Build a parking lot

• A lot has multiple levels; each level has spots of types (motorcycle, compact, large), and a vehicle (motorcycle, car, truck) fits only compatible spot types (for example a car fits compact or large)
• park(vehicle) allocates the nearest suitable free spot (lowest level, then lowest number), issues a ticket with id, vehicle plate, spot and entry time, and fails clearly when the lot is full or the vehicle is already parked
• unpark(ticketId) frees the spot, computes the fee from the elapsed time, and returns a receipt; a ticket cannot be used twice
• Fee calculation is a swappable strategy (for example hourly with a first-hour rate, rounding up partial hours, with per-vehicle-type rates), and time comes from an injectable clock
• availability() reports free spots by type and by level
• Concurrent calls must not assign the same spot twice (document how: single-threaded event loop with synchronous allocation, or a lock per lot if awaits are introduced)
• Entry and exit through gates are modelled or at least the interfaces are in place (an entry gate calls park; an exit gate takes a ticket and a payment)

Only after every requirement works.

• Reserve or pre-book a spot
• Handicapped and electric charging spot types with priority rules
• Lost ticket handling with a flat penalty