:- use_module(library(lists), [append/3]).

shunt(In,Out,Moves) :-
	StartNode = node(In,[],[]),
	EndNode = node(Out,[],[]),
	LastTrack = two,
	length(Moves,_),
	path(Moves,StartNode,EndNode,LastTrack),
	!.

path([],Node,Node,_).
path([OtherTrack/N|Moves],Node,EndNode,LastTrack) :-
	opposite(LastTrack,OtherTrack),
	edge(Node,NextNode,OtherTrack,N),
	path(Moves,NextNode,EndNode,OtherTrack).

edge(node(X1,A1,B), node(X2,A2,B), one, N) :- pop(A1,A2,X1,X2,N).
edge(node(X1,A1,B), node(X2,A2,B), one, N) :- push(A1,A2,X1,X2,N).
edge(node(X1,A,B1), node(X2,A,B2), two, N) :- pop(B1,B2,X1,X2,N).
edge(node(X1,A,B1), node(X2,A,B2), two, N) :- push(B1,B2,X1,X2,N).

pop(A1,A2,X1,X2,N) :-
	append(F,A2,A1),
	F \== [],
	append(X1,F,X2),
	length(F,M),
	N is -M.

push(A1,A2,X1,X2,N) :-
	append(X2,F,X1),
	F \== [],
	append(F,A1,A2),
	length(F,N).

opposite(one,two).
opposite(two,one).