Design a bank system with scheduled transfers

Bank System (OOD): Accounts, Transfers, Scheduling, Top Spending, Merge

Design an in-memory banking system that supports:

• account creation
• deposits/withdrawals
• immediate transfers
• scheduled (delayed) transfers with cancellation
• querying top spenders
• merging accounts

Core rules

• Each account has a balance (integer, starts at 0).
• Each operation is called with a timestamp .
• Before executing any operation at time t , the system must process all scheduled transfers whose execution time <= t .
• “Spending” for a customer is the total money sent out (withdrawals + transfers out, including scheduled transfers once executed; depending on your interpretation, you may also count scheduled transfers as spending when reserved—state your choice and be consistent).

API

1. create_account(timestamp, customer_id) -> bool
   • Create if not exists.
2. deposit(timestamp, customer_id, amount) -> bool
   • Fail if account doesn’t exist.
3. withdraw(timestamp, customer_id, amount) -> bool
   • Fail if account doesn’t exist or insufficient funds.
   • Counts toward spending.
4. transfer(timestamp, from_id, to_id, amount) -> bool
   • Immediate transfer.
   • Fail if either account missing or insufficient funds.
   • Counts amount toward from_id spending.
5. top_spending(timestamp, n) -> list<string>
   • Return top n customers formatted "customerId(total)" .
   • Sort by total spending desc, then customerId asc.
6. schedule_transfer(timestamp, from_id, to_id, amount, delay) -> string | null
   • Create a transfer that will execute at timestamp + delay .
   • If invalid (missing accounts or insufficient funds), return null .
   • Return a unique transfer_id for cancellation.
   • Funds may be “reserved” immediately or deducted at execution; choose one and document it.
7. cancel_transfer(timestamp, customer_id, transfer_id) -> bool
   • Cancel only if:
     • transfer_id exists
     • status is still pending (not executed)
     • the customer_id is the sender
   • Return whether cancellation succeeded.
8. merge_account(timestamp, from_id, to_id) -> bool
   • Merge from_id into to_id :
     • Move remaining balance into to_id
     • Add spending totals
     • Update any pending scheduled transfers that reference from_id as sender or receiver to reference to_id
     • Delete from_id

Notes

• Assume up to ~1e5 operations.
• You may use a min-heap/priority queue for scheduled transfers.