% farmer, wolf, cabbage, and goat problem
% simplified from text by using a list instead of a stack.

% a state is represented by state(Farmer, Wolf, Goat, Cabbage)
% and w and e mean the west and east river banks

move(state(X,X,G,C), state(Y,Y,G,C)):-
  opp(X,Y), not(unsafe(state(Y,Y,G,C))),
  writelist(['try farmer takes wolf ', Y, Y, G, C]), nl.

move(state(X,W,X,C), state(Y,W,Y,C)):-
  opp(X,Y), not(unsafe(state(Y,W,Y,C))),
  writelist(['try farmer takes goat ', Y, W, Y, C]), nl.

move(state(X,W,G,X), state(Y,W,G,Y)):-
  opp(X,Y), not(unsafe(state(Y,W,G,Y))),
  writelist(['try farmer takes cabbage ', Y, W, G, Y]), nl.

move(state(X,W,G,C), state(Y,W,G,C)):-
  opp(X,Y), not(unsafe(state(Y,W,G,C))),
  writelist(['try farmer takes himself ', Y, W, G, C]), nl.

move(state(F,W,G,C), state(F,W,G,C)):-
  writelist(['backtrack from ', F, W, G, C]), nl.

unsafe(state(X,Y,Y,C)) :- opp(X,Y).
unsafe(state(X,W,Y,Y)) :- opp(X,Y).

opp(e,w).
opp(w,e).

path(X, X, L):-
  nl,
  write('The solution path is: '), 
  nl,
  writelist(L).

path(X, Y, L):-
  move(X, Z),
  not(member(Z, L)),
  path(Z, Y, [Z|L]).

writelist([]).
writelist([H|T]) :-
  write(H),
  write(' '),
  writelist(T).

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

go(Start, Goal):-
  path(Start, Goal, [Start]).

test:- 
  write('starting at state(e,e,e,e) '), nl,
  go(state(e,e,e,e), state(w,w,w,w)).