%% file:    map.pl
%% author:  Robert Keller
%% purpose: coloring a map using Prolog

map([A,B,C,D,E,F,G], Colors) :-
    next(A,B, Colors),
    next(A,C, Colors),
    next(A,D, Colors),
    next(A,E, Colors),
    next(B,D, Colors),
    next(B,F, Colors),
    next(C,D, Colors),
    next(C,E, Colors),
    next(C,F, Colors),
    next(D,F, Colors),
    next(E,F, Colors),
    next(E,G, Colors),
    next(F,G, Colors).

next(X, Y, Colors) :- 		%% two adjacent regions are colored differently
    member(X, Colors), 
    member(Y, Colors), 
    X \== Y.

test2 :-
    map(Solution, [red, blue]),
    write(Solution).

test3 :-
    map(Solution, [red, blue, green]),
    write(Solution).