person(intern, 1).
person(assistant, 2).
person(janitor, 5).
person(old_professor, 10).

belongs(X, [X | _]).
belongs(X, [_ | T]) :- belongs(X, T).

concat([], L, L).
concat([H | T], L, [H | C]) :- concat(T, L, C).

not_belongs(X, [H|T]) :- not(belongs(X, [H|T])).

delete(X, [X | L], L).
delete(X, [Y | L], [Y | Tail]) :- delete(X, L, Tail).

insert(X, L, [X | L]).
insert(X, [H | T], [H | L]) :- insert(X, T, L).

max(X, Y, X) :-
    X >= Y.
max(X, Y, Y) :-
    X < Y.

findtime([X],Time):- person(X,Time).
findtime([A,B],Time):- person(A,Ta),person(B,Tb), max(Ta,Tb, Time).

go_through_bridge(state(PeopleNearTower1, nearTower1, PeopleNearTower2, Time),
 move(delete([X, Y], PeopleNearTower1, New_PeopleNearTower1),
	concat([X, Y], PeopleNearTower2, New_PeopleNearTower2),
	findtime([X, Y], T), New_Time is Time + T),
	state(New_PeopleNearTower1, nearTower2, New_PeopleNearTower2, New_Time)),
		nl, write(X), write(" and "),
		write(Y), write("cross the bridge. Time left: "), write(New_Time).

go_through_bridge(state(PeopleNearTower1, nearTower2, PeopleNearTower2, Time),
 move(delete(Crossing_People, PeopleNearTower2, New_PeopleNearTower2),
	insert(Crossing_People, PeopleNearTower1, New_PeopleNearTower1),
	findtime(Crossing_People, T), New_Time is Time + T),
	state(New_PeopleNearTower1, nearTower1, New_PeopleNearTower2, New_Time)),
	nl, write(Crossing_People), write(" returns to tower 1. Time left: "), write(New_Time).

goal(state([], nearTower2, [intern, assistant, janitor, old_professor], 17)).
goal(state(PeopleNearTower1, LightPos, PeopleNearTower2, T < 18)) :-
go_through_bridge(state(PeopleNearTower1, LightPos, PeopleNearTower2, T < 18), move(_, _, _, _),
State), goal(State).

%?-goal(state([intern, assistant, janitor, old_professor], nearTower1, [], 0)).