Bank Automation: Simulate ATM Transactions with Balance and Overdraft Rules

Question

## Round 1 - Coding / OA

## Problem

Simulate a bank account system. Implement `deposit`, `withdraw`, and `transfer`. Withdrawals fail if the account balance would fall below zero. Transfers atomically move funds between two accounts — if the source lacks funds, the transfer fails entirely.

```python
class BankAccount:
    def __init__(self, account_id: str, balance: float):
        ...
    def deposit(self, amount: float) -> float:    # returns new balance
        ...
    def withdraw(self, amount: float) -> float:   # raises InsufficientFundsError if fails
        ...

class Bank:
    def __init__(self):
        ...
    def add_account(self, account: BankAccount) -> None:
        ...
    def transfer(self, from_id: str, to_id: str, amount: float) -> bool:
        ...
    def get_balance(self, account_id: str) -> float:
        ...
```

## Example

```
bank = Bank()
bank.add_account(BankAccount("ACC1", 500))
bank.add_account(BankAccount("ACC2", 100))
bank.transfer("ACC1", "ACC2", 200)  -> True
bank.get_balance("ACC1")            -> 300
bank.get_balance("ACC2")            -> 300
bank.transfer("ACC2", "ACC1", 500)  -> False  # insufficient
```

## Follow-ups

1. How do you prevent race conditions if two threads transfer from the same account simultaneously?
2. How would you implement a transaction history log with timestamps?
3. What happens if `from_id` or `to_id` doesn't exist in the bank?
4. How would you add an overdraft limit so accounts can go negative up to a set threshold?

Bank Automation: Simulate ATM Transactions with Balance and Overdraft Rules

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