feat: add wallet ledger with idempotent credit and debit operations

test/wallet.test.js

@@ -0,0 +1,63 @@
+"use strict";
+
+const test = require("node:test");
+const assert = require("node:assert/strict");
+const { WalletStore } = require("../src/lib/wallet");
+
+test("applies credit and debit transactions", () => {
+  const wallet = new WalletStore();
+
+  wallet.applyTransaction({
+    userId: "u1",
+    type: "credit",
+    amount: 5,
+    reason: "topup",
+    idempotencyKey: "evt-1",
+  });
+
+  wallet.applyTransaction({
+    userId: "u1",
+    type: "debit",
+    amount: 2,
+    reason: "article_generation",
+    idempotencyKey: "evt-2",
+  });
+
+  assert.equal(wallet.getBalance("u1"), 3);
+});
+
+test("prevents overdraft debits", () => {
+  const wallet = new WalletStore();
+  assert.throws(() => {
+    wallet.applyTransaction({
+      userId: "u1",
+      type: "debit",
+      amount: 1,
+      reason: "article_generation",
+      idempotencyKey: "evt-3",
+    });
+  }, /insufficient_credits/);
+});
+
+test("is idempotent by idempotency key", () => {
+  const wallet = new WalletStore();
+
+  const first = wallet.applyTransaction({
+    userId: "u1",
+    type: "credit",
+    amount: 4,
+    reason: "topup",
+    idempotencyKey: "evt-4",
+  });
+
+  const second = wallet.applyTransaction({
+    userId: "u1",
+    type: "credit",
+    amount: 999,
+    reason: "topup",
+    idempotencyKey: "evt-4",
+  });
+
+  assert.equal(first.id, second.id);
+  assert.equal(wallet.getBalance("u1"), 4);
+});