In-Memory Bank System: API, Data Model, Invariants, Semantics, and Edge Cases

Goal

Design and implement an in-memory bank system that supports accounts, deposits, and payments/transfers. The system must be hash-map-based for O(1) average-time lookups and must define behavior for edge cases (timestamps, idempotency, unknown accounts, insufficient funds) and basic concurrency.

API

Interface/class BankSystem with these methods:

• bool createAccount(long timestamp, String customerId)
• int deposit(long timestamp, String customerId, int amount)
• int pay(long timestamp, String sourceAccountId, String targetAccountId, int amount)

Requirements

1. Data model
   • Define account and transaction structures suitable for O(1) lookups via hash maps.
2. Invariants
   • No negative balances.
   • Per-account timestamps are monotonic (reject stale/out-of-order ops).
3. Return semantics
   • Specify what the methods return on success and on failure (e.g., new balance vs error codes).
4. Complexity
   • Time and space complexity for each operation.
5. Edge cases to handle
   • Duplicate or out-of-order timestamps.
   • Insufficient funds.
   • Unknown accounts.
   • Idempotency under retries.
   • Basic concurrency assumptions.

Constraints

• Use hash maps (not lists) for constant-time account and transaction lookups.
• The system is in-memory (single process), but may be accessed concurrently by multiple threads.