diff --git a/README.md b/README.md
new file mode 100644
index 0000000..c4dd9a4
--- /dev/null
+++ b/README.md
@@ -0,0 +1,14 @@
+# cart-app
+
+## `src/lib/cart.js`
+
+- `subtotal(items)` — sum of `price * qty` across items, before tax.
+- `itemCount(items)` — total quantity of all items.
+- `discount(items, percent)` — subtotal reduced by `percent`%, then taxed and rounded to cents.
+- `total(items)` — subtotal plus tax (`TAX_RATE`), rounded to cents.
+
+## Running the checks
+
+- `npm test` — runs the unit tests (`node --test "tests/**/*.test.js"`).
+- `npm run check` — runs `scripts/check.mjs`, then the tests.
+- `npm run gen` — runs `scripts/gen.mjs` (generates files such as `src/generated/rates.js`).
diff --git a/src/lib/cart.js b/src/lib/cart.js
index 385fd69..696ac0d 100644
--- a/src/lib/cart.js
+++ b/src/lib/cart.js
@@ -5,6 +5,15 @@ export function subtotal(items) {
   return items.reduce((sum, item) => sum + item.price * item.qty, 0);
 }
 
+export function itemCount(items) {
+  return items.reduce((sum, item) => sum + item.qty, 0);
+}
+
+export function discount(items, percent) {
+  const discounted = subtotal(items) * (1 - percent / 100);
+  return roundCents(discounted * (1 + TAX_RATE));
+}
+
 export function total(items) {
   return roundCents(subtotal(items) * (1 + TAX_RATE));
 }
diff --git a/tests/lib/cart.test.js b/tests/lib/cart.test.js
index 7a9eab3..93dbb3f 100644
--- a/tests/lib/cart.test.js
+++ b/tests/lib/cart.test.js
@@ -1,6 +1,6 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
-import { subtotal, total } from '../../src/lib/cart.js';
+import { subtotal, total, discount, itemCount } from '../../src/lib/cart.js';
 
 const items = [{ price: 10, qty: 2 }, { price: 5, qty: 1 }];
 
@@ -8,6 +8,16 @@ test('subtotal adds price times quantity', () => {
   assert.equal(subtotal(items), 25);
 });
 
+test('itemCount sums quantities across items', () => {
+  assert.equal(itemCount(items), 3);
+  assert.equal(itemCount([]), 0);
+});
+
 test('total applies tax and rounds to cents', () => {
   assert.equal(total(items), 27);
 });
+
+test('discount is taken off the subtotal before tax', () => {
+  assert.equal(discount(items, 20), 21.6);
+  assert.equal(discount(items, 0), total(items));
+});