Add cart pricing module

src/cart.ts

@@ -0,0 +1,25 @@
+export interface CartItem {
+  sku: string
+  name: string
+  unitPriceCents: number
+  quantity: number
+}
+
+export interface Discount {
+  code: string
+  percentOff: number
+}
+
+export function subtotalCents(items: CartItem[]): number {
+  return items.reduce((sum, item) => sum + item.unitPriceCents * item.quantity, 0)
+}
+
+export function applyDiscount(subtotal: number, discount?: Discount): number {
+  if (!discount) return subtotal
+  const factor = 1 - discount.percentOff / 100
+  return Math.round(subtotal * factor)
+}
+
+export function totalCents(items: CartItem[], discount?: Discount): number {
+  return applyDiscount(subtotalCents(items), discount)
+}