% Zebra puzzle, adapted from Eyelet's input/zebra.pl.
%
% Five houses are represented as house(Color, Nationality, Pet, Beverage,
% Cigarette).  Each clue is a member/2, first/2, third/2, next_to/3, or adjacent/3
% constraint over that list.  The answer is the classic one: the Norwegian drinks
% water and the Japanese owns the zebra.

% Output declarations: query/1 selects the relations written to this example's golden output.
query(waterDrinker(X0, X1)).
query(zebraOwner(X0, X1)).
query(solved(X0, X1)).

% The single zebra/2 rule is a finite constraint model over the five house slots.
zebra(Waterdrinker, Zebraowner) :-
  eq(Houses, [_, _, _, _, _]),
  first(Houses, house(_, norwegian, _, _, _)),
  third(Houses, house(_, _, _, milk, _)),
  adjacent(house(_, norwegian, _, _, _), house(blue, _, _, _, _), Houses),
  next_to(house(ivory, _, _, _, _), house(green, _, _, _, _), Houses),
  member(house(red, english, _, _, _), Houses),
  member(house(green, _, _, coffee, _), Houses),
  member(house(yellow, _, _, _, kools), Houses),
  member(house(_, spanish, dog, _, _), Houses),
  member(house(_, ukrainian, _, tea, _), Houses),
  member(house(_, _, snail, _, old_gold), Houses),
  adjacent(house(_, _, _, _, chesterfields), house(_, _, fox, _, _), Houses),
  adjacent(house(_, _, _, _, kools), house(_, _, horse, _, _), Houses),
  member(house(_, _, _, orange_juice, lucky_strike), Houses),
  member(house(_, japanese, _, _, parliaments), Houses),
  member(house(_, Waterdrinker, _, water, _), Houses),
  member(house(_, Zebraowner, zebra, _, _), Houses).

% Positional and neighborhood helpers keep the clue encoding readable.
first([X|_], X).
third([_, _, X|_], X).

adjacent(A, B, List) :- next_to(A, B, List).
adjacent(A, B, List) :- next_to(B, A, List).

next_to(X, Y, [X, Y|_]).
next_to(X, Y, [_|Zs]) :- next_to(X, Y, Zs).

waterDrinker(zebraPuzzle, Waterdrinker) :- zebra(Waterdrinker, _zebraowner).
zebraOwner(zebraPuzzle, Zebraowner) :- zebra(_waterdrinker, Zebraowner).
solved(zebraPuzzle, true) :- zebra(norwegian, japanese).