NASDAQ Market Data Price-Level Book

Maintain the AAPL order book from insert, modify, and cancel callbacks, then implement `GetPriceLevel(side, level_index)` to return the price and aggregate quantity at the requested zero-based bid or ask level. Requirements Maintain market data for exactly one symbol, AAPL. Consume these callbacks: OnOrderInsert(id, side, price, qty) OnOrderModify(id, price, qty) OnOrderCancel(id) Implement GetPriceLevel(side, level_index), where level_index is an unsigned, zero-based index and the return value is (price, total_qty). Aggregate all live orders at the same price into one price level. For buys, level 0 is the highest price; for sells, level 0 is the lowest price. Higher indices walk away from the best price in the corresponding direction. A modification must remove the order's previous contribution before adding its new price and quantity; a cancellation removes it completely. Notes Keep an ID-to-order map so modify and cancel operations can locate the old side, price, and quantity. Remove a price level when its aggregate quantity reaches zero; otherwise later level indices can point at empty levels. Preserve side-specific ordering: descending bids and ascending asks. Preparation Implement the four-method interface with an order map plus side-specific ordered price-level structures. Drill inserts at an existing price, moves between prices, quantity-only modifications, cancellations of the last order at a level, and retrieval beyond the populated depth according to the prompt's required behavior.