Concurrent Restaurant Order and Pickup Simulator

Design and implement a thread-safe simulation for a restaurant system that manages the lifecycle of orders from placement to pickup. The system must support concurrent operations across multiple stages:

• Receiving incoming orders (multiple producer threads).
• Preparing orders (one or more worker/chef threads).
• Allowing customers to pick up orders (multiple consumer threads).

Requirements

1. Order Structure: Each order must have:

   • order_id: A unique integer identifier.
   • item_name: A string representing the item.
   • cook_time_ms: A positive integer indicating the time required to cook the item in milliseconds.

2. API / Operations: The system must expose the following operations (names can be adapted):

   • placeOrder(order_id, item_name, cook_time_ms): Submits a new order to the system.
   • startKitchen(num_chefs): Initializes and starts background processing threads for cooking, with num_chefs specifying the number of concurrent chef workers.
   • pickupOrder(order_id): Blocks until the specified order is ready for pickup, then returns the order.
   • shutdown(): Gracefully stops all background processing threads.

3. Correctness Constraints:

   • An order cannot be picked up before it has been fully cooked.
   • Each order must be cooked exactly once.
   • Each order can be picked up at most once.
   • The system must be free of deadlocks and avoid busy-waiting.
   • The design should robustly handle N concurrent order placements and pickups.

Discussion and Implementation Focus

Candidates should be prepared to discuss and implement:

• Their choice of synchronization primitives (e.g., mutexes, locks, condition variables, semaphores, blocking queues).
• The data structures used to track and manage order states throughout their lifecycle.
• Strategies to ensure fairness among waiting orders/customers and prevent race conditions.

Assumptions

• The simulation operates within a single process with shared memory.
• No networking or distributed system concerns are required.