Zen-ZZen-Z

Deno.serve(async (req) => {
  const p = Object.fromEntries(await req.formData());

  // PayU's docs show a 5-udf formula. Payment Links really use all ten,
  // confirmed byte for byte against a live ₹1 transaction.
  const expected = await sha512([
    SALT, p.status,
    p.udf10, p.udf9, p.udf8, p.udf7, p.udf6,
    p.udf5, p.udf4, p.udf3, p.udf2, p.udf1,
    p.email, p.firstname, p.productinfo, p.amount, p.txnid, KEY,
  ].join("|"));

  // Constant-time compare: a bad signature confirms nothing.
  if (!timingSafeEqual(expected, p.hash)) return reject(p, "invalid_signature");

  // Cancelled, then paid on a stale tab? Logged, not confirmed.
  const outcome = await settleBooking(p);

  await logEvent(p, outcome);
  return ok();
});

// Best-effort: logging never changes the response.
async function logEvent(p, outcome) {
  try {
    await db.from("payment_webhook_events").insert({ txnid: p.txnid, outcome });
  } catch {}
}