func (tkr *Tracker) handlePeerEvent(ann *models.Announce, p *models.Peer) (snatched bool, err error) {
	p, t := ann.Peer, ann.Torrent

	switch {
	case ann.Event == "stopped" || ann.Event == "paused":
		// updateSwarm checks if the peer is active on the torrent,
		// so one of these branches must be followed.
		if t.Seeders.Contains(p.Key()) {
			err = tkr.DeleteSeeder(t.Infohash, p)
			if err != nil {
				return
			}
			stats.RecordPeerEvent(stats.DeletedSeed, p.HasIPv6())

		} else if t.Leechers.Contains(p.Key()) {
			err = tkr.DeleteLeecher(t.Infohash, p)
			if err != nil {
				return
			}
			stats.RecordPeerEvent(stats.DeletedLeech, p.HasIPv6())
		}

	case ann.Event == "completed":
		tkr.leecherFinished(t, p)
		snatched = true

	case t.Leechers.Contains(p.Key()) && ann.Left == 0:
		// A leecher completed but the event was never received.
		err = tkr.leecherFinished(t, p)
	}

	return
}

// leecherFinished moves a peer from the leeching pool to the seeder pool.
func (tkr *Tracker) leecherFinished(t *models.Torrent, p *models.Peer) error {
	if err := tkr.DeleteLeecher(t.Infohash, p); err != nil {
		return err
	}

	if err := tkr.PutSeeder(t.Infohash, p); err != nil {
		return err
	}

	stats.RecordPeerEvent(stats.Completed, p.HasIPv6())
	return nil
}

func (s *Storage) DeleteSeeder(infohash string, p *models.Peer) error {
	shard := s.getTorrentShard(infohash, false)
	defer shard.Unlock()

	torrent, exists := shard.torrents[infohash]
	if !exists {
		return models.ErrTorrentDNE
	}

	torrent.Seeders.Delete(p.Key())

	return nil
}

func (tkr *Tracker) handlePeerEvent(ann *models.Announce, p *models.Peer) (snatched bool, err error) {
	p, t := ann.Peer, ann.Torrent

	switch {
	case ann.Event == "stopped" || ann.Event == "paused":
		// updateSwarm checks if the peer is active on the torrent,
		// so one of these branches must be followed.
		if t.Seeders.Contains(p.Key()) {
			err = tkr.DeleteSeeder(t.Infohash, p)
			if err != nil {
				return
			}
			stats.RecordPeerEvent(stats.DeletedSeed, p.HasIPv6())

		} else if t.Leechers.Contains(p.Key()) {
			err = tkr.DeleteLeecher(t.Infohash, p)
			if err != nil {
				return
			}
			stats.RecordPeerEvent(stats.DeletedLeech, p.HasIPv6())
		}

	case t.Leechers.Contains(p.Key()) && (ann.Event == "completed" || ann.Left == 0):
		// A leecher has completed or this is the first time we've seen them since
		// they've completed.
		err = tkr.leecherFinished(t, p)
		if err != nil {
			return
		}

		// Only mark as snatched if we receive the completed event.
		if ann.Event == "completed" {
			snatched = true
		}
	}

	return
}