refactor/checkout (#8)

Co-authored-by: matst80 <mats.tornberg@gmail.com>
Reviewed-on: #8
Co-authored-by: Mats Törnberg <mats@tornberg.me>
Co-committed-by: Mats Törnberg <mats@tornberg.me>

pkg/checkout/mutation_payment_started.go

@@ -0,0 +1,84 @@
+package checkout
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	messages "git.k6n.net/go-cart-actor/proto/checkout"
+)
+
+// PaymentStarted registers the beginning of a payment attempt for a checkout.
+// It either upserts the payment entry (based on paymentId) or creates a new one,
+// marks the checkout as having a payment in progress.
+func HandlePaymentStarted(g *CheckoutGrain, m *messages.PaymentStarted) error {
+	if m == nil {
+		return fmt.Errorf("PaymentStarted: nil payload")
+	}
+	paymentID := strings.TrimSpace(m.PaymentId)
+	if paymentID == "" {
+		return fmt.Errorf("PaymentStarted: missing paymentId")
+	}
+	if m.Amount < 0 {
+		return fmt.Errorf("PaymentStarted: amount cannot be negative")
+	}
+
+	currency := strings.TrimSpace(m.Currency)
+	provider := strings.TrimSpace(m.Provider)
+	method := copyOptionalString(m.Method)
+
+	startedAt := time.Now().UTC()
+	if m.StartedAt != nil {
+		startedAt = m.StartedAt.AsTime()
+	}
+
+	payment, found := g.FindPayment(paymentID)
+
+	if found {
+		if payment.Status != "pending" {
+			return fmt.Errorf("PaymentStarted: payment already started")
+		}
+		if payment.PaymentId != paymentID {
+			payment.PaymentId = paymentID
+		}
+		payment.Status = "pending"
+		payment.Amount = m.Amount
+		if currency != "" {
+			payment.Currency = currency
+		}
+		if provider != "" {
+			payment.Provider = provider
+		}
+		if method != nil {
+			payment.Method = method
+		}
+		payment.StartedAt = &startedAt
+		payment.CompletedAt = nil
+		payment.ProcessorReference = nil
+	} else {
+		g.PaymentInProgress++
+		g.Payments = append(g.Payments, &Payment{
+			PaymentId: paymentID,
+			Status:    "pending",
+			Amount:    m.Amount,
+			Currency:  currency,
+			Provider:  provider,
+			Method:    method,
+			StartedAt: &startedAt,
+		})
+	}
+
+	return nil
+}
+
+func copyOptionalString(src *string) *string {
+	if src == nil {
+		return nil
+	}
+	trimmed := strings.TrimSpace(*src)
+	if trimmed == "" {
+		return nil
+	}
+	dst := trimmed
+	return &dst
+}