package eventstream

import (
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"strconv"
	"strings"
	"time"
)

type Event struct {
	ID   int64
	Kind string
	Data any
}

func ParseSinceID(r *http.Request) int64 {
	sinceStr := strings.TrimSpace(r.URL.Query().Get("since"))
	if sinceStr == "" {
		sinceStr = strings.TrimSpace(r.Header.Get("Last-Event-ID"))
	}
	if sinceStr == "" {
		return 0
	}
	if v, err := strconv.ParseInt(sinceStr, 10, 64); err == nil && v >= 0 {
		return v
	}
	return 0
}

func Serve(w http.ResponseWriter, r *http.Request, pollEvery, heartbeat time.Duration, loadSince func(int64) []Event) {
	if r.Method != http.MethodGet {
		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
		return
	}
	flusher, ok := w.(http.Flusher)
	if !ok {
		http.Error(w, "streaming unsupported", http.StatusBadRequest)
		return
	}

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

	if pollEvery <= 0 {
		pollEvery = 500 * time.Millisecond
	}
	if heartbeat <= 0 {
		heartbeat = 15 * time.Second
	}

	ctx := r.Context()
	since := ParseSinceID(r)

	send := func(ev Event) error {
		payload, err := json.Marshal(ev.Data)
		if err != nil {
			return err
		}
		if _, err := fmt.Fprintf(w, "id: %d\nevent: %s\ndata: %s\n\n", ev.ID, ev.Kind, string(payload)); err != nil {
			return err
		}
		flusher.Flush()
		return nil
	}

	if loadSince != nil {
		for _, ev := range loadSince(since) {
			if err := send(ev); err != nil {
				return
			}
			since = ev.ID
		}
	}

	pollTicker := time.NewTicker(pollEvery)
	pingTicker := time.NewTicker(heartbeat)
	defer pollTicker.Stop()
	defer pingTicker.Stop()

	for {
		select {
		case <-ctx.Done():
			return
		case <-pingTicker.C:
			_, _ = io.WriteString(w, ": ping\n\n")
			flusher.Flush()
		case <-pollTicker.C:
			if loadSince == nil {
				continue
			}
			evs := loadSince(since)
			if len(evs) == 0 {
				continue
			}
			for _, ev := range evs {
				if err := send(ev); err != nil {
					return
				}
				since = ev.ID
			}
		}
	}
}