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.

1. deposit(timestamp, customer_id, amount) -> bool

• Fail if account doesn’t exist.

1. withdraw(timestamp, customer_id, amount) -> bool

• Fail if account doesn’t exist or insufficient funds.
• Counts toward spending.

1. transfer(timestamp, from_id, to_id, amount) -> bool

• Immediate transfer.
• Fail if either account missing or insufficient funds.
• Counts amount toward from_id spending.

1. top_spending(timestamp, n) -> list<string>

• Return top n customers formatted "customerId(total)".
• Sort by total spending desc, then customerId asc.

1. 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.

1. 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.

1. 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.