Wallet

// A wallet that follows the contract, for tests: balances and receipts live in memory.
// A production wallet stores each receipt in the same database transaction as the balance change.
import { createHash, createHmac, timingSafeEqual } from "node:crypto";
import { createServer } from "node:http";

const WALLET_SECRET = process.env.WALLET_SECRET;
const accounts = new Map(); // "player:currency" -> { balance, revision }
const receipts = new Map(); // id -> { fingerprint, receipt }

function account(player, currency) {
  const key = `${player}:${currency}`;
  if (!accounts.has(key)) accounts.set(key, { balance: 1_000_000n, revision: 0n });
  return accounts.get(key);
}

// The game server signs `timestamp + "." + body` with WALLET_SECRET.
function signed(headers, body) {
  const timestamp = headers["x-kinematic-timestamp"] ?? "";
  const signature = headers["x-kinematic-signature"] ?? "";
  if (!/^[0-9]{13}$/.test(timestamp) || Math.abs(Date.now() - Number(timestamp)) > 60_000) return false;
  if (!/^[0-9a-f]{64}$/.test(signature)) return false;
  const expected = createHmac("sha256", WALLET_SECRET).update(`${timestamp}.${body}`).digest();
  return timingSafeEqual(expected, Buffer.from(signature, "hex"));
}

function apply(request, body) {
  const a = account(request.player, request.currency);
  const receipt = (status) => ({
    id: request.id,
    status,
    balance: String(a.balance),
    revision: String(a.revision),
    currency: request.currency,
  });
  if (request.kind === "balance") return receipt("applied");
  // A retry repeats the ID with the same body: answer the first receipt again.
  const fingerprint = createHash("sha256").update(body).digest("hex");
  const earlier = receipts.get(request.id);
  if (earlier) return earlier.fingerprint === fingerprint ? earlier.receipt : null;
  const amount = BigInt(request.amount);
  let status = "applied";
  if (request.kind === "debit" && a.balance < amount) status = "declined"; // the whole batch or nothing
  else {
    a.balance += request.kind === "debit" ? -amount : amount; // credits are gross, never declined
    a.revision += 1n;
  }
  const result = receipt(status);
  receipts.set(request.id, { fingerprint, receipt: result });
  return result;
}

createServer((req, res) => {
  let body = "";
  req.on("data", (chunk) => (body += chunk));
  req.on("end", () => {
    const send = (status, json) =>
      res.writeHead(status, { "content-type": "application/json" }).end(JSON.stringify(json));
    const kind = new URL(req.url, "https://wallet.invalid").pathname.split("/").pop();
    if (req.method !== "POST" || !["debit", "credit", "balance"].includes(kind)) return send(404, { error: "not_found" });
    if (!signed(req.headers, body)) return send(401, { error: "unauthorized" });
    let request;
    try {
      request = JSON.parse(body);
    } catch {
      return send(400, { error: "invalid_json" });
    }
    if (request.kind !== kind || req.headers["idempotency-key"] !== request.id) return send(400, { error: "invalid_request" });
    const receipt = apply(request, body);
    return receipt ? send(200, receipt) : send(409, { error: "idempotency_conflict" });
  });
}).listen(Number(process.env.PORT ?? 8080));