Design Add, Overwrite, Undo, and Redo for Billing State

Problem Statement

In financial and billing systems, balance updates must be strictly auditable, deterministic, and reversible. You are tasked with designing an in-memory billing ledger class, BillingAccount, that tracks customer balances in integer cents (to avoid floating-point inaccuracies).

The ledger must support incremental updates (add_delta), absolute adjustments (overwrite_balance), and full reversible history (undo and redo).

Implement the BillingAccount class with the following methods:

• __init__(initial_balance_cents: int = 0): Initializes the account. Raises ValueError if initial_balance_cents int: Adjusts current balance by amount_cents (can be positive or negative). If the resulting balance would be negative, the transaction must be rejected, the state unchanged, and a ValueError raised. On success, any existing redo history is invalidated, and the new balance is returned.
• overwrite_balance(new_balance_cents: int) -> int: Explicitly sets the balance to new_balance_cents. Raises ValueError if new_balance_cents bool: Reverts the most recent mutation (add_delta or overwrite_balance). Returns True if an action was reverted, or False if there are no actions to undo.
• redo() -> bool: Reapplies the most recently undone action. Returns True if an action was reapplied, or False if there are no actions to redo.
• get_balance() -> int: Returns the current balance in integer cents.

All operations (add_delta, overwrite_balance, undo, redo, get_balance) must run in O(1) time.

Example

account = BillingAccount(100) # Initial balance: $1.00 (100 cents)

account.add_delta(50)         # Balance: 150 cents
account.overwrite_balance(300)# Balance: 300 cents
account.add_delta(-100)       # Balance: 200 cents

account.undo()                # Undoes -100 delta -> Balance: 300 cents. Returns True.
account.undo()                # Undoes overwrite -> Balance: 150 cents. Returns True.
account.redo()                # Redoes overwrite -> Balance: 300 cents. Returns True.

account.add_delta(20)         # Balance: 320 cents. Redo branch is now invalidated.
account.redo()                # Returns False (redo history was cleared)
account.get_balance()         # Returns 320

Constraints

• 0 <= balance_cents <= 10^12
• -10^10 <= amount_cents <= 10^10
• At most 10^5 operations will be called across any test run.
• Balance cannot drop below 0 at any point.