/*  -*- prolog -*-
 ******************************************************************************
  File: queue.pro
  Description: The queue ADT. From Luger & Stubblefield. A queue can be
  represented in Prolog by defining the enqueue() and dequeue() predicates,
  which take the following form:
  
      enqueue(Element, Queue, NewQueue).
      dequeue(Element, Queue, NewQueue).
 
  As in the stack, we use a Prolog list to store the queue's elements.

****************************************************************************** */

empty_queue([]).

enqueue(E,[],[E]).
enqueue(E,[H|T],[H|TNew]) :-
	enqueue(E,T,TNew).

dequeue(E,[E|T],T).
dequeue(E,[E|T],_).       %% peek predicate

%% Add List to the end  of the Queue.

add_list_to_queue(List,Queue,NewQueue) :- 
	append(Queue,List, NewQueue).

%% append() utility predicate is built-in in some Prologs.

append([], List, List).
append([Head|TailofList1], List2, [Head|TailofList3]) :- 
    append(TailofList1, List2, TailofList3).

%% member() utility predicate is built-in in some Prologs.

member(H,[H|T]).
member(X,[_|T]) :-
	member(X,T).