Exchange Order Matching Engine

Implement an order book that supports BUY, SELL, CANCEL, MODIFY, and PRINT commands, with IOC/GFD order types and order-id based updates. Requirements Write an exchange order matching engine. Input lines begin with one of these operations: BUY <IOC|GFD> <price> <quantity> <order_id> SELL <IOC|GFD> <price> <quantity> <order_id> CANCEL <order_id> MODIFY <order_id> <BUY|SELL> <new_price> <new_quantity> PRINT Rules visible from the prompt: price and quantity are positive integers. order_id can be any printable-character sequence. CANCEL removes the order id if it exists; do nothing if it does not. MODIFY changes an existing order's side, price, and quantity; do nothing if the id does not exist. BUY and SELL update the order book; PRINT prints current book state. Notes The missing part of the prompt is the exact print format and matching priority, so treat this as a partial card. The standard exchange-engine skeleton is still clear: Maintain buy levels in descending price and sell levels in ascending price. Within a price level, use FIFO queues for time priority. Keep an order_id -> order_handle index so CANCEL and MODIFY do not scan the book. IOC orders match immediately and discard residual quantity; GFD orders rest on the book if not fully filled. A buy crosses while best_ask <= buy_price; a sell crosses while best_bid >= sell_price. MODIFY is easiest to implement as cancel-then-new-order with the same id and a fresh time priority, unless the interviewer states that priority must be preserved. The same engine recurs in C++ form as a "write a Buy/Sell matching-engine POC from scratch and run it" task. The HackerRank harness does not feed stdin in that variant; a common workaround is to inspect the provided input through stderr. The C++ sittings are time-pressured (one notes a deliberately absurd 4320-minute duration) and several candidates time out on two cases, so prioritize a correct, fully-running core over edge-case completeness. A live pair-coding follow-up after the OA has asked candidates to implement a fixed-capacity ring buffer. Preparation Write a minimal matching engine with two ordered maps and per-level deques; do not start with a single sorted list. Drill CANCEL cleanup: remove the order from its level queue and delete empty price levels. Practice explaining IOC vs GFD and price-time priority in plain language before coding. Add tests for partial fill, full fill, cancel missing id, modify missing id, and modify that crosses the spread.