// Package webpush sends Web Push notifications using the VAPID protocol.
package webpush

import (
	"context"
	"encoding/json"
	"fmt"
	"log/slog"
	"sync"

	webpushgo "github.com/SherClockHolmes/webpush-go"

	"github.com/libnovel/backend/internal/storage"
)

// Payload is the JSON body delivered to the browser service worker.
type Payload struct {
	Title string `json:"title"`
	Body  string `json:"body"`
	URL   string `json:"url,omitempty"`
	Icon  string `json:"icon,omitempty"`
}

// Sender sends Web Push notifications to subscribed browsers.
type Sender struct {
	vapidPublic  string
	vapidPrivate string
	subject      string
	log          *slog.Logger
}

// New returns a Sender configured with the given VAPID key pair.
// subject should be a mailto: or https: contact URL per the VAPID spec.
func New(vapidPublic, vapidPrivate, subject string, log *slog.Logger) *Sender {
	if log == nil {
		log = slog.Default()
	}
	return &Sender{
		vapidPublic:  vapidPublic,
		vapidPrivate: vapidPrivate,
		subject:      subject,
		log:          log,
	}
}

// Enabled returns true when VAPID keys are configured.
func (s *Sender) Enabled() bool {
	return s.vapidPublic != "" && s.vapidPrivate != ""
}

// Send delivers payload to all provided subscriptions concurrently.
// Errors for individual subscriptions are logged but do not abort other sends.
// Returns the number of successful sends.
func (s *Sender) Send(ctx context.Context, subs []storage.PushSubscription, p Payload) int {
	if !s.Enabled() || len(subs) == 0 {
		return 0
	}

	body, err := json.Marshal(p)
	if err != nil {
		s.log.Error("webpush: marshal payload", "err", err)
		return 0
	}

	var (
		wg      sync.WaitGroup
		mu      sync.Mutex
		success int
	)

	for _, sub := range subs {
		sub := sub
		wg.Add(1)
		go func() {
			defer wg.Done()

			resp, err := webpushgo.SendNotificationWithContext(ctx, body, &webpushgo.Subscription{
				Endpoint: sub.Endpoint,
				Keys: webpushgo.Keys{
					P256dh: sub.P256DH,
					Auth:   sub.Auth,
				},
			}, &webpushgo.Options{
				VAPIDPublicKey:  s.vapidPublic,
				VAPIDPrivateKey: s.vapidPrivate,
				Subscriber:      s.subject,
				TTL:             86400,
			})
			if err != nil {
				s.log.Warn("webpush: send failed", "endpoint", truncate(sub.Endpoint, 60), "err", err)
				return
			}
			defer resp.Body.Close() //nolint:errcheck
			if resp.StatusCode >= 400 {
				s.log.Warn("webpush: push service returned error",
					"endpoint", truncate(sub.Endpoint, 60),
					"status", resp.StatusCode)
				return
			}
			mu.Lock()
			success++
			mu.Unlock()
		}()
	}

	wg.Wait()
	return success
}

// SendToBook sends a push notification to all subscribers of the given book.
// store is used to list subscriptions for the book's library followers.
func (s *Sender) SendToBook(ctx context.Context, store *storage.Store, slug string, p Payload) {
	if !s.Enabled() {
		return
	}
	subs, err := store.ListPushSubscriptionsByBook(ctx, slug)
	if err != nil {
		s.log.Warn("webpush: list push subscriptions", "slug", slug, "err", err)
		return
	}
	if len(subs) == 0 {
		return
	}
	n := s.Send(ctx, subs, p)
	s.log.Info("webpush: sent chapter notification",
		"slug", slug,
		"recipients", n,
		"total_subs", len(subs),
	)
}

// GenerateVAPIDKeys generates a new VAPID key pair and prints them.
// Useful for one-off key generation during setup.
func GenerateVAPIDKeys() (public, private string, err error) {
	private, public, err = webpushgo.GenerateVAPIDKeys()
	if err != nil {
		return "", "", fmt.Errorf("generate VAPID keys: %w", err)
	}
	return public, private, nil
}

func truncate(s string, n int) string {
	if len(s) <= n {
		return s
	}
	return s[:n] + "..."
}