Build a vending machine

• Slots hold a product (code, name, price in cents) with a quantity; an admin can restock and set prices
• States: Idle, HasMoney, Dispensing, OutOfService; operations (insertCoin, selectProduct, cancel, dispense) behave differently per state and invalid operations return clear errors
• Accept coins in denominations (for example 5, 10, 25, 100 cents) and keep a running balance for the current session
• On selectProduct verify that the product exists, is in stock, and the balance covers the price; otherwise report the reason and stay in the same state
• Change making: return exact change using the coins the machine holds (greedy with the available inventory, falling back to a search if greedy fails); if exact change is impossible, reject the purchase and refund
• cancel refunds the inserted balance as coins and returns to Idle
• Machine coin inventory and product stock update atomically on a successful sale, with no partial updates on failure

Only after every requirement works.

• Sales report and low-stock alerts through an event emitter
• Accept notes and card payments through a PaymentMethod interface
• A maintenance mode that requires an admin PIN