package main

import (
	"context"
	"encoding/json"
	"fmt"
	"log"
	"os"
	"os/exec"
	"path/filepath"
	"runtime"
	"strconv"
	"sync"
	"syscall"
	"time"
)

var (
	tsCmd      *exec.Cmd
	tsCmdMu    sync.Mutex
	tsIP       string
	tsDataDir  string
	tsSocket   string
)

type tailscaleStatus struct {
	Self struct {
		TailscaleIPs []string `json:"TailscaleIPs"`
	} `json:"Self"`
}

func tailscaleBin(name string) string {
	// Prefer bundled binaries (tailscale-bin/<os>/tailscaled etc.).
	ex, err := os.Executable()
	if err == nil {
		bundled := filepath.Join(filepath.Dir(ex), "tailscale-bin", runtime.GOOS, name)
		if runtime.GOOS == "windows" {
			bundled += ".exe"
		}
		if _, err := os.Stat(bundled); err == nil {
			return bundled
		}
	}
	if p, err := exec.LookPath(name); err == nil {
		return p
	}
	return name
}

func startTailscale(dataDir, nodeID, headscaleURL, authKey string) (string, error) {
	tsCmdMu.Lock()
	defer tsCmdMu.Unlock()

	if tsCmd != nil {
		return tsIP, nil
	}

	if dataDir == "" {
		return "", fmt.Errorf("tailscale data dir is empty")
	}
	tsDataDir = filepath.Join(dataDir, "tailscale")
	if err := os.MkdirAll(tsDataDir, 0700); err != nil {
		return "", fmt.Errorf("create tailscale dir: %w", err)
	}
	// Make sure a previous tailscaled (e.g. left behind after a crash or
	// force-kill) does not block the new daemon on the same socket/state.
	killStaleTailscaled(tsDataDir)
	if runtime.GOOS == "windows" {
		// Windows uses named pipes for tailscaled IPC, not Unix sockets.
		tsSocket = `\\.\pipe\studioe5-tailscaled`
	} else {
		tsSocket = filepath.Join(tsDataDir, "tailscaled.sock")
	}
	stateFile := filepath.Join(tsDataDir, "tailscaled.state")

	log.Printf("Starting tailscaled for node %s (socket=%s)", nodeID, tsSocket)
	tsCmd = exec.Command(tailscaleBin("tailscaled"),
		"--state="+stateFile,
		"--socket="+tsSocket,
		"--tun=userspace-networking",
	)
	hideWindow(tsCmd)
	// Redirect tailscaled output to a dedicated log file.
	tsLogPath := filepath.Join(tsDataDir, "tailscaled.log")
	if tsLogFile, err := os.OpenFile(tsLogPath, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0644); err == nil {
		tsCmd.Stdout = tsLogFile
		tsCmd.Stderr = tsLogFile
	} else {
		log.Printf("Cannot open tailscaled log file %s: %v", tsLogPath, err)
	}
	if err := tsCmd.Start(); err != nil {
		tsCmd = nil
		return "", fmt.Errorf("start tailscaled: %w", err)
	}
	if err := os.WriteFile(filepath.Join(tsDataDir, "tailscaled.pid"), []byte(strconv.Itoa(tsCmd.Process.Pid)), 0644); err != nil {
		log.Printf("Cannot write tailscaled pid file: %v", err)
	}

	// Give tailscaled a moment to start listening.
	time.Sleep(1 * time.Second)

	// Bring the interface up with the auth key.
	ctx, cancel := context.WithTimeout(context.Background(), 120*time.Second)
	defer cancel()

	upArgs := []string{
		"--socket=" + tsSocket,
		"up",
		"--login-server=" + headscaleURL,
		"--hostname=" + nodeID,
		"--accept-dns=false",
	}
	// The auth key is omitted on reconnect: Tailscale reuses the existing state.
	if authKey != "" {
		upArgs = append(upArgs, "--authkey="+authKey)
	}
	if runtime.GOOS == "windows" {
		// On Windows, keep the VPN up even after the tailscale.exe CLI client disconnects.
		upArgs = append(upArgs, "--unattended")
	} else {
		// --operator is only meaningful on Unix systems.
		upArgs = append(upArgs, "--operator=root")
	}
	upCmd := exec.CommandContext(ctx, tailscaleBin("tailscale"), upArgs...)
	hideWindow(upCmd)
	upCmd.Stdout = log.Writer()
	upCmd.Stderr = log.Writer()
	if err := upCmd.Run(); err != nil {
		_ = tsCmd.Process.Kill()
		_ = tsCmd.Wait()
		tsCmd = nil
		return "", fmt.Errorf("tailscale up: %w", err)
	}

	// Wait for an IP address.
	for {
		statusCmd := exec.CommandContext(ctx, tailscaleBin("tailscale"),
			"--socket="+tsSocket,
			"status", "--json",
		)
		hideWindow(statusCmd)
		out, err := statusCmd.Output()
		if err != nil {
			select {
			case <-ctx.Done():
				_ = tsCmd.Process.Kill()
				_ = tsCmd.Wait()
				tsCmd = nil
				return "", fmt.Errorf("tailscale status: %w", err)
			default:
				time.Sleep(1 * time.Second)
				continue
			}
		}
		var st tailscaleStatus
		if err := json.Unmarshal(out, &st); err == nil && len(st.Self.TailscaleIPs) > 0 {
			tsIP = st.Self.TailscaleIPs[0]
			break
		}
		select {
		case <-ctx.Done():
			_ = tsCmd.Process.Kill()
			_ = tsCmd.Wait()
			tsCmd = nil
			return "", fmt.Errorf("tailscale IP timeout")
		default:
			time.Sleep(1 * time.Second)
		}
	}

	log.Printf("Tailscale started with IP: %s", tsIP)
	return tsIP, nil
}

func stopTailscale() {
	tsCmdMu.Lock()
	defer tsCmdMu.Unlock()
	stopTailscaleLocked()
}

func stopTailscaleLocked() {
	if tsCmd == nil || tsCmd.Process == nil {
		return
	}
	if tsSocket != "" {
		downCmd := exec.Command(tailscaleBin("tailscale"), "--socket="+tsSocket, "down")
		hideWindow(downCmd)
		_ = downCmd.Run()
	}
	_ = tsCmd.Process.Kill()
	_ = tsCmd.Wait()
	tsCmd = nil
	tsIP = ""
	if tsDataDir != "" {
		_ = os.Remove(filepath.Join(tsDataDir, "tailscaled.pid"))
	}
	log.Printf("Tailscale stopped")
}

func isTailscaleRunning() bool {
	tsCmdMu.Lock()
	defer tsCmdMu.Unlock()
	if tsCmd == nil || tsCmd.Process == nil {
		return false
	}
	// Signal 0 checks process existence without affecting it.
	return tsCmd.Process.Signal(syscall.Signal(0)) == nil
}

func getTailscaleIP() string {
	tsCmdMu.Lock()
	defer tsCmdMu.Unlock()
	return tsIP
}

// isTailscaleReady reports whether tailscaled is running and has successfully
// joined the tailnet (i.e. it has a Tailscale IP). It does not rely on
// isTailscaleRunning because tailscaled may have been started by a previous
// agent run or externally; the important thing is that the socket responds.
func isTailscaleReady() bool {
	tsCmdMu.Lock()
	socket := tsSocket
	tsCmdMu.Unlock()
	if socket == "" {
		return false
	}

	statusCmd := exec.Command(tailscaleBin("tailscale"), "--socket="+socket, "status", "--json")
	hideWindow(statusCmd)
	out, err := statusCmd.Output()
	if err != nil {
		return false
	}
	var st tailscaleStatus
	if err := json.Unmarshal(out, &st); err != nil {
		return false
	}
	return len(st.Self.TailscaleIPs) > 0
}

// setupTailscaleServe configures Tailscale to proxy inbound Tailnet traffic
// on the given TCP port to localhost:<port>. This is required on Windows
// because userspace networking does not forward incoming connections to
// loopback by default.
func setupTailscaleServe(port int) error {
	tsCmdMu.Lock()
	defer tsCmdMu.Unlock()
	if tsSocket == "" {
		return fmt.Errorf("tailscale socket not initialized")
	}

	portStr := strconv.Itoa(port)
	// Clean up any stale config for this port first.
	offCmd := exec.Command(tailscaleBin("tailscale"), "--socket="+tsSocket, "serve", "--bg", "--tcp="+portStr, "off")
	hideWindow(offCmd)
	_ = offCmd.Run()

	serveCmd := exec.Command(tailscaleBin("tailscale"), "--socket="+tsSocket, "serve", "--bg", "--tcp="+portStr, "tcp://localhost:"+portStr)
	hideWindow(serveCmd)
	out, err := serveCmd.CombinedOutput()
	if err != nil {
		return fmt.Errorf("tailscale serve: %w: %s", err, string(out))
	}
	log.Printf("Tailscale serve configured for port %s", portStr)
	return nil
}

// removeTailscaleServe removes the Tailscale serve proxy for a port when an
// instance is stopped or deleted.
func removeTailscaleServe(port int) {
	tsCmdMu.Lock()
	defer tsCmdMu.Unlock()
	if tsSocket == "" {
		return
	}
	portStr := strconv.Itoa(port)
	offCmd := exec.Command(tailscaleBin("tailscale"), "--socket="+tsSocket, "serve", "--bg", "--tcp="+portStr, "off")
	hideWindow(offCmd)
	_ = offCmd.Run()
	log.Printf("Tailscale serve removed for port %s", portStr)
}

// killStaleTailscaled terminates a previously started tailscaled process that
// may have been left running after the agent was force-killed.
func killStaleTailscaled(tsDataDir string) {
	pidFile := filepath.Join(tsDataDir, "tailscaled.pid")
	data, err := os.ReadFile(pidFile)
	if err != nil {
		return
	}
	var pid int
	if _, err := fmt.Sscanf(string(data), "%d", &pid); err != nil {
		return
	}
	proc, err := os.FindProcess(pid)
	if err != nil {
		return
	}
	if err := proc.Signal(syscall.Signal(0)); err == nil {
		log.Printf("Killing stale tailscaled process %d", pid)
		_ = proc.Kill()
		_, _ = proc.Wait()
	}
	_ = os.Remove(pidFile)
}