add cancel

pkg/checkout/mutation_cancel_payment.go

@@ -0,0 +1,30 @@
+package checkout
+
+import (
+	"errors"
+	"slices"
+
+	messages "git.k6n.net/go-cart-actor/proto/checkout"
+)
+
+func HandlePaymentCancelled(g *CheckoutGrain, m *messages.PaymentDeclined) error {
+
+	payment, found := g.FindPayment(m.PaymentId)
+	if !found {
+		return ErrPaymentNotFound
+	}
+	if payment.CompletedAt != nil {
+		return errors.New("payment already completed")
+	}
+	g.PaymentInProgress--
+	g.AmountInCentsStarted -= payment.Amount
+	g.Payments = removePayment(g.Payments, payment.PaymentId)
+
+	return nil
+}
+
+func removePayment(payment []*Payment, s string) []*Payment {
+	return slices.DeleteFunc(payment, func(p *Payment) bool {
+		return p.PaymentId == s
+	})
+}