;;; queue.el --- Queue data structure -*- lexical-binding: t; -*-
;; Copyright (C) 1991-1995, 2008-2009, 2012, 2017 Free Software Foundation, Inc
;; Author: Inge Wallin <inge@lysator.liu.se>
;; Toby Cubitt <toby-predictive@dr-qubit.org>
;; Maintainer: Toby Cubitt <toby-predictive@dr-qubit.org>
;; Version: 0.2
;; Keywords: extensions, data structures, queue
;; URL: http://www.dr-qubit.org/emacs.php
;; Repository: http://www.dr-qubit.org/git/predictive.git
;; This file is part of Emacs.
;;
;; GNU Emacs is free software: you can redistribute it and/or modify it under
;; the terms of the GNU General Public License as published by the Free
;; Software Foundation, either version 3 of the License, or (at your option)
;; any later version.
;;
;; GNU Emacs is distributed in the hope that it will be useful, but WITHOUT
;; ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
;; FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
;; more details.
;;
;; You should have received a copy of the GNU General Public License along
;; with GNU Emacs. If not, see <http://www.gnu.org/licenses/>.
;;; Commentary:
;;
;; These queues can be used both as a first-in last-out (FILO) and as a
;; first-in first-out (FIFO) stack, i.e. elements can be added to the front or
;; back of the queue, and can be removed from the front. (This type of data
;; structure is sometimes called an "output-restricted deque".)
;;
;; You create a queue using `make-queue', add an element to the end of the
;; queue using `queue-enqueue', and push an element onto the front of the
;; queue using `queue-prepend'. To remove the first element from a queue, use
;; `queue-dequeue'. A number of other queue convenience functions are also
;; provided, all starting with the prefix `queue-'. Functions with prefix
;; `queue--' are for internal use only, and should never be used outside this
;; package.
;;; Code:
(eval-when-compile (require 'cl))
(defmacro queue--when-generators (then)
"Evaluate THEN if `generator' library is available."
(declare (debug t))
(if (require 'generator nil 'noerror) then))
(defstruct (queue
;; A tagged list is the pre-defstruct representation.
;; (:type list)
:named
(:constructor nil)
(:constructor queue-create ())
(:copier nil))
head tail)
;;;###autoload
(defalias 'make-queue 'queue-create
"Create an empty queue data structure.")
(defun queue-enqueue (queue element)
"Append an ELEMENT to the end of the QUEUE."
(if (queue-head queue)
(setcdr (queue-tail queue)
(setf (queue-tail queue) (cons element nil)))
(setf (queue-head queue)
(setf (queue-tail queue) (cons element nil)))))
(defalias 'queue-append 'queue-enqueue)
(defun queue-prepend (queue element)
"Prepend an ELEMENT to the front of the QUEUE."
(if (queue-head queue)
(push element (queue-head queue))
(setf (queue-head queue)
(setf (queue-tail queue) (cons element nil)))))
(defun queue-dequeue (queue)
"Remove the first element of QUEUE and return it.
Returns nil if the queue is empty."
(unless (cdr (queue-head queue)) (setf (queue-tail queue) nil))
(pop (queue-head queue)))
(defun queue-empty (queue)
"Return t if QUEUE is empty, otherwise return nil."
(null (queue-head queue)))
(defun queue-first (queue)
"Return the first element of QUEUE or nil if it is empty,
without removing it from the QUEUE."
(car (queue-head queue)))
(defun queue-nth (queue n)
"Return the nth element of a queue, without removing it.
If the length of the queue is less than N, return nil. The first
element in the queue has index 0."
(nth n (queue-head queue)))
(defun queue-last (queue)
"Return the last element of QUEUE, without removing it.
Returns nil if the QUEUE is empty."
(car (queue-tail queue)))
(defun queue-all (queue)
"Return a list of all elements of QUEUE or nil if it is empty.
The oldest element in the queue is the first in the list."
(queue-head queue))
(defun queue-copy (queue)
"Return a copy of QUEUE.
The new queue contains the elements of QUEUE in the same
order. The elements themselves are *not* copied."
(let ((q (queue-create))
(list (queue-head queue)))
(when (queue-head queue)
(setf (queue-head q) (cons (car (queue-head queue)) nil)
(queue-tail q) (queue-head q))
(while (setq list (cdr list))
(setf (queue-tail q)
(setcdr (queue-tail q) (cons (car list) nil)))))
q))
(defun queue-length (queue)
"Return the number of elements in QUEUE."
(length (queue-head queue)))
(defun queue-clear (queue)
"Remove all elements from QUEUE."
(setf (queue-head queue) nil
(queue-tail queue) nil))
(queue--when-generators
(iter-defun queue-iter (queue)
"Return a queue iterator object.
Calling `iter-next' on this object will retrieve the next element
from the queue. The queue itself is not modified."
(let ((list (queue-head queue)))
(while list (iter-yield (pop list))))))
;;;; ChangeLog:
;; 2017-08-16 Toby S. Cubitt <tsc25@cantab.net>
;;
;; Upgrade data structure packages to latest versions.
;;
;; 2014-05-15 Toby S. Cubitt <tsc25@cantab.net>
;;
;; queue.el: fix buggy queue-first and queue-empty definitions.
;;
;; 2012-04-30 Toby S. Cubitt <tsc25@cantab.net>
;;
;; Minor fixes to commentaries, package headers, and whitespace
;;
;; * queue.el: fix description of data structure in Commentary; add
;; Maintainer
;; header.
;;
;; * queue.el, heap.el, tNFA.el, trie.el, dict-tree.el: trivial whitespace
;; fixes.
;;
;; 2012-04-29 Toby S. Cubitt <tsc25@cantab.net>
;;
;; Add queue.el
;;
(provide 'queue)
;;; queue.el ends here
