// backend\sse.go

package main

import (
	"encoding/json"
	"fmt"
	"net/http"
	"sync"
	"time"
)

// -------------------- SSE primitives --------------------

type sseEvent struct {
	name string
	data []byte
}

type sseHub struct {
	mu      sync.Mutex
	clients map[chan sseEvent]struct{}
}

func newSSEHub() *sseHub {
	return &sseHub{clients: map[chan sseEvent]struct{}{}}
}

func (h *sseHub) add(ch chan sseEvent) {
	h.mu.Lock()
	h.clients[ch] = struct{}{}
	h.mu.Unlock()
}

func (h *sseHub) remove(ch chan sseEvent) {
	h.mu.Lock()
	delete(h.clients, ch)
	h.mu.Unlock()
	close(ch)
}

func (h *sseHub) broadcastEvent(name string, b []byte) {
	h.mu.Lock()
	defer h.mu.Unlock()

	ev := sseEvent{name: name, data: b}

	for ch := range h.clients {
		select {
		case ch <- ev:
		default:
		}
	}
}

// -------------------- only perf SSE --------------------

var (
	perfHub = newSSEHub()
)

// initSSE startet nur noch den perf broadcaster.
func initSSE() {
	go func() {
		t := time.NewTicker(3 * time.Second)
		defer t.Stop()

		for range t.C {
			b := perfSnapshotJSON()
			if len(b) > 0 {
				perfHub.broadcastEvent("perf", b)
			}
		}
	}()
}

// perfSnapshotJSON liefert einen Snapshot für das Frontend (PerformanceMonitor).
func perfSnapshotJSON() []byte {
	payload := buildPerfSnapshot()
	b, _ := json.Marshal(payload)
	return b
}

// -------------------- SSE: /api/stream --------------------
//
// events:
// - perf -> PerfSnapshot
func appStream(w http.ResponseWriter, r *http.Request) {
	if r.Method != http.MethodGet {
		http.Error(w, "Nur GET erlaubt", http.StatusMethodNotAllowed)
		return
	}

	flusher, ok := w.(http.Flusher)
	if !ok {
		http.Error(w, "Streaming nicht unterstützt", http.StatusInternalServerError)
		return
	}

	h := w.Header()
	h.Set("Content-Type", "text/event-stream; charset=utf-8")
	h.Set("Cache-Control", "no-cache, no-transform")
	h.Set("Connection", "keep-alive")
	h.Set("X-Accel-Buffering", "no")

	w.WriteHeader(http.StatusOK)

	writeEvent := func(event string, data []byte) bool {
		if event != "" {
			if _, err := fmt.Fprintf(w, "event: %s\n", event); err != nil {
				return false
			}
		}
		if len(data) > 0 {
			if _, err := fmt.Fprintf(w, "data: %s\n\n", data); err != nil {
				return false
			}
		} else {
			if _, err := fmt.Fprint(w, "\n"); err != nil {
				return false
			}
		}
		flusher.Flush()
		return true
	}

	writeComment := func(msg string) bool {
		if _, err := fmt.Fprintf(w, ": %s\n\n", msg); err != nil {
			return false
		}
		flusher.Flush()
		return true
	}

	if _, err := fmt.Fprintf(w, "retry: 3000\n\n"); err != nil {
		return
	}
	flusher.Flush()

	perfCh := make(chan sseEvent, 32)
	perfHub.add(perfCh)
	defer perfHub.remove(perfCh)

	// optional: direkt initialen perf snapshot schicken
	if b := perfSnapshotJSON(); len(b) > 0 {
		if !writeEvent("perf", b) {
			return
		}
	}

	ctx := r.Context()
	ping := time.NewTicker(15 * time.Second)
	defer ping.Stop()

	for {
		select {
		case <-ctx.Done():
			return

		case ev, ok := <-perfCh:
			if !ok {
				return
			}
			if len(ev.data) == 0 {
				continue
			}

			eventName := ev.name
			if eventName == "" {
				eventName = "perf"
			}

			if !writeEvent(eventName, ev.data) {
				return
			}

		case <-ping.C:
			if !writeComment(fmt.Sprintf("ping %d", time.Now().Unix())) {
				return
			}
		}
	}
}