System Design: In-Memory Account Management with Transfers, Payments, and Cashback

Design and implement an in-memory account management service that supports atomic balance updates, delayed cashback, and a leaderboard of top transfer spenders. All timestamps should be treated as integers (e.g., seconds since epoch) and all monetary amounts as integers representing the smallest currency unit (e.g., cents).

Explicit Assumptions

• Amounts must be positive integers; operations with non-positive amounts will fail.
• Timestamps are provided by the caller and are used to determine when cashback is due.
• 2% cashback is computed using integer math: floor(0.02 × amount) which can be implemented as (amount * 2) / 100.
• For any failure condition (e.g., missing account, insufficient funds, invalid input), Optional.empty() should be returned, unless explicitly stated otherwise.
• Transfer spending for the topSpenders leaderboard should only count outgoing transfer amounts from the transfer operation and explicitly exclude amounts from pay operations and cashback credits.

Required Operations

Implement the following operations with their specified semantics:

1. createAccount(int timestamp, String accountId)

• Creates a new account with an initial balance of 0.
• Returns false if an account with the given accountId already exists; otherwise, returns true.

2. deposit(int timestamp, String accountId, int amount)

• Deposits the specified amount into an existing account.
• Returns Optional.of(newBalance) on successful deposit.
• Returns Optional.empty() if the account does not exist or if amount is less than or equal to 0.

3. transfer(int timestamp, String sourceAccountId, String targetAccountId, int amount)

• Transfers the specified amount between two distinct, existing accounts, provided the source account has a sufficient balance.
• On success, returns Optional.of(sourceNewBalance).
• Returns Optional.empty() if any precondition fails (e.g., missing source or target accounts, identical source and target accounts, amount less than or equal to 0, or insufficient funds in the source account).
• This operation must increase only the source account's cumulative outgoing transfer total, which is used for the topSpenders leaderboard.

4. topSpenders(int timestamp, int n)

• Returns a List<String> containing up to n accountIds with the highest cumulative outgoing transfer totals.
• If fewer than n accounts exist, all available accounts should be returned.
• In case of a tie in cumulative outgoing transfer totals, use a deterministic tie-break (e.g., lexicographic accountId ascending).

5. pay(int timestamp, String accountId, int amount)

• Deducts the specified amount from the account if sufficient funds are available.
• On success, returns Optional.of(uniquePaymentId).
• Returns Optional.empty() on failure (e.g., missing account, amount less than or equal to 0, or insufficient funds).
• Automatically schedules a 2% cashback credit to the account after 24 hours (86,400 seconds) from the provided timestamp.
• The cashback credit resulting from this operation must not affect the topSpenders leaderboard calculations.

6. getPaymentStatus(int timestamp, String accountId, String paymentId)

• Returns Optional.of("IN_PROGRESS") if the cashback associated with the paymentId has not yet been applied.
• Returns Optional.of("CASHBACK_RECEIVED") after the cashback has been successfully applied.
• Returns Optional.empty() if the accountId or paymentId is invalid or if the paymentId does not match the accountId.

Additional Requirements

• Ensure atomicity and correctness under concurrency; funds must not be lost or double-counted during operations.
• Each API call must first process any due cashbacks up to the timestamp provided in that call before executing its primary operation.