Watch for Payments

export async function watchPayments(
  token: Address,
  merchant: Address,
  startBlock: bigint,
  store: PaymentEventStore,
) {
  async function backfillConfirmed() {
    const head = await client.getBlockNumber();
    if (head <= confirmations) return;
    const toBlock = head - confirmations;
    const cursor = await store.lastScannedBlock();
    const fromBlock = cursor && cursor > startBlock + confirmations
      ? cursor - confirmations
      : startBlock;
    const logs = await client.getLogs({
      address: token,
      event: transferEvent,
      args: { to: merchant },
      fromBlock,
      toBlock,
      strict: true,
    });

    // In one database transaction, replace this overlap window and advance the cursor.
    // Key each row by (blockHash, transactionHash, logIndex) so retries stay idempotent.
    await store.replaceRange(fromBlock, toBlock, logs);
  }

  await backfillConfirmed();
  return client.watchEvent({
    address: token,
    event: transferEvent,
    args: { to: merchant },
    onLogs: backfillConfirmed,
    onError: (error) => console.error("Payment watcher failed", error),
  });
}