billing(Customer_ID,Kwh,Amount) :-
    customer(Customer_ID,Customer_Kind,Customer_Type,Territory),
    print('customer_ID: ',Customer_ID),
    print('customer_kind: ',Customer_Kind),
    print('customer_type: ',Customer_Type),
    print('territory; ',Territory),
    bill(Customer_Kind,Customer_Type,Territory,Kwh,Amount),!.
% normal residential customers
bill(residential,normal,Territory,Kwh,Amount) :-
    current_season(Season),
    residential_rate(Territory,Season,Rate),
    Amount is Kwh * Rate.
% lifeline residential customers
bill(residential,lifeline,Territory,Kwh,Amount) :-
    Kwh <= 100, Amount is Kwh * 3,!.
bill(residential,lifeline,Territory,Kwh,Amount) :-
    Kwh <= 200, 
    Amount is 300 + (Kwh - 100) * 5,!.
bill(residential,lifeline,Territory,Kwh,Amount) :-
    Kwh > 200, bill(residential,normal,Territory,Kwh,Amount),!.
% business customers
bill(business,_,_,Kwh,Amount) :-
    Kwh <= 1000,
    Amount is Kwh * (9 - Kwh / 500),!.
bill(business,_,_,Kwh,Amount) :-
    Kwh > 1000,
    Amount is 7000 + (Kwh - 1000) * 5,!.
% industrial customers
bill(industrial,any_interrupt,_,Kwh,Amount) :-
    bill(business,_,_,Kwh,BusBill),
    Discount is BusBill / 5,
    Amount is BusBill - Discount,!.
%    discount(20),!.
bill(industrial,one_hour_interrupt,_,Kwh,Amount) :-
    bill(business,_,_,Kwh,BusBill),
    Discount is BusBill / 10,
    Amount is BusBill - Discount,!.
%    discount(10),!,
bill(industrial,two_hour_interrupt,_,Kwh,Amount) :-
    bill(business,_,_,Kwh,BusBill),
    Discount is BusBill / 20,
    Amount is BusBill - Discount,!.
%    discount(5),!,
bill(industrial,none,_,Kwh,BusBill) :-
    bill(business,_,_,Kwh,BusBill),!.

% test database
% customer(Customer_ID,Customer_Kind,Customer_Type,Territory).
customer(tom,residential,normal,terr_1).
customer(joe,residential,lifeline,terr_1).
customer(bell,business,_,_).
customer(factory1,industrial,any_interrupt,_).

% current_season(Season).
current_season(summer).

% residential_rate(Territory,Season,Rate).
residential_rate(terr_1,summer,6).
residential_rate(terr_1,winter,7).
residential_rate(terr_2,summer,7).
residential_rate(terr_2,winter,6).
residential_rate(terr_3,_,6).

test(Amount) :-
    billing(joe,500,Amount).