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yap-6.3/library/queues.yap

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% This file has been included as an YAP library by Vitor Santos Costa, 1999
% File : QUEUES.PL
% Author : R.A.O'Keefe
% Updated: Friday November 18th, 1983, 8:09:31 pm
% Purpose: define queue operations
% Needs : lib(lists) for append/3.
2014-09-11 20:06:57 +01:00
/** @defgroup Queues Queues
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@ingroup library
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@{
The following queue manipulation routines are available once
included with the `use_module(library(queues))` command. Queues are
implemented with difference lists.
*/
/**
@pred make_queue(+ _Queue_)
Creates a new empty queue. It should only be used to create a new queue.
*/
/** @pred empty_queue(+ _Queue_)
Tests whether the queue is empty.
*/
/** @pred head_queue(+ _Queue_, ? _Head_)
Unifies Head with the first element of the queue.
*/
/** @pred join_queue(+ _Element_, + _OldQueue_, - _NewQueue_)
Adds the new element at the end of the queue.
*/
/** @pred jump_queue(+ _Element_, + _OldQueue_, - _NewQueue_)
Adds the new element at the front of the list.
*/
/** @pred length_queue(+ _Queue_, - _Length_)
Counts the number of elements currently in the queue.
*/
/** @pred list_join_queue(+ _List_, + _OldQueue_, - _NewQueue_)
Ads the new elements at the end of the queue.
*/
/** @pred list_jump_queue(+ _List_, + _OldQueue_, + _NewQueue_)
Adds all the elements of _List_ at the front of the queue.
*/
/** @pred list_to_queue(+ _List_, - _Queue_)
Creates a new queue with the same elements as _List._
*/
/** @pred queue_to_list(+ _Queue_, - _List_)
Creates a new list with the same elements as _Queue_.
*/
/** @pred serve_queue(+ _OldQueue_, + _Head_, - _NewQueue_)
Removes the first element of the queue for service.
*/
:- module(queues, [
make_queue/1, % create empty queue
join_queue/3, % add element to end of queue
list_join_queue/3, % add many elements to end of queue
jump_queue/3, % add element to front of queue
list_jump_queue/3, % add many elements to front of queue
head_queue/2, % look at first element of queue
serve_queue/3, % remove first element of queue
length_queue/2, % count elements of queue
empty_queue/1, % test whether queue is empty
list_to_queue/2, % convert list to queue
queue_to_list/2 % convert queue to list
]).
:- use_module(library(lists), [append/3]).
/*
:- mode
make_queue(-),
join_queue(+, +, -),
list_join_queue(+, +, -),
jump_queue(+, +, -),
list_jump_queue(+, +, -),
head_queue(+, ?),
serve_queue(+, ?, -),
length_queue(+, ?),
length_queue(+, +, +, -),
empty_queue(+),
list_to_queue(+, -),
queue_to_list(+, -),
queue_to_list(+, +, -).
*/
/* In this package, a queue is represented as a term Front-Back, where
Front is a list and Back is a tail of that list, and is normally a
variable. join_queue will only work when the Back is a variable,
the other routines will accept any tail. The elements of the queue
are the list difference, that is, all the elements starting at Front
and stopping at Back. Examples:
[a,b,c,d,e|Z]-Z has elements a,b,c,d,e
[a,b,c,d,e]-[d,e] has elements a,b,c
Z-Z has no elements
[1,2,3]-[1,2,3] has no elements
*/
% make_queue(Queue)
% creates a new empty queue. It will also match empty queues, but
% because Prolog doesn't do the occurs check, it will also match
% other queues, creating circular lists. So this should ONLY be
% used to make new queues.
make_queue(X-X).
% join_queue(Element, OldQueue, NewQueue)
% adds the new element at the end of the queue. The old queue is
% side-effected, so you *can't* do
% join_queue(1, OldQ, NewQ1),
% join_queue(2, OldQ, NewQ2).
% There isn't any easy way of doing that, sensible though it might
% be. You *can* do
% join_queue(1, OldQ, MidQ),
% join_queue(2, MidQ, NewQ).
% See list_join_queue.
join_queue(Element, Front-[Element|Back], Front-Back).
% list_join_queue(List, OldQueue, NewQueue)
% adds the new elements at the end of the queue. The elements are
% added in the same order that they appear in the list, e.g.
% list_join_queue([y,z], [a,b,c|M]-M, [a,b,c,y,z|N]-N).
list_join_queue(List, Front-OldBack, Front-NewBack) :-
append(List, OldBack, NewBack).
% jump_queue(Element, OldQueue, NewQueue)
% adds the new element at the front of the list. Unlike join_queue,
% jump_queue(1, OldQ, NewQ1),
% jump_queue(2, OldQ, NewQ2)
% *does* work, though if you add things at the end of NewQ1 they
% will also show up in NewQ2. Note that
% jump_queue(1, OldQ, MidQ),
% jump_queue(2, MidQ, NewQ)
% makes NewQ start 2, 1, ...
jump_queue(Element, Front-Back, [Element|Front]-Back).
% list_jump_queue(List, OldQueue, NewQueue)
% adds all the elements of List at the front of the queue. There are
% two ways we might do this. We could add all the elements one at a
% time, so that they would appear at the beginning of the queue in the
% opposite order to the order they had in the list, or we could add
% them in one lump, so that they have the same order in the queue as
% in the list. As you can easily add the elements one at a time if
% that is what you want, I have chosen the latter.
list_jump_queue(List, OldFront-Back, NewFront-Back) :-
append(List, OldFront, NewFront).
% reverse(List, OldFront, NewFront). % for the other definition
% head_queue(Queue, Head)
% unifies Head with the first element of the queue. The tricky part
% is that we might be at the end of a queue: Back-Back, with Back a
% variable, and in that case this predicate should not succeed, as we
% don't know what that element is or whether it exists yet.
head_queue(Front-Back, Head) :-
Front \== Back, % the queue is not empty
Front = [Head|_].
% serve_queue(OldQueue, Head, NewQueue)
% removes the first element of the queue for service.
serve_queue(OldFront-Back, Head, NewFront-Back) :-
OldFront \== Back,
OldFront = [Head|NewFront].
% empty_queue(Queue)
% tests whether the queue is empty. If the back of a queue were
% guaranteed to be a variable, we could have
% empty_queue(Front-Back) :- var(Front).
% but I don't see why you shouldn't be able to treat difference
% lists as queues if you want to.
empty_queue(Front-Back) :-
Front == Back.
% length_queue(Queue, Length)
% counts the number of elements currently in the queue. Note that
% we have to be careful in checking for the end of the list, we
% can't test for [] the way length(List) does.
length_queue(Front-Back, Length) :-
length_queue(Front, Back, 0, N),
Length = N.
length_queue(Front, Back, N, N) :-
Front == Back, !.
length_queue([_|Front], Back, K, N) :-
L is K+1,
length_queue(Front, Back, L, N).
% list_to_queue(List, Queue)
% creates a new queue with the same elements as List.
list_to_queue(List, Front-Back) :-
append(List, Back, Front).
% queue_to_list(Queue, List)
% creates a new list with the same elements as Queue.
queue_to_list(Front-Back, List) :-
queue_to_list(Front, Back, List).
queue_to_list(Front, Back, Ans) :-
Front == Back, !, Ans = [].
queue_to_list([Head|Front], Back, [Head|Tail]) :-
queue_to_list(Front, Back, Tail).