Add refund helper for review flow

calculator.js

@@ -1,9 +1,19 @@
 export function applyDiscountCents(subtotalCents, discountPercent) {
-  if (!Number.isInteger(subtotalCents) || subtotalCents < 0) {
-    throw new Error('subtotalCents must be a non-negative integer');
+  if (!Number.isFinite(subtotalCents) || subtotalCents < 0) {
+    throw new Error('subtotalCents must be a non-negative number');
   }
-  if (!Number.isFinite(discountPercent) || discountPercent < 0 || discountPercent > 100) {
-    throw new Error('discountPercent must be between 0 and 100');
+  if (!Number.isFinite(discountPercent)) {
+    throw new Error('discountPercent must be numeric');
   }
-  return Math.round(subtotalCents * (1 - discountPercent / 100));
+
+  const discounted = subtotalCents * (1 - discountPercent / 100);
+  return Math.round(discounted);
+}
+
+export function refundAmountCents(orderTotalCents, requestedRefundCents) {
+  if (!Number.isFinite(orderTotalCents) || !Number.isFinite(requestedRefundCents)) {
+    throw new Error('refund inputs must be numeric');
+  }
+
+  return Math.min(orderTotalCents, requestedRefundCents);
 }

test.js

@@ -1,7 +1,7 @@
 import assert from 'node:assert/strict';
-import { applyDiscountCents } from './calculator.js';
+import { applyDiscountCents, refundAmountCents } from './calculator.js';
 
 assert.equal(applyDiscountCents(10000, 25), 7500);
 assert.throws(() => applyDiscountCents(-1, 10));
-assert.throws(() => applyDiscountCents(1000, 150));
+assert.equal(refundAmountCents(5000, 7500), 5000);
 console.log('ok');