% customer activities
order_book(Customer, Book) :-
	print(Customer," is ordering ", Book),
	receive_order(Customer, Book).
send_payment(Customer, Invoice) :-
	print(Customer, " is paying invoice ", Invoice),
	receive_payment(Customer, Invoice).
% dealer activities
receive_order(Customer, Book) :-
	book_available(Book),
	invoice_book(Customer, Book),
	ship_book(Customer, Book), !.
receive_order(Customer, Book) :-
	print("backorder for ", Book).
receive_payment(Customer, Invoice) :-
	adjust_account(Customer, Invoice).
invoice_book(Customer, Book) :-
	get_invoice_number(Invoice),
	book(Book, _, Price),
	assert(account(Customer, Invoice, Price)),
	send_invoice(Invoice, Customer, Book, Price).
send_invoice(Invoice, Customer, Book, Price) :-
	print("invoice ", Invoice, " sent to ", Customer).
ship_book(Customer, Book) :-
	adjust_inventory(Book, -1),
	print(Book, " shipped to ", Customer).
% dealer support operations
book_available(Book) :-
	book(Book, Qty, _),
	Qty >= 1.
get_invoice_number(Next_number) :-
	last_invoice(Old_number),
	Next_number is Old_number + 1,
	retract(last_invoice(Old_number)),
	assert(last_invoice(Next_number)).
adjust_inventory(Book, Adjustment) :-
	book(Book, Qty, Price),
	NewQty is Qty + Adjustment,
	retract(book(Book, Qty, Price)),
	assert(book(Book, NewQty, Price)).
adjust_account(Customer,Invoice) :-
	account(Customer, Invoice, Price),
	print(Customer, " has paid invoice ", Invoice),
	retract(account(Customer, Invoice, Price)),!.
% sample database
% book(Title,Qty,Price)	
book(dictionary, 2, 15).
book(novel, 1, 5).
% last invoice number used
last_invoice(100).
% test
test :-
	order_book(joe, dictionary),
	order_book(jill, novel),
	send_payment(joe, _),
	send_payment(jill, _).