Design a BankSystem Class (Take‑home Project)

Goal

Design and document a BankSystem class that supports three operations:

• createAccount(timestamp, customerId) -> bool
• deposit(timestamp, customerId, amount) -> int
• pay(timestamp, sourceAccountId, targetAccountId, amount) -> int

Assume this is an in-memory service intended for correctness and clarity (not persistence or distributed systems).

What to Produce

Provide a complete specification and design that covers:

1. Data Models

• Define the core entities (e.g., Account, Transaction) and their fields.
• Specify how balances and transaction records are stored and updated.

2. Identifier Semantics

• Clarify what customerId and accountId represent and how they relate (e.g., 1:1 vs. 1:many).
• Reconcile that deposit takes customerId while pay uses accountIds.

3. Validation Rules

• Duplicate vs. nonexistent accounts.
• Nonpositive amounts (zero or negative).
• Insufficient funds for payments.
• Any other constraints you deem important (e.g., overflow, same source/target).

4. Return Values

• Define exactly what the bool and int returns mean in all cases (success and error).
• If using error codes, enumerate them.

5. Data Structures

• Describe the primary data structures (e.g., hash maps keyed by accountId) used to implement O(1) operations.

6. Operational Semantics

• For each API, describe step-by-step how state changes and transaction records are appended.
• Include how timestamps are used in records.

7. Complexity

• Time and space complexity for each operation and overall storage.

8. Assumptions

• State any reasonable assumptions you make (e.g., single-threaded, amounts in cents, integer timestamps).

You may include concise pseudocode to illustrate the implementation.