% Sate representation, use a functor state/4
% state(Monkey_horiz_pos, Monkey_vert_pos, Box_pos, Has_banana)

% To generate actions which change state use move/3
% move(CurrentSate, Action, NewState)


% Four possible actions: walk, climb, push, grasp

% grasp banana
move(state(middle,onbox,middle,hasnot), grasp, state(middle,onbox,middle,has)).

% climb
move(state(P,onfloor,P,H), climb, state(P,onbox,P,H)).

% push box
move(state(P1,onfloor,P1,H), push(P1,P2), state(P2,onfloor,P2,H)).

% walk
move(state(P1,onfloor,B,H), walk(P1,P2), state(P2,onfloor,B,H)).

% recursive "depth first search" predicate canget(State) 
% which tells you if a solution is possible from State
canget(state(_, _, _, has)).

canget(State1) :-
    move(State1, Action, State2),
    showAction(State1, Action, State2),
    canget(State2).

% this predicate is just to show the step by step moves that Prolog 
% tries as it blindly tries to find a path thruough the search 
% space, including backtracking when it reaches a cul de sac.
showAction(S1, A, S2) :-
    nl, write(S1), write(' -> '), write(A), 
    write( ' -> '), write(S2), nl, write('  press c to continue '),
    read(P).