better

cmd/cart/mutation_change_quantity.go

@@ -25,34 +25,29 @@ import (
 //     the grain's implicit expectation that higher layers control access.
 //     (If strict locking is required around every mutation, wrap logic in
 //     an explicit g.mu.Lock()/Unlock(), but current model mirrors prior code.)
-func init() {
-	RegisterMutation[messages.ChangeQuantity](
-		"ChangeQuantity",
-		func(g *CartGrain, m *messages.ChangeQuantity) error {
-			if m == nil {
-				return fmt.Errorf("ChangeQuantity: nil payload")
-			}
 
-			foundIndex := -1
-			for i, it := range g.Items {
-				if it.Id == int(m.Id) {
-					foundIndex = i
-					break
-				}
-			}
-			if foundIndex == -1 {
-				return fmt.Errorf("ChangeQuantity: item id %d not found", m.Id)
-			}
+func ChangeQuantity(g *CartGrain, m *messages.ChangeQuantity) error {
+	if m == nil {
+		return fmt.Errorf("ChangeQuantity: nil payload")
+	}
 
-			if m.Quantity <= 0 {
-				// Remove the item
-				g.Items = append(g.Items[:foundIndex], g.Items[foundIndex+1:]...)
-				return nil
-			}
+	foundIndex := -1
+	for i, it := range g.Items {
+		if it.Id == int(m.Id) {
+			foundIndex = i
+			break
+		}
+	}
+	if foundIndex == -1 {
+		return fmt.Errorf("ChangeQuantity: item id %d not found", m.Id)
+	}
 
-			g.Items[foundIndex].Quantity = int(m.Quantity)
-			return nil
-		},
-		WithTotals(),
-	)
+	if m.Quantity <= 0 {
+		// Remove the item
+		g.Items = append(g.Items[:foundIndex], g.Items[foundIndex+1:]...)
+		return nil
+	}
+
+	g.Items[foundIndex].Quantity = int(m.Quantity)
+	return nil
 }