====== Wolf goat cabbage ======
{{tag>puzzle}}
===== Description =====
Solving the wolf, goat, and cabbage problem

**Source**:  The Art of Prolog
===== Download =====
Program source code: {{wolf_goat_cabbage.pl}}
===== Listing =====
<code prolog>
/*
   States for the wolf, goat and cabbage problem are a structure
   wgc(Boat,Left,Right), where Boat is the bank on which the boat 
   currently is, Left is the list of occupants on the left bank of 
   the river, and Right is the list of occupants on the right bank.
*/
     initial_state(wgc,wgc(left,[wolf,goat,cabbage],[])).

     final_state(wgc(right,[],[wolf,goat,cabbage])).

     move(wgc(left,L,R),Cargo) :- member(Cargo,L).
     move(wgc(right,L,R),Cargo) :- member(Cargo,R).
     move(wgc(B,L,R),alone).

     update(wgc(B,L,R),Cargo,wgc(B1,L1,R1)) :-
         update_boat(B,B1), update_banks(Cargo,B,L,R,L1,R1).

     update_boat(left,right).        
     update_boat(right,left).
        
     update_banks(alone,B,L,R,L,R).
     update_banks(Cargo,left,L,R,L1,R1) :- 
	select(Cargo,L,L1), insert(Cargo,R,R1).
     update_banks(Cargo,right,L,R,L1,R1) :- 
	select(Cargo,R,R1), insert(Cargo,L,L1).

     insert(X,[Y|Ys],[X,Y|Ys]) :-
	precedes(X,Y).
     insert(X,[Y|Ys],[Y|Zs]) :-
	precedes(Y,X), insert(X,Ys,Zs).
     insert(X,[],[X]).

     precedes(wolf,X).
     precedes(X,cabbage).

     legal(wgc(left,L,R)) :- not(illegal(R)).
     legal(wgc(right,L,R)) :- not(illegal(L)).

	illegal(Bank) :- member(wolf,Bank), member(goat,Bank).
        illegal(Bank) :- member(goat,Bank), member(cabbage,Bank).

	select(X,[X|Xs],Xs).
	select(X,[Y|Ys],[Y|Zs]) :-
		select(X,Ys,Zs).

%    Program 20.2:  Solving the wolf, goat, and cabbage problem
</code>
===== Comments =====