Implementing NIP-50 search support

The nostr.Filter type has a Search field, so you basically just has to handle that if it's present.

It can be tricky to implement fulltext search properly though, so some eventstores implement it natively, such as Bluge, OpenSearch and ElasticSearch (although for the last two you'll need an instance of these database servers running, while with Bluge it's embedded).

If you have any of these you can just use them just like any other eventstore:

func main () {
    // other stuff here

	normal := &lmdb.LMDBBackend{Path: "data"}
	os.MkdirAll(normal.Path, 0755)
	if err := normal.Init(); err != nil {
		panic(err)
	}

	search := bluge.BlugeBackend{Path: "search", RawEventStore: normal}
	if err := search.Init(); err != nil {
		panic(err)
	}

	relay.StoreEvent = func(ctx context.Context, evt nostr.Event) error {
		if err := normal.SaveEvent(evt); err != nil {
			return err
		}
		return search.SaveEvent(evt)
	}
	relay.QueryStored = func(ctx context.Context, filter nostr.Filter) iter.Seq[nostr.Event] {
		if filter.Search != "" {
			return search.QueryEvents(filter)
		}
		return normal.QueryEvents(filter)
	}
	relay.DeleteEvent = func(ctx context.Context, id nostr.ID) error {
		if err := normal.DeleteEvent(id); err != nil {
			return err
		}
		return search.DeleteEvent(id)
	}

    // other stuff here
}

Note that in this case we're using the LMDB adapter for normal queries and it explicitly rejects any filter that contains a Search field, while Bluge rejects any filter without a Search value, which make them pair well together.