Simulate the functionality of “Gas & Co.”

You are asked to simulate the functionality of “Gas & Co.” fuel station. The station consists of a number of fuel pumps, initially three of them. Each pump contains an input area, a display (status, amount being dispensed, amount of money), and a dispenser, and gives out one type of fuel (diesel, gasoline, etc.). Each type of fuel has a variable price that is configured on the fuel station price list. The fuel contained in a pump is of finite size and needs to be refilled periodically in order to serve the customers. “Gas & Co.” also registers regular customers and rewards them with special discounts according to flexible business rules. The station operator starts by selecting the pump, inputting the customer identity (if any, otherwise considered an anonymous customer), and then entering either the amount of fuel (depending on the pump configuration, can be litres, gallons, etc.) or the amount of money. If there is available fuel, the dispenser starts dispensing fuel at a fixed rate, reflected on the fuel station display (both amount and money), until the amount requested is completely dispensed. Thus, the display is continuously notified as the state of the fuel pump changes. Otherwise, if the requested fuel amount is not available, an error is displayed on the display. When the new customer arrives, the values are reset to 0. The pump makes sure it does not accept new customer and fuel amount values while it is dispensing fuel, thus it switches between different “states”. The customer of this application may require in the future to upgrade the displays to fancier ones, thus replacing displays should not be too difficult for programmers. For anonymous customers, the charge is calculated from the amount of fuel (or, vice versa, the amount of fuel is calculated from the amount of money) based on the simple formula charge = (amount in units) x (price/unit). However, for registered customers, special discounts may apply. The discount strategies are variable over time, such as fixed percentage discount, fixed amount of discount if a certain amount threshold is passed, free first litres/gallons, etc. These discount rules should be easily selected during run time and new ones easily added to the application in the future.