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:- object(queue,
implements(queuep),
extends(compound)).
:- info([
version is 1.0,
author is 'Paulo Moura',
date is 2000/7/24,
comment is 'Queue predicates implemented using difference lists.']).
as_list(Queue-Back, List) :-
Queue == Back ->
List = []
;
List = [Head| Tail],
Queue = [Head| Rest],
as_list(Rest-Back, Tail).
empty(Front-Back) :-
Front == Back.
head(Front-Back, Head) :-
Front \== Back,
Front = [Head| _].
join(Element, Front-[Element| Back], Front-Back).
join_all([], Queue, Queue).
join_all([Head| Tail], Queue1, Queue3) :-
join(Head, Queue1, Queue2),
join_all(Tail, Queue2, Queue3).
jump(Element, Front-Back, [Element| Front]-Back).
jump_all([], Queue, Queue).
jump_all([Head| Tail], Queue1, Queue3) :-
jump(Head, Queue1, Queue2),
jump_all(Tail, Queue2, Queue3).
length(Front-Back, Length) :-
length(Front, Back, 0, N),
Length = N.
length(Front, Back, N, N) :-
Front == Back, !.
length([_|Front], Back, K, N) :-
L is K+1,
length(Front, Back, L, N).
new(Back-Back).
serve(OldFront-Back, Head, NewFront-Back) :-
OldFront \== Back,
OldFront = [Head| NewFront].
valid(Queue) :-
nonvar(Queue),
valid2(Queue).
valid2(Queue-Back) :-
Queue == Back,
!.
valid2(Queue-Back) :-
nonvar(Queue),
Queue = [_| Tail],
valid2(Tail-Back).
:- end_object.
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