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% This file has been included as an YAP library by Vitor Santos Costa, 1999
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% File : QUEUES.PL
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% Author : R.A.O'Keefe
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% Updated: Friday November 18th, 1983, 8:09:31 pm
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% Purpose: define queue operations
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% Needs : lib(lists) for append/3.
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/** @defgroup Queues Queues
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@ingroup library
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@{
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The following queue manipulation routines are available once
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included with the `use_module(library(queues))` command. Queues are
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implemented with difference lists.
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*/
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/**
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@pred make_queue(+ _Queue_)
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Creates a new empty queue. It should only be used to create a new queue.
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*/
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/** @pred empty_queue(+ _Queue_)
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Tests whether the queue is empty.
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*/
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/** @pred head_queue(+ _Queue_, ? _Head_)
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Unifies Head with the first element of the queue.
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*/
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/** @pred join_queue(+ _Element_, + _OldQueue_, - _NewQueue_)
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Adds the new element at the end of the queue.
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*/
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/** @pred jump_queue(+ _Element_, + _OldQueue_, - _NewQueue_)
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Adds the new element at the front of the list.
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*/
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/** @pred length_queue(+ _Queue_, - _Length_)
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Counts the number of elements currently in the queue.
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*/
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/** @pred list_join_queue(+ _List_, + _OldQueue_, - _NewQueue_)
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Ads the new elements at the end of the queue.
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*/
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/** @pred list_jump_queue(+ _List_, + _OldQueue_, + _NewQueue_)
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Adds all the elements of _List_ at the front of the queue.
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*/
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/** @pred list_to_queue(+ _List_, - _Queue_)
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Creates a new queue with the same elements as _List._
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*/
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/** @pred queue_to_list(+ _Queue_, - _List_)
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Creates a new list with the same elements as _Queue_.
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*/
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/** @pred serve_queue(+ _OldQueue_, + _Head_, - _NewQueue_)
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Removes the first element of the queue for service.
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*/
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:- module(queues, [
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make_queue/1, % create empty queue
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join_queue/3, % add element to end of queue
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list_join_queue/3, % add many elements to end of queue
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jump_queue/3, % add element to front of queue
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list_jump_queue/3, % add many elements to front of queue
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head_queue/2, % look at first element of queue
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serve_queue/3, % remove first element of queue
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length_queue/2, % count elements of queue
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empty_queue/1, % test whether queue is empty
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list_to_queue/2, % convert list to queue
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queue_to_list/2 % convert queue to list
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]).
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:- use_module(library(lists), [append/3]).
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/*
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:- mode
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make_queue(-),
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join_queue(+, +, -),
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list_join_queue(+, +, -),
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jump_queue(+, +, -),
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list_jump_queue(+, +, -),
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head_queue(+, ?),
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serve_queue(+, ?, -),
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length_queue(+, ?),
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length_queue(+, +, +, -),
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empty_queue(+),
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list_to_queue(+, -),
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queue_to_list(+, -),
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queue_to_list(+, +, -).
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*/
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/* In this package, a queue is represented as a term Front-Back, where
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Front is a list and Back is a tail of that list, and is normally a
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variable. join_queue will only work when the Back is a variable,
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the other routines will accept any tail. The elements of the queue
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are the list difference, that is, all the elements starting at Front
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and stopping at Back. Examples:
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[a,b,c,d,e|Z]-Z has elements a,b,c,d,e
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[a,b,c,d,e]-[d,e] has elements a,b,c
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Z-Z has no elements
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[1,2,3]-[1,2,3] has no elements
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*/
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% make_queue(Queue)
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% creates a new empty queue. It will also match empty queues, but
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% because Prolog doesn't do the occurs check, it will also match
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% other queues, creating circular lists. So this should ONLY be
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% used to make new queues.
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make_queue(X-X).
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% join_queue(Element, OldQueue, NewQueue)
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% adds the new element at the end of the queue. The old queue is
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% side-effected, so you *can't* do
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% join_queue(1, OldQ, NewQ1),
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% join_queue(2, OldQ, NewQ2).
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% There isn't any easy way of doing that, sensible though it might
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% be. You *can* do
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% join_queue(1, OldQ, MidQ),
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% join_queue(2, MidQ, NewQ).
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% See list_join_queue.
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join_queue(Element, Front-[Element|Back], Front-Back).
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% list_join_queue(List, OldQueue, NewQueue)
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% adds the new elements at the end of the queue. The elements are
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% added in the same order that they appear in the list, e.g.
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% list_join_queue([y,z], [a,b,c|M]-M, [a,b,c,y,z|N]-N).
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list_join_queue(List, Front-OldBack, Front-NewBack) :-
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append(List, OldBack, NewBack).
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% jump_queue(Element, OldQueue, NewQueue)
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% adds the new element at the front of the list. Unlike join_queue,
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% jump_queue(1, OldQ, NewQ1),
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% jump_queue(2, OldQ, NewQ2)
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% *does* work, though if you add things at the end of NewQ1 they
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% will also show up in NewQ2. Note that
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% jump_queue(1, OldQ, MidQ),
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% jump_queue(2, MidQ, NewQ)
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% makes NewQ start 2, 1, ...
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jump_queue(Element, Front-Back, [Element|Front]-Back).
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% list_jump_queue(List, OldQueue, NewQueue)
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% adds all the elements of List at the front of the queue. There are
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% two ways we might do this. We could add all the elements one at a
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% time, so that they would appear at the beginning of the queue in the
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% opposite order to the order they had in the list, or we could add
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% them in one lump, so that they have the same order in the queue as
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% in the list. As you can easily add the elements one at a time if
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% that is what you want, I have chosen the latter.
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list_jump_queue(List, OldFront-Back, NewFront-Back) :-
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append(List, OldFront, NewFront).
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% reverse(List, OldFront, NewFront). % for the other definition
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% head_queue(Queue, Head)
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% unifies Head with the first element of the queue. The tricky part
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% is that we might be at the end of a queue: Back-Back, with Back a
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% variable, and in that case this predicate should not succeed, as we
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% don't know what that element is or whether it exists yet.
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head_queue(Front-Back, Head) :-
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Front \== Back, % the queue is not empty
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Front = [Head|_].
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% serve_queue(OldQueue, Head, NewQueue)
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% removes the first element of the queue for service.
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serve_queue(OldFront-Back, Head, NewFront-Back) :-
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OldFront \== Back,
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OldFront = [Head|NewFront].
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% empty_queue(Queue)
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% tests whether the queue is empty. If the back of a queue were
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% guaranteed to be a variable, we could have
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% empty_queue(Front-Back) :- var(Front).
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% but I don't see why you shouldn't be able to treat difference
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% lists as queues if you want to.
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empty_queue(Front-Back) :-
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Front == Back.
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% length_queue(Queue, Length)
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% counts the number of elements currently in the queue. Note that
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% we have to be careful in checking for the end of the list, we
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% can't test for [] the way length(List) does.
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length_queue(Front-Back, Length) :-
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length_queue(Front, Back, 0, N),
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Length = N.
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length_queue(Front, Back, N, N) :-
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Front == Back, !.
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length_queue([_|Front], Back, K, N) :-
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L is K+1,
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length_queue(Front, Back, L, N).
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% list_to_queue(List, Queue)
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% creates a new queue with the same elements as List.
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list_to_queue(List, Front-Back) :-
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append(List, Back, Front).
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% queue_to_list(Queue, List)
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% creates a new list with the same elements as Queue.
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queue_to_list(Front-Back, List) :-
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queue_to_list(Front, Back, List).
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queue_to_list(Front, Back, Ans) :-
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Front == Back, !, Ans = [].
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queue_to_list([Head|Front], Back, [Head|Tail]) :-
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queue_to_list(Front, Back, Tail).
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William Tumeo