feature/backoffice (#6)

Co-authored-by: matst80 <mats.tornberg@gmail.com>
Reviewed-on: https://git.tornberg.me/mats/go-cart-actor/pulls/6
Co-authored-by: Mats Törnberg <mats@tornberg.me>
Co-committed-by: Mats Törnberg <mats@tornberg.me>

pkg/cart/mutation_change_quantity.go

@@ -0,0 +1,54 @@
+package cart
+
+import (
+	"fmt"
+
+	messages "git.tornberg.me/go-cart-actor/pkg/messages"
+)
+
+// mutation_change_quantity.go
+//
+// Registers the ChangeQuantity mutation.
+//
+// Behavior:
+//   - Locates an item by its cart-local line item Id (not source item_id).
+//   - If requested quantity <= 0 the line is removed.
+//   - Otherwise the line's Quantity field is updated.
+//   - Totals are recalculated (WithTotals).
+//
+// Error handling:
+//   - Returns an error if the item Id is not found.
+//   - Returns an error if payload is nil (defensive).
+//
+// Concurrency:
+//   - Uses the grain's RW-safe mutation pattern: we mutate in place under
+//     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 ChangeQuantity(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 == uint32(m.Id) {
+			foundIndex = i
+			break
+		}
+	}
+	if foundIndex == -1 {
+		return fmt.Errorf("ChangeQuantity: item id %d not found", m.Id)
+	}
+
+	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)
+	g.UpdateTotals()
+	return nil
+}