% user-initiated events
need_car(Who,Where) :- 
	print(Who, " is requesting a reservation in ", Where),
	make_reservation(Who,Where).
ready_for_trip(Who,Where) :- 
	print(Who, " is ready to pick up a car in ", Where),
	pick_up_car(Who,Where).
trip_cancelled(Who,Where) :-
	print(Who, " is requesting reservation cancellation in ", Where),
	cancel_reservation(Who,Where).
trip_completed(Who,Where) :- 
	print(Who, " is returning car to ", Where),
	return_car(Who,Where).
% rental agency events
order_cars(Where,How_many) :-
	print(How_many," cars ordered for ",Where),
	deliver_cars(Where,How_many),!.
transfer_cars(From,To,Requested_number) :-
	print("Request for ",Requested_number," car transfer from ",From," to ",To),
	car_inventory(From,On_lot),
	car_reservations(From,Reserved),
	Available_for_transfer is On_lot - Reserved,
	Available_for_transfer >= Requested_number,
	adjust_car_inventory(From,-Requested_number),
	adjust_car_inventory(To,Requested_number),
	print(Requested_number," car(s) transferred from ",From," to ",To),!.
transfer_cars(From,To,Requested_number) :-
	print(From," - requested number of cars are not available").
% car manufacturer events
deliver_cars(Where,How_many) :-
	print(How_many," cars shipped for ",Where),
	receive_cars(Where,How_many),!.
% rental agency events
make_reservation(Who,Where) :-
	car_available(Where),
	assert(reservation(Who,Where)),
	print("A car is reserved for ",Who," in ", Where),
	adjust_car_reservations(Where, 1),!.
make_reservation(Who,Where) :-
	print(Who, " - Sorry, a car is not available in ",Where).
cancel_reservation(Who,Where) :-
	reservation(Who,Where),
	retract(reservation(Who,Where)),
	print("Car reservation is cancelled for ",Who," in ",Where),
	adjust_car_reservations(Where, -1),!.
cancel_reservation(Who,Where) :-
	print(Who, "you did not make a reservation in ",Where).
pick_up_car(Who,Where) :-
	reservation(Who,Where),
	retract(reservation(Who,Where)),
	assert(car_out(Who,Where)),
	adjust_car_inventory(Where, -1),
	adjust_car_reservations(Where, -1),
	print(Who," - Your car is ready."),!.
pick_up_car(Who,Where) :-
	print(Who, " - you do not have a reservation in ",Where).
return_car(Who,Where) :-
	car_out(Who,Where),
	retract(car_out(Who,Where)),
	adjust_car_inventory(Where,1),
	print(Who," - Return OK, please pay the invoice"),!.
return_car(Who,Where) :-
	print(Who, " - The car you are returning to ",Where, " is not ours").
% car rental service events
car_available(Where) :-
	car_inventory(Where,Cars_available),
	car_reservations(Where,Cars_reserved),
	Cars_available > Cars_reserved,!.
receive_cars(Where,Qty_received) :-
	print(Qty_received," cars received at ",Where),
	adjust_car_inventory(Where,Qty_received),!.
adjust_car_inventory(Where, Adjustment) :-
	car_inventory(Where,Old_inventory),
	New_inventory is Old_inventory + Adjustment,
	retract(car_inventory(Where,Old_inventory)),
	assert(car_inventory(Where,New_inventory)),!.
adjust_car_reservations(Where, Adjustment) :-
	car_reservations(Where,Old_reservations),
	New_reservations is Old_reservations + Adjustment,
	retract(car_reservations(Where,Old_reservations)),
	assert(car_reservations(Where,New_reservations)),!.
% sample facts for testing
car_inventory(atlanta,2).
car_inventory(chicago,3).
car_reservations(atlanta,0).
car_reservations(chicago,0).
% sample test
test:-
	need_car(jim,chicago),
	need_car(julie,atlanta),
	ready_for_trip(jim,chicago),
	ready_for_trip(julie,atlanta),
	trip_completed(julie,atlanta),
	ready_for_trip(bill,chicago),
	trip_completed(jim,chicago),
%	pp car_inventory,
	order_cars(atlanta,2),
%	pp car_inventory,
	transfer_cars(atlanta,chicago,1).