Idempotent Ledger

Implement a small account ledger that applies deposits and withdrawals exactly once per request ID.

Requirements

• Define Ledger.
• apply(request_id, account_id, amount) applies amount to the account and returns the resulting balance.
• Positive amounts are deposits. Negative amounts are withdrawals.
• Request IDs are global to the ledger (not scoped per account).
• A duplicate request_id returns the same result as the first successful application and must not apply twice.
• On a duplicate request_id, ignore the retry payload (account_id / amount) and return the stored result.
• Only successful applications are recorded for idempotency. Failed overdrafts are not recorded, so a later retry with the same id may still succeed after the balance changes.
• Withdrawals that would make the balance negative raise ValueError.
• balance(account_id) returns the current account balance, defaulting to zero.

Example

Constraints

• Use standard-library Python only.
• Keep all state in memory.