package main

import (
	"context"
	"fmt"
	"io"
	"log"
	"net"
	"net/http"
	"os"
	"os/exec"
	"os/signal"
	"runtime"
	"strings"
	"syscall"
	"time"
)

// language=html
const injectScript = `
<script>
	const sendShutdown = () => navigator.sendBeacon('/shutdown');

	window.addEventListener('beforeunload', sendShutdown);
	window.addEventListener('unload', sendShutdown);
</script>
`

func main() {
	if err := run(); err != nil {
		log.Fatal(err)
	}
}

func newHandler(html string, onShutdown func()) *http.ServeMux {
	handler := http.NewServeMux()

	handler.HandleFunc("POST /shutdown", func(w http.ResponseWriter, r *http.Request) {
		onShutdown()
	})

	handler.HandleFunc("GET /", func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("Content-Type", "text/html")
		w.Write([]byte(html))
	})

	return handler
}

func run() error {
	html, err := io.ReadAll(os.Stdin)
	if err != nil {
		return fmt.Errorf("failed to read HTML from stdin: %w", err)
	}

	htmlStr := string(html)
	if strings.Contains(htmlStr, "</body>") {
		htmlStr = strings.ReplaceAll(string(html), "</body>", injectScript+"</body>")
	} else {
		htmlStr += injectScript
	}

	listener, err := net.Listen("tcp", ":0")
	if err != nil {
		return fmt.Errorf("failed to create listener: %w", err)
	}

	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
	defer cancel()

	port := listener.Addr().(*net.TCPAddr).Port

	server := &http.Server{Addr: listener.Addr().String()}
	server.Handler = newHandler(htmlStr, func() {
		cancel()
	})

	servedURL := fmt.Sprintf("http://localhost:%d", port)

	go func() {
		log.Printf("serving at %s", servedURL)
		if err := server.Serve(listener); err != nil && err != http.ErrServerClosed {
			log.Fatalf("failed to serve: %v", err)
		}
	}()

	go func() {
		// wait for server to start
		time.Sleep(250 * time.Millisecond)
		if err := openBrowser(servedURL); err != nil {
			log.Fatalf("failed to open browser: %v", err)
		}
	}()

	<-ctx.Done()

	log.Printf("shutting down server")

	if err := server.Shutdown(ctx); err != nil {
		return fmt.Errorf("failed to shutdown server: %w", err)
	}

	return nil
}

func openBrowser(url string) error {
	var cmd *exec.Cmd
	switch runtime.GOOS {
	case "darwin":
		cmd = exec.Command("open", url)
	case "linux":
		cmd = exec.Command("xdg-open", url)
	case "windows":
		cmd = exec.Command("cmd", "/c", "start", url)
	default:
		return fmt.Errorf("invalid URL: %s", url)
	}
	return cmd.Run()
}