package main

import (
	"encoding/json"
	"fmt"
	"log/slog"
	"net/http"
)

func sseHandler(broker *Broker) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		flusher, ok := w.(http.Flusher)
		if !ok {
			http.Error(w, "streaming not supported", http.StatusInternalServerError)
			return
		}

		w.Header().Set("Content-Type", "text/event-stream")
		w.Header().Set("Cache-Control", "no-cache")
		w.Header().Set("Connection", "keep-alive")
		w.Header().Set("X-Accel-Buffering", "no") // disable nginx buffering

		ch := broker.Subscribe()
		defer broker.Unsubscribe(ch)

		slog.Debug("SSE client connected")

		ctx := r.Context()
		for {
			select {
			case <-ctx.Done():
				slog.Debug("SSE client disconnected")
				return

			case evt, ok := <-ch:
				if !ok {
					// Broker shut down
					return
				}
				writeSSEEvent(w, flusher, "push", evt)
			}
		}
	}
}

func writeSSEEvent(w http.ResponseWriter, flusher http.Flusher, eventType string, data any) {
	jsonData, err := json.Marshal(data)
	if err != nil {
		slog.Warn("failed to marshal SSE event", "error", err)
		return
	}

	_, err = fmt.Fprintf(w, "event: %s\ndata: %s\n\n", eventType, jsonData)
	if err != nil {
		slog.Debug("failed to write SSE event, client likely disconnected", "error", err)
		return
	}
	flusher.Flush()
}