complete rewrite to grpc

cart-grain.go

@@ -14,6 +14,22 @@ import (
 
 type CartId [16]byte
 
+// String returns the cart id as a trimmed UTF-8 string (trailing zero bytes removed).
+func (id CartId) String() string {
+	n := 0
+	for n < len(id) && id[n] != 0 {
+		n++
+	}
+	return string(id[:n])
+}
+
+// ToCartId converts an arbitrary string to a fixed-size CartId (truncating or padding with zeros).
+func ToCartId(s string) CartId {
+	var id CartId
+	copy(id[:], []byte(s))
+	return id
+}
+
 func (id CartId) MarshalJSON() ([]byte, error) {
 	return json.Marshal(id.String())
 }