Implement an in-memory banking system

Implement an in-memory banking system that manages user accounts, transfers, rankings by spending, scheduled payments, and account merging.

Design a class/module with methods to support the following features (you may choose exact method signatures, but behavior must be clear and testable):

1. Basic account operations

• addUser(userId) : Create a new user/account with zero balance.
• deposit(userId, amount) : Add funds to a user’s balance.
• transfer(fromUserId, toUserId, amount) : Move funds between two users if the sender has sufficient balance.

2. Rank accounts by spending

• Track each user’s total spending (money they paid out).
• Provide a query like topSpenders(k) that returns the top k users ranked by total spending (ties broken deterministically, e.g., by userId).

3. Schedule and cancel payments

• Support scheduling a payment to happen at a future time: e.g., schedulePayment(fromUserId, toUserId, amount, executeAtTimestamp) .
• Support cancellation: e.g., cancelPayment(paymentId) (only if it has not executed yet).
• Ensure scheduled payments execute at or after their scheduled time when time advances (you may model this with a processDuePayments(currentTimestamp) call).

4. Merge users while preserving payment history

• Support mergeUsers(sourceUserId, targetUserId) that merges sourceUserId into targetUserId .
• After merging:
  • Balances are combined into the target.
  • Spending totals and relevant histories are preserved correctly.
  • Scheduled payments referencing the source user are handled consistently (e.g., re-pointed to the target or canceled—state your choice and implement it).

Assume all amounts are positive integers. Define clear return values for success/failure cases (e.g., boolean, exceptions, or error codes).