package flamegraph

import (
	"context"
	"fmt"
	"net/http"
	"time"
)

// ServeLive starts the live flamegraph HTTP server and blocks until ctx is canceled.
func ServeLive(ctx context.Context, lt *LiveTrie, interval time.Duration) error {
	mux := http.NewServeMux()
	mux.HandleFunc("/", handleLivePage())
	mux.HandleFunc("/events", handleSSE(lt, interval))
	srv := &http.Server{Handler: mux}

	listener, err := listenRandomPort()
	if err != nil {
		return err
	}
	defer listener.Close()

	hostname, port := serverHostPort(listener)
	fmt.Printf("Live flamegraph available at http://%s:%d/\n", hostname, port)

	errCh := make(chan error, 1)
	go func() {
		errCh <- srv.Serve(listener)
	}()

	select {
	case <-ctx.Done():
	case serveErr := <-errCh:
		if serveErr != nil && serveErr != http.ErrServerClosed {
			return serveErr
		}
		return nil
	}

	shutdownCtx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
	defer cancel()
	if err := srv.Shutdown(shutdownCtx); err != nil {
		return fmt.Errorf("shutdown live web server: %w", err)
	}

	serveErr := <-errCh
	if serveErr != nil && serveErr != http.ErrServerClosed {
		return serveErr
	}
	return nil
}

func handleLivePage() http.HandlerFunc {
	return func(w http.ResponseWriter, _ *http.Request) {
		w.Header().Set("Content-Type", "text/html; charset=utf-8")
		_, _ = w.Write([]byte(liveHTML))
	}
}

func handleSSE(lt *LiveTrie, interval time.Duration) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		flusher, ok := w.(http.Flusher)
		if !ok {
			http.Error(w, "streaming unsupported", http.StatusInternalServerError)
			return
		}
		if interval <= 0 {
			interval = time.Second
		}

		w.Header().Set("Content-Type", "text/event-stream")
		w.Header().Set("Cache-Control", "no-cache")

		lastVersion, err := sendSnapshot(w, flusher, lt, ^uint64(0))
		if err != nil {
			return
		}

		ticker := time.NewTicker(interval)
		defer ticker.Stop()

		for {
			select {
			case <-r.Context().Done():
				return
			case <-ticker.C:
				if lt.Version() == lastVersion {
					continue
				}
				lastVersion, err = sendSnapshot(w, flusher, lt, lastVersion)
				if err != nil {
					return
				}
			}
		}
	}
}

func sendSnapshot(w http.ResponseWriter, flusher http.Flusher, lt *LiveTrie, lastVersion uint64) (uint64, error) {
	payload, version := lt.SnapshotJSON()
	if version == lastVersion {
		return lastVersion, nil
	}
	if _, err := fmt.Fprintf(w, "data: %s\n\n", payload); err != nil {
		return lastVersion, err
	}
	flusher.Flush()
	return version, nil
}