test: add unit tests for frontend top-down auto-reset and backend split logic

shop/lib/checkout-split.test.ts

@@ -0,0 +1,42 @@
+import { describe, it, expect } from 'vitest';
+import { splitCart, CartItem } from './checkout-split';
+
+describe('Checkout Split Logic', () => {
+  it('should place purchase items only into purchase group', () => {
+    const cart: CartItem[] = [
+      { billingInterval: 'one_time', selections: {} },
+      { billingType: 'purchase', selections: {} }
+    ];
+
+    const { purchaseItems, subscriptionItems } = splitCart(cart);
+
+    expect(purchaseItems.length).toBe(2);
+    expect(subscriptionItems.length).toBe(0);
+  });
+
+  it('should place subscription items only into subscription group', () => {
+    const cart: CartItem[] = [
+      { billingInterval: 'monthly', selections: {} },
+      { billingType: 'subscription', selections: {} }
+    ];
+
+    const { purchaseItems, subscriptionItems } = splitCart(cart);
+
+    expect(purchaseItems.length).toBe(0);
+    expect(subscriptionItems.length).toBe(2);
+  });
+
+  it('should split mixed cart into separate purchase and subscription groups', () => {
+    const cart: CartItem[] = [
+      { billingInterval: 'one_time', selections: {} },
+      { billingInterval: 'monthly', selections: {} },
+      { billingType: 'purchase', selections: {} },
+      { billingType: 'subscription', selections: {} }
+    ];
+
+    const { purchaseItems, subscriptionItems } = splitCart(cart);
+
+    expect(purchaseItems.length).toBe(2);
+    expect(subscriptionItems.length).toBe(2);
+  });
+});