# Add Nested Begin, Commit, and Abort Semantics to a Shopping Cart An in-memory shopping cart already supports `add(item)`, `remove(item)`, and a pure `discount(cart_state)` calculation. Extend it with transaction methods: - `begin()`: start a new transaction scope. - `commit()`: close the innermost active scope while preserving its changes. - `abort()`: close the innermost active scope and undo only changes made since its matching `begin()`. - `get_total_discount()`: calculate from the currently visible state, including uncommitted changes. Transactions can be nested. For example, after `begin(); add(A); begin(); add(B)`, aborting once removes only `B`; committing once preserves `B` inside the still-active outer scope. Aborting when no scope is active is a no-op. State and justify the behavior of `commit()` with no active scope. Operations performed outside a transaction take effect immediately. Design the data structures and implement the transaction layer. The item and discount representations may be treated as existing abstractions; focus on state semantics. ### Constraints & Assumptions - Calls are single-threaded for the base problem. - An item may have a quantity greater than one. - `remove` cannot produce a negative quantity; define whether it rejects or clamps an invalid request. - The cart can receive many operations between transaction boundaries. ### Clarifying Questions to Ask - Does nested `commit` merge into its parent or make changes irrevocable immediately? - Must an invalid `remove` fail atomically? - Is `get_total_discount()` pure, and may its result be cached? - Are transaction scopes required to survive process failure? ### What a Strong Answer Covers - Precise nested-scope semantics and visibility - A stack-based implementation using snapshots or reversible changes - Correct handling of repeated changes to the same item - Complexity and memory trade-offs - Tests for nested commit/abort combinations and invalid operations ### Follow-up Questions - How would you add thread safety? - How would the design change if transactions had to survive a crash? - Can an undo log avoid copying the full cart at every `begin()`? - When is cache invalidation for the discount total safe?