shortcut quantity change

cart-grain.go

@@ -160,6 +160,17 @@ func (c *CartGrain) ItemsWithoutDelivery() []int {
 	return ret
 }
 
+func (c *CartGrain) FindItemWithSku(sku string) (*CartItem, bool) {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+	for _, item := range c.Items {
+		if item.Sku == sku {
+			return item, true
+		}
+	}
+	return nil, false
+}
+
 func (c *CartGrain) HandleMessage(message *Message, isReplay bool) ([]byte, error) {
 	if message.TimeStamp == nil {
 		now := time.Now().Unix()
@@ -173,26 +184,28 @@ func (c *CartGrain) HandleMessage(message *Message, isReplay bool) ([]byte, erro
 		if !ok {
 			err = fmt.Errorf("expected AddRequest")
 		} else {
-			return c.AddItem(msg.Sku, 1) // extent AddRequest to include quantity
+			existingItem, found := c.FindItemWithSku(msg.Sku)
+			if found {
+				existingItem.Quantity += int(msg.Quantity)
+				c.TotalPrice += existingItem.Price * int64(msg.Quantity)
+			} else {
+				return c.AddItem(msg.Sku, int(msg.Quantity)) // extent AddRequest to include quantity
+			}
 		}
 	case AddItemType:
 		msg, ok := message.Content.(*messages.AddItem)
 		if !ok {
 			err = fmt.Errorf("expected AddItem")
 		} else {
-			found := false
+
 			if msg.Quantity < 1 {
 				return nil, fmt.Errorf("invalid quantity")
 			}
-			for _, item := range c.Items {
-				if item.Sku == msg.Sku {
-					found = true
-					item.Quantity += int(msg.Quantity)
-					c.TotalPrice += item.Price * int64(msg.Quantity)
-					break
-				}
-			}
-			if !found {
+			existingItem, found := c.FindItemWithSku(msg.Sku)
+			if found {
+				existingItem.Quantity += int(msg.Quantity)
+				c.TotalPrice += existingItem.Price * int64(msg.Quantity)
+			} else {
 				c.mu.Lock()
 				c.lastItemId++
 				c.Items = append(c.Items, &CartItem{