       bool   `json:"running"` // MikroTik 'R' flag
	RxPackets     uint64 `json:"rx_packets"`
	TxPackets     uint64 `json:"tx_packets"`
	RxBytes       uint64 `json:"rx_bytes"`
	TxBytes       uint64 `json:"tx_bytes"`
	RxHuman       string `json:"rx_human,omitempty"`
	TxHuman       string `json:"tx_human,omitempty"`
	LastError     string `json:"last_error,omitempty"`
}

type EoIPManager struct {
	mu        sync.RWMutex
	storePath string
	tunnels   map[string]*EoIPInstance
	workers   map[string]*eoipTunnelWorker
}

func NewEoIPManager() *EoIPManager {
	mgr := &EoIPManager{
		storePath: "/etc/proxmox-router/vpn/eoip.json",
		tunnels:   make(map[string]*EoIPInstance),
		workers:   make(map[string]*eoipTunnelWorker),
	}
	mgr.load()
	mgr.syncAll()
	return mgr
}

func (m *EoIPManager) load() {
	if data, err := os.ReadFile(m.storePath); err == nil {
		var list []*EoIPInstance
		if err := json.Unmarshal(data, &list); err == nil {
			for _, t := range list {
				m.tunnels[t.ID] = t
			}
		}
	}
}

func (m *EoIPManager) save() error {
	_ = os.MkdirAll(filepath.Dir(m.storePath), 0755)
	var list []*EoIPInstance
	for _, t := range m.tunnels {
		list = append(list, t)
	}
	data, err := json.MarshalIndent(list, "", "  ")
	if err != nil {
		return err
	}
	return os.WriteFile(m.storePath, data, 0644)
}

func (m *EoIPManager) List() []*EoIPInstance {
	m.mu.Lock()
	defer m.mu.Unlock()

	list := make([]*EoIPInstance, 0, len(m.tunnels))
	for _, t := range m.tunnels {
		m.updateStats(t)
		list = append(list, t)
	}

	sort.Slice(list, func(i, j int) bool {
		if list[i].TunnelID != list[j].TunnelID {
			return list[i].TunnelID < list[j].TunnelID
		}
		return list[i].Name < list[j].Name
	})

	return list
}

func (m *EoIPManager) Get(id string) (*EoIPInstance, bool) {
	m.mu.Lock()
	defer m.mu.Unlock()
	t, ok := m.tunnels[id]
	if ok {
		m.updateStats(t)
	}
	return t, ok
}

func (m *EoIPManager) Save(inst *EoIPInstance) error {
	m.mu.Lock()
	defer m.mu.Unlock()

	if inst.Name == "" {
		inst.Name = fmt.Sprintf("eoip-tunnel%d", inst.TunnelID)
	}
	if inst.TunnelID <= 0 || inst.TunnelID > 65535 {
		return fmt.Errorf("tunnel_id must be between 1 and 65535")
	}
	if inst.RemoteAddress == "" {
		return fmt.Errorf("remote_address is required")
	}
	if inst.MTU <= 0 {
		inst.MTU = 1500
	}
	if inst.MACAddress == "" {
		inst.MACAddress = generateMAC()
	}

	if inst.ID == "" {
		inst.ID = fmt.Sprintf("eoip-%d", time.Now().UnixNano())
	}

	// Smart detection: if user put CIDR into LocalAddress by mistake, move it to IPAddress
	if strings.Contains(inst.LocalAddress, "/") && inst.IPAddress == "" {
		inst.IPAddress = inst.LocalAddress
		inst.LocalAddress = ""
	}

	// Stop previous worker if exists
	if w, exists := m.workers[inst.ID]; exists {
		w.stop()
		delete(m.workers, inst.ID)
	}

	m.tunnels[inst.ID] = inst
	_ = m.save()

	if !inst.Disabled {
		worker, err := newEoIPTunnelWorker(inst)
		if err != nil {
			inst.LastError = err.Error()
			inst.Running = false
			return err
		}
		m.workers[inst.ID] = worker
		go worker.start()
	} else {
		inst.Running = false
	}

	return nil
}

func (m *EoIPManager) SetState(id string, disabled bool) error {
	m.mu.Lock()
	defer m.mu.Unlock()

	t, ok := m.tunnels[id]
	if !ok {
		return fmt.Errorf("tunnel not found")
	}
	t.Disabled = disabled
	_ = m.save()

	if w, exists := m.workers[id]; exists {
		w.stop()
		delete(m.workers, id)
	}

	if disabled {
		t.Running = false
		return nil
	}

	worker, err := newEoIPTunnelWorker(t)
	if err != nil {
		t.LastError = err.Error()
		t.Running = false
		return err
	}
	m.workers[id] = worker
	go worker.start()
	return nil
}

func (m *EoIPManager) Delete(id string) error {
	m.mu.Lock()
	defer m.mu.Unlock()

	if _, ok := m.tunnels[id]; !ok {
		return fmt.Errorf("tunnel not found")
	}

	if w, exists := m.workers[id]; exists {
		w.stop()
		delete(m.workers, id)
	}

	delete(m.tunnels, id)
	return m.save()
}

func (m *EoIPManager) syncAll() {
	m.mu.Lock()
	defer m.mu.Unlock()

	for id, t := range m.tunnels {
		if !t.Disabled {
			if w, exists := m.workers[id]; exists {
				w.stop()
				delete(m.workers, id)
			}
			worker, err := newEoIPTunnelWorker(t)
			if err != nil {
				t.LastError = err.Error()
				t.Running = false
				continue
			}
			m.workers[id] = worker
			go worker.start()
		} else {
			t.Running = false
		}
	}
}

func (m *EoIPManager) updateStats(t *EoIPInstance) {
	if w, exists := m.workers[t.ID]; exists {
		t.Running = w.isRunning()
		t.RxBytes = atomic.LoadUint64(&w.rxBytes)
		t.RxPackets = atomic.LoadUint64(&w.rxPackets)
		t.TxBytes = atomic.LoadUint64(&w.txBytes)
		t.TxPackets = atomic.LoadUint64(&w.txPackets)
	} else {
		t.Running = false
	}
	t.RxHuman = formatBytes(t.RxBytes)
	t.TxHuman = formatBytes(t.TxBytes)

	// Detect any bound IPv4 if not explicitly saved in JSON
	if t.IPAddress == "" {
		if ifi, err := net.InterfaceByName(t.Name); err == nil {
			if addrs, err := ifi.Addrs(); err == nil {
				for _, a := range addrs {
					if ipNet, ok := a.(*net.IPNet); ok && ipNet.IP.To4() != nil {
						t.IPAddress = ipNet.String()
						break
					}
				}
			}
		}
	}
}

// -------------------------------------------------------------
// Native MikroTik EoIP Engine Worker
// -------------------------------------------------------------

type eoipTunnelWorker struct {
	inst           *EoIPInstance
	peerIfName     string
	peerIfIndex    int
	greFd          int
	pktFd          int
	stopChan       chan struct{}
	lastRx         int64 // unix nano
	rxBytes        uint64
	rxPackets      uint64
	txBytes        uint64
	txPackets      uint64
	remoteAddrInet syscall.SockaddrInet4
}

func newEoIPTunnelWorker(inst *EoIPInstance) (*eoipTunnelWorker, error) {
	// Parse Remote IPv4
	remoteIP := net.ParseIP(inst.RemoteAddress).To4()
	if remoteIP == nil {
		return nil, fmt.Errorf("invalid remote address: %s", inst.RemoteAddress)
	}

	// Internal peer interface name (max 15 chars for Linux ifnames)
	peerName := fmt.Sprintf("ep_%s", inst.Name)
	if len(peerName) > 15 {
		peerName = peerName[:15]
	}

	// 1. Clean up existing links if present
	_ = exec.Command("ip", "link", "del", inst.Name).Run()
	_ = exec.Command("ip", "link", "del", peerName).Run()

	// 2. Create veth pair: inst.Name <--> peerName
	cmdAdd := exec.Command("ip", "link", "add", inst.Name, "type", "veth", "peer", "name", peerName)
	if out, err := cmdAdd.CombinedOutput(); err != nil {
		return nil, fmt.Errorf("failed to create veth pair: %s - %w", string(out), err)
	}

	// 3. Set MAC address and MTU
	if inst.MACAddress != "" {
		_ = exec.Command("ip", "link", "set", inst.Name, "address", inst.MACAddress).Run()
	}
	mtuStr := strconv.Itoa(inst.MTU)
	_ = exec.Command("ip", "link", "set", inst.Name, "mtu", mtuStr).Run()
	_ = exec.Command("ip", "link", "set", peerName, "mtu", mtuStr).Run()

	// 4. Bring both interfaces UP
	_ = exec.Command("ip", "link", "set", inst.Name, "up").Run()
	_ = exec.Command("ip", "link", "set", peerName, "up").Run()

	// 4b. Assign Overlay IP address if specified
	if inst.IPAddress != "" {
		_ = exec.Command("ip", "addr", "add", inst.IPAddress, "dev", inst.Name).Run()
	}

	// 5. TCP MSS Clamping
	if inst.ClampMSS {
		_ = exec.Command("iptables", "-t", "mangle", "-A", "FORWARD", "-o", inst.Name, "-p", "tcp", "--tcp-flags", "SYN,RST", "SYN", "-j", "TCPMSS", "--clamp-mss-to-pmtu").Run()
	}

	// 6. Find interface index of peerName
	peerIf, err := net.InterfaceByName(peerName)
	if err != nil {
		_ = exec.Command("ip", "link", "del", inst.Name).Run()
		return nil, fmt.Errorf("cannot find peer interface %s: %w", peerName, err)
	}

	// 7. Open AF_PACKET raw socket on peer interface
	pktFd, err := syscall.Socket(syscall.AF_PACKET, syscall.SOCK_RAW, int(htons(syscall.ETH_P_ALL)))
	if err != nil {
		_ = exec.Command("ip", "link", "del", inst.Name).Run()
		return nil, fmt.Errorf("cannot create AF_PACKET socket: %w", err)
	}
	pktAddr := syscall.SockaddrLinklayer{
		Protocol: htons(syscall.ETH_P_ALL),
		Ifindex:  peerIf.Index,
	}
	if err := syscall.Bind(pktFd, &pktAddr); err != nil {
		syscall.Close(pktFd)
		_ = exec.Command("ip", "link", "del", inst.Name).Run()
		return nil, fmt.Errorf("cannot bind AF_PACKET socket to %s: %w", peerName, err)
	}

	// 8. Open raw GRE socket (IPPROTO_GRE = 47)
	greFd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_RAW, syscall.IPPROTO_GRE)
	if err != nil {
		syscall.Close(pktFd)
		_ = exec.Command("ip", "link", "del", inst.Name).Run()
		return nil, fmt.Errorf("cannot create raw GRE socket: %w", err)
	}

	// Bind GRE socket to LocalAddress if specified
	if inst.LocalAddress != "" {
		localIP := net.ParseIP(inst.LocalAddress).To4()
		if localIP != nil {
			localAddr := &syscall.SockaddrInet4{
				Addr: [4]byte{localIP[0], localIP[1], localIP[2], localIP[3]},
			}
			_ = syscall.Bind(greFd, localAddr)
		}
	}

	w := &eoipTunnelWorker{
		inst:        inst,
		peerIfName:  peerName,
		peerIfIndex: peerIf.Index,
		greFd:       greFd,
		pktFd:       pktFd,
		stopChan:    make(chan struct{}),
		remoteAddrInet: syscall.SockaddrInet4{
			Addr: [4]byte{remoteIP[0], remoteIP[1], remoteIP[2], remoteIP[3]},
		},
		rxBytes:   inst.RxBytes,
		rxPackets: inst.RxPackets,
		txBytes:   inst.TxBytes,
		txPackets: inst.TxPackets,
	}

	return w, nil
}

func (w *eoipTunnelWorker) start() {
	log.Printf("[EoIP] Started tunnel '%s' (ID: %d, Remote: %s)", w.inst.Name, w.inst.TunnelID, w.inst.RemoteAddress)

	// Goroutine 1: Forward packets from Local veth -> Remote MikroTik via GRE
	go w.txLoop()

	// Goroutine 2: Receive packets from Remote MikroTik via GRE -> Inject to Local veth
	go w.rxLoop()

	// Goroutine 3: Periodic MikroTik EoIP Keepalive
	go w.keepaliveLoop()
}

func (w *eoipTunnelWorker) isRunning() bool {
	last := atomic.LoadInt64(&w.lastRx)
	if last == 0 {
		return false
	}
	return time.Since(time.Unix(0, last)) < 15*time.Second
}

func (w *eoipTunnelWorker) sendKeepalive() {
	// MikroTik EoIP Keepalive packet:
	// GRE Flags: 0x2001
	// Protocol:  0x6400
	// Length:    0x0000
	// Tunnel ID: Little-Endian
	tid := uint16(w.inst.TunnelID)
	pkt := []byte{
		0x20, 0x01,
		0x64, 0x00,
		0x00, 0x00,
		byte(tid & 0xff), byte(tid >> 8),
	}
	_ = syscall.Sendto(w.greFd, pkt, 0, &w.remoteAddrInet)
}

func (w *eoipTunnelWorker) keepaliveLoop() {
	ticker := time.NewTicker(5 * time.Second)
	defer ticker.Stop()

	// Send immediate initial keepalive
	w.sendKeepalive()

	for {
		select {
		case <-w.stopChan:
			return
		case <-ticker.C:
			w.sendKeepalive()
		}
	}
}

func (w *eoipTunnelWorker) txLoop() {
	buf := make([]byte, 2048)
	tid := uint16(w.inst.TunnelID)

	for {
		select {
		case <-w.stopChan:
			return
		default:
		}

		n, from, err := syscall.Recvfrom(w.pktFd, buf, 0)
		if err != nil {
			select {
			case <-w.stopChan:
				return
			default:
				time.Sleep(10 * time.Millisecond)
				continue
			}
		}

		// Filter out outgoing packets to prevent reflection
		if fromLink, ok := from.(*syscall.SockaddrLinklayer); ok {
			if fromLink.Pkttype == syscall.PACKET_OUTGOING {
				continue
			}
		}

		// Must have at least minimum Ethernet header length (14 bytes)
		if n < 14 {
			continue
		}

		// Construct MikroTik EoIP Header (8 bytes) + Ethernet Frame
		// [0..1] = 0x2001 (Flags)
		// [2..3] = 0x6400 (Protocol)
		// [4..5] = payload length (Big-Endian)
		// [6..7] = tunnel ID (Little-Endian)
		grePkt := make([]byte, 8+n)
		grePkt[0] = 0x20
		grePkt[1] = 0x01
		grePkt[2] = 0x64
		grePkt[3] = 0x00
		binary.BigEndian.PutUint16(grePkt[4:6], uint16(n))
		binary.LittleEndian.PutUint16(grePkt[6:8], tid)
		copy(grePkt[8:], buf[:n])

		// Send over raw GRE socket
		if err := syscall.Sendto(w.greFd, grePkt, 0, &w.remoteAddrInet); err == nil {
			atomic.AddUint64(&w.txPackets, 1)
			atomic.AddUint64(&w.txBytes, uint64(n))
		}
	}
}

func (w *eoipTunnelWorker) rxLoop() {
	buf := make([]byte, 4096)
	expectedTid := uint16(w.inst.TunnelID)
	remoteIP := net.ParseIP(w.inst.RemoteAddress).To4()

	injectAddr := syscall.SockaddrLinklayer{
		Protocol: htons(syscall.ETH_P_ALL),
		Ifindex:  w.peerIfIndex,
		Halen:    6,
	}

	for {
		select {
		case <-w.stopChan:
			return
		default:
		}

		n, from, err := syscall.Recvfrom(w.greFd, buf, 0)
		if err != nil {
			select {
			case <-w.stopChan:
				return
			default:
				time.Sleep(10 * time.Millisecond)
				continue
			}
		}

		// RAW IPv4 socket Recvfrom returns [IPv4 Header (20+ bytes)] + [GRE Header (8 bytes)] + [Payload]
		if n < 28 {
			continue
		}

		// Verify remote sender IP
		if fromIP, ok := from.(*syscall.SockaddrInet4); ok {
			if fromIP.Addr[0] != remoteIP[0] || fromIP.Addr[1] != remoteIP[1] ||
				fromIP.Addr[2] != remoteIP[2] || fromIP.Addr[3] != remoteIP[3] {
				continue
			}
		}

		// Calculate IPv4 Header Length
		ipHdrLen := int(buf[0]&0x0f) * 4
		if n < ipHdrLen+8 {
			continue
		}

		gre := buf[ipHdrLen:n]
		// Verify MikroTik EoIP GRE Header
		// Flags: 0x2001 (or 0x2000)
		// Proto: 0x6400
		if gre[0] != 0x20 || (gre[1] != 0x01 && gre[1] != 0x00) {
			continue
		}
		if gre[2] != 0x64 || gre[3] != 0x00 {
			continue
		}

		frameLen := binary.BigEndian.Uint16(gre[4:6])
		tunnelID := binary.LittleEndian.Uint16(gre[6:8])

		if tunnelID != expectedTid {
			continue
		}

		// Record successful packet reception
		now := time.Now().UnixNano()
		atomic.StoreInt64(&w.lastRx, now)

		if frameLen == 0 {
			// Keepalive probe from MikroTik: reply with keepalive
			w.sendKeepalive()
			continue
		}

		// Valid Ethernet frame received from MikroTik EoIP tunnel
		ethFrame := gre[8:]
		if len(ethFrame) < int(frameLen) {
			continue
		}
		payload := ethFrame[:frameLen]

		// Inject Ethernet frame into veth peer -> kernel delivers it to user interface (w.inst.Name)
		if err := syscall.Sendto(w.pktFd, payload, 0, &injectAddr); err == nil {
			atomic.AddUint64(&w.rxPackets, 1)
			atomic.AddUint64(&w.rxBytes, uint64(frameLen))
		}
	}
}

func (w *eoipTunnelWorker) stop() {
	close(w.stopChan)
	_ = syscall.Close(w.pktFd)
	_ = syscall.Close(w.greFd)

	// Clean up interfaces & iptables
	if w.inst.ClampMSS {
		_ = exec.Command("iptables", "-t", "mangle", "-D", "FORWARD", "-o", w.inst.Name, "-p", "tcp", "--tcp-flags", "SYN,RST", "SYN", "-j", "TCPMSS", "--clamp-mss-to-pmtu").Run()
	}
	_ = exec.Command("ip", "link", "del", w.inst.Name).Run()
	log.Printf("[EoIP] Stopped tunnel '%s'", w.inst.Name)
}

func htons(n uint16) uint16 {
	return (n<<8)&0xff00 | (n>>8)&0x00ff
}

func generateMAC() string {
	buf := make([]byte, 3)
	_, _ = rand.Read(buf)
	return fmt.Sprintf("02:64:%02x:%02x:%02x:%02x", buf[0], buf[1], buf[2], time.Now().UnixNano()%256)
}

func formatBytes(b uint64) string {
	const unit = 1024
	if b < unit {
		return fmt.Sprintf("%d B", b)
	}
	div, exp := uint64(unit), 0
	for n := b / unit; n >= unit; n /= unit {
		div *= unit
		exp++
	}
	return fmt.Sprintf("%.1f %cB", float64(b)/float64(div), "KMGTPE"[exp])
}