add nip40 helper.

nip40/nip40.go

@@ -0,0 +1,22 @@
+package nip40
+
+import (
+	"strconv"
+
+	"github.com/nbd-wtf/go-nostr"
+)
+
+// GetExpiration returns the expiration timestamp for this event, or -1 if no "expiration" tag exists or
+// if it is invalid.
+func GetExpiration(tags nostr.Tags) nostr.Timestamp {
+	for _, tag := range tags {
+		if len(tag) >= 2 && tag[0] == "expiration" {
+			if ts, err := strconv.ParseInt(tag[1], 10, 64); err == nil {
+				return nostr.Timestamp(ts)
+			} else {
+				return -1
+			}
+		}
+	}
+	return -1
+}