move actor to pkg

cmd/cart/mutation_change_quantity.go

@@ -0,0 +1,58 @@
+package main
+
+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 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)
+			}
+
+			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
+		},
+		WithTotals(),
+	)
+}