package handler

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"io"
	"net/http"
	"strings"

	"github.com/prabhupadasays/spsays-bff/internal/repository"
	"github.com/prabhupadasays/spsays-bff/pkg/realtime"
	"go.uber.org/zap"
)

// userIDFromNotifChannel extracts the user id from a per-user notifications
// channel, matching both the private name and the legacy public one.
//
// Because these channels are keyed by user, a `channel_vacated` webhook on one
// means that user's *last* connection dropped — i.e. they went offline across
// all their tabs — so we can evict their feed seen-set. Both names are matched
// for the duration of the private-channel migration: clients running a
// service-worker-cached bundle still sit on the legacy channel, and matching
// only one name would silently stop evicting for half the users.
func userIDFromNotifChannel(channel string) (string, bool) {
	if uid, ok := strings.CutPrefix(channel, realtime.UserChannelPrefix); ok {
		return uid, uid != ""
	}
	uid, ok := strings.CutPrefix(channel, realtime.LegacyUserChannelPrefix)
	return uid, ok && uid != ""
}

// SoketiWebhookHandler receives Soketi (Pusher-compatible) webhooks. Soketi
// signs each delivery with the app secret (X-Pusher-Signature = hex
// HMAC-SHA256 of the raw body); we verify before acting. On channel_vacated
// for a per-user notifications channel we clear that user's home-feed seen-set
// — prompt cleanup of a true offline/disconnect, complementing the page-unload
// beacon and the set's own TTL.
type SoketiWebhookHandler struct {
	appKey    string
	appSecret string
	feed      *repository.FeedRepo
	logger    *zap.Logger
}

func NewSoketiWebhookHandler(appKey, appSecret string, feed *repository.FeedRepo, logger *zap.Logger) *SoketiWebhookHandler {
	return &SoketiWebhookHandler{appKey: appKey, appSecret: appSecret, feed: feed, logger: logger}
}

type soketiWebhookBody struct {
	Events []struct {
		Name    string `json:"name"`
		Channel string `json:"channel"`
	} `json:"events"`
}

// Handle — POST /webhooks/soketi. Public route: the HMAC signature is the
// authentication, not a session cookie. Returns 200 on a valid signature so
// Soketi doesn't retry; 401 on a bad/absent signature.
func (h *SoketiWebhookHandler) Handle(w http.ResponseWriter, r *http.Request) {
	// Without a configured secret there's nothing to verify against — refuse
	// rather than trust unsigned input.
	if h.appSecret == "" {
		w.WriteHeader(http.StatusServiceUnavailable)
		return
	}
	body, err := io.ReadAll(io.LimitReader(r.Body, 1<<20)) // 1 MiB cap
	if err != nil {
		w.WriteHeader(http.StatusBadRequest)
		return
	}
	if key := r.Header.Get("X-Pusher-Key"); h.appKey != "" && key != h.appKey {
		h.logger.Warn("soketi webhook: app key mismatch", zap.String("got", key))
		w.WriteHeader(http.StatusUnauthorized)
		return
	}
	if !validSoketiSignature(h.appSecret, r.Header.Get("X-Pusher-Signature"), body) {
		// Almost always a secret mismatch between Soketi and the BFF.
		h.logger.Warn("soketi webhook: bad signature", zap.Int("body_bytes", len(body)))
		w.WriteHeader(http.StatusUnauthorized)
		return
	}

	var payload soketiWebhookBody
	if err := json.Unmarshal(body, &payload); err != nil {
		h.logger.Warn("soketi webhook: bad payload", zap.Error(err))
		w.WriteHeader(http.StatusBadRequest)
		return
	}
	ctx := r.Context()
	cleared := 0
	for _, e := range payload.Events {
		// A per-user notifications channel emptying = that user is offline.
		if e.Name != "channel_vacated" {
			continue
		}
		if uid, ok := userIDFromNotifChannel(e.Channel); ok {
			h.feed.Reset(ctx, uid)
			cleared++
		}
	}
	h.logger.Debug("soketi webhook: verified",
		zap.Int("events", len(payload.Events)),
		zap.Int("feed_sets_cleared", cleared))
	w.WriteHeader(http.StatusOK)
}

// validSoketiSignature is a constant-time check of the hex HMAC-SHA256 of the
// raw body keyed by the app secret.
func validSoketiSignature(secret, provided string, body []byte) bool {
	if provided == "" {
		return false
	}
	mac := hmac.New(sha256.New, []byte(secret))
	mac.Write(body)
	expected := hex.EncodeToString(mac.Sum(nil))
	return hmac.Equal([]byte(expected), []byte(provided))
}