package main

import (
	"context"
	"errors"
	"flag"
	"fmt"
	"log"
	"net/http"
	"os"
	"os/exec"
	"os/signal"
	"path/filepath"
	"strings"
	"sync"
	"syscall"
	"time"

	"github.com/fsnotify/fsnotify"
)

// serveDist starts an HTTP file server rooted at distPath on the given address.
func serveDist(ctx context.Context, distPath, addr string, wg *sync.WaitGroup) *http.Server {
	fs := http.FileServer(http.Dir(distPath))
	srv := &http.Server{Addr: addr, Handler: fs}

	wg.Add(1)
	go func() {
		defer wg.Done()
		if err := srv.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
			log.Printf("http server error: %v", err)
		}
	}()

	go func() {
		<-ctx.Done()
		shutdownCtx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
		defer cancel()
		_ = srv.Shutdown(shutdownCtx)
	}()

	return srv
}

// runMake executes `make` in the provided project directory.
func runMake(projectDir string) error {
	cmd := exec.Command("make")
	cmd.Dir = projectDir
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr
	log.Printf("running make in %s", projectDir)
	return cmd.Run()
}

func shouldSkipPath(projectDir, path string) bool {
	rel, err := filepath.Rel(projectDir, path)
	if err != nil {
		return false
	}
	if rel == "." {
		return false
	}
	// Normalize to forward slashes for simple contains checks.
	rel = filepath.ToSlash(rel)
	// Skip changes inside these directories to avoid rebuild loops and noise.
	skipPrefixes := []string{
		"_dist/",
		".git/",
	}
	for _, p := range skipPrefixes {
		if strings.HasPrefix(rel, p) {
			return true
		}
	}
	return false
}

// addWatchersRecursively walks projectDir and adds fsnotify watchers to all directories except skipped ones.
func addWatchersRecursively(w *fsnotify.Watcher, projectDir string) error {
	return filepath.WalkDir(projectDir, func(path string, d os.DirEntry, err error) error {
		if err != nil {
			return err
		}
		if !d.IsDir() {
			return nil
		}
		if shouldSkipPath(projectDir, path) {
			return filepath.SkipDir
		}
		if err := w.Add(path); err != nil {
			return fmt.Errorf("add watcher for %s: %w", path, err)
		}
		return nil
	})
}

func main() {
	log.SetFlags(log.LstdFlags | log.Lmicroseconds)

	// Support optional --port flag while keeping the requested positional path arg.
	port := flag.Int("port", 8000, "port to serve _dist on")
	flag.Parse()

	if flag.NArg() < 1 {
		fmt.Fprintf(os.Stderr, "usage: %s [--port PORT] <project-path-with-Makefile>\n", filepath.Base(os.Args[0]))
		os.Exit(2)
	}
	projectDir := flag.Arg(0)
	absProjectDir, err := filepath.Abs(projectDir)
	if err != nil {
		log.Fatalf("resolve project path: %v", err)
	}

	// Validate Makefile presence.
	if _, err := os.Stat(filepath.Join(absProjectDir, "Makefile")); err != nil {
		log.Fatalf("Makefile not found in %s: %v", absProjectDir, err)
	}

	distDir := filepath.Join(absProjectDir, "_dist")
	if err := os.MkdirAll(distDir, 0o755); err != nil {
		log.Fatalf("ensure _dist exists: %v", err)
	}

	// Initial build.
	if err := runMake(absProjectDir); err != nil {
		log.Printf("initial make failed: %v", err)
	}

	// Context and signal handling.
	ctx, cancel := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
	defer cancel()

	// Start HTTP server.
	var wg sync.WaitGroup
	addr := fmt.Sprintf(":%d", *port)
	_ = serveDist(ctx, distDir, addr, &wg)
	log.Printf("serving %s at http://localhost%s/", distDir, addr)

	// Set up the file watcher with debounced rebuilds.
	watcher, err := fsnotify.NewWatcher()
	if err != nil {
		log.Fatalf("create watcher: %v", err)
	}
	defer watcher.Close()

	if err := addWatchersRecursively(watcher, absProjectDir); err != nil {
		log.Fatalf("add watchers: %v", err)
	}

	rebuildRequests := make(chan struct{}, 1)

	// Watch events and coalesce rebuild requests.
	go func() {
		for {
			select {
			case <-ctx.Done():
				return
			case ev, ok := <-watcher.Events:
				if !ok {
					return
				}
				// Track newly created directories to watch them.
				if (ev.Op & fsnotify.Create) == fsnotify.Create {
					info, err := os.Stat(ev.Name)
					if err == nil && info.IsDir() && !shouldSkipPath(absProjectDir, ev.Name) {
						_ = watcher.Add(ev.Name)
					}
				}

				if shouldSkipPath(absProjectDir, ev.Name) {
					continue
				}
				// Signal a rebuild (non-blocking).
				select {
				case rebuildRequests <- struct{}{}:
				default:
				}

			case err, ok := <-watcher.Errors:
				if !ok {
					return
				}
				log.Printf("watcher error: %v", err)
			}
		}
	}()

	// Debounce and run make.
	go func() {
		for {
			select {
			case <-ctx.Done():
				return
			case <-rebuildRequests:
				// Debounce window.
				time.Sleep(200 * time.Millisecond)
				// Drain any queued requests.
				for {
					select {
					case <-rebuildRequests:
					default:
						goto START_BUILD
					}
				}
			START_BUILD:
				if err := runMake(absProjectDir); err != nil {
					log.Printf("make failed: %v", err)
				}
			}
		}
	}()

	// Block until signal then wait for server shutdown.
	<-ctx.Done()
	cancel()
	wg.Wait()
}