%%%% Zebra puzzle
%
% shirt: black, blue, pink, yellow
% name: andrew, brian, samuel, yousef
% produce: apples, corn, onions, tomatoes
% price: $2.0, $2.5, $4.0, $6.0
%
% The farmer charging $2.00 per pound is in the third position.
% The farmer charging $2.50 per pound is in the second position.
% The farmer selling Onions is immediately before the farmer wearing the Black shirt.
% Brian is the farmer selling Apples.
% The farmer wearing the Blue shirt is in the last position.
% The farmer wearing the Pink shirt is immediately after the one charging $2.50 per pound.
% Andrew is the vendor in the second position.
% The farmer charging $6.00 per pound is in the first position.
% Yousef is the vendor in the last position.
% The farmer wearing the Black shirt is selling Tomatoes.
% The vendor selling Corn is at one of the ends.

in_position([E|_],E,1).
in_position([_|T],E,N) :- N1 is N - 1, in_position(T,E,N1). 

immediately_before([E1|_],E1,[_,E2|_],E2).
immediately_before([_|T1],E1,[_|T2],E2) :- immediately_before(T1,E1,T2,E2).

immediately_after(L1,E1,L2,E2) :- immediately_before(L2,E2,L1,E1).

the_same([E1|_],E1,[E2|_],E2).
the_same([_|T1],E1,[_|T2],E2) :- the_same(T1,E1,T2,E2).

is_last([E],E).
is_last([_|T],E) :- is_last(T,E).

at_one_end([E|_],E).
at_one_end(L,E) :- is_last(L,E).

zebra(Shirt,Name,Produce,Price) :-
    permutation(Shirt,[black, blue, pink, yellow]),
    permutation(Name,[andrew, brian, samuel, yousef]),
    permutation(Produce,[apples, corn, onions, tomatoes]),
    permutation(Price,[$2.0, $2.5, $4.0, $6.0]),
    in_position(Price,$2.0,3),
    in_position(Price,$2.5,2),
    immediately_before(Produce,onions,Shirt,black),
    the_same(Name,brian,Produce,apples),
    is_last(Shirt,blue),
    immediately_after(Shirt,pink,Price,$2.5),
    in_position(Name,andrew,2),
    in_position(Price,$6.0,1),
    is_last(Name,yousef),
    the_same(Shirt,black,Produce,tomatoes),
    at_one_end(Produce,corn).

zebra(Shirt,Name,Produce,Price).