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|
;;;; q.scm --- Queues
;;;;
;;;; Copyright (C) 1995, 2001, 2004, 2006 Free Software Foundation, Inc.
;;;;
;;;; This library is free software; you can redistribute it and/or
;;;; modify it under the terms of the GNU Lesser General Public
;;;; License as published by the Free Software Foundation; either
;;;; version 3 of the License, or (at your option) any later version.
;;;;
;;;; This library 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
;;;; Lesser General Public License for more details.
;;;;
;;;; You should have received a copy of the GNU Lesser General Public
;;;; License along with this library; if not, write to the Free Software
;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
;;;;
;;; Commentary:
;;; Q: Based on the interface to
;;;
;;; "queue.scm" Queues/Stacks for Scheme
;;; Written by Andrew Wilcox (awilcox@astro.psu.edu) on April 1, 1992.
;;; {Q}
;;;
;;; A list is just a bunch of cons pairs that follows some constrains,
;;; right? Association lists are the same. Hash tables are just
;;; vectors and association lists. You can print them, read them,
;;; write them as constants, pun them off as other data structures
;;; etc. This is good. This is lisp. These structures are fast and
;;; compact and easy to manipulate arbitrarily because of their
;;; simple, regular structure and non-disjointedness (associations
;;; being lists and so forth).
;;;
;;; So I figured, queues should be the same -- just a "subtype" of cons-pair
;;; structures in general.
;;;
;;; A queue is a cons pair:
;;; ( <the-q> . <last-pair> )
;;;
;;; <the-q> is a list of things in the q. New elements go at the end
;;; of that list.
;;;
;;; <last-pair> is #f if the q is empty, and otherwise is the last
;;; pair of <the-q>.
;;;
;;; q's print nicely, but alas, they do not read well because the
;;; eq?-ness of <last-pair> and (last-pair <the-q>) is lost by read.
;;;
;;; All the functions that aren't explicitly defined to return
;;; something else (a queue element; a boolean value) return the queue
;;; object itself.
;;; Code:
(define-module (ice-9 q)
:export (sync-q! make-q q? q-empty? q-empty-check q-front q-rear
q-remove! q-push! enq! q-pop! deq! q-length))
;;; sync-q!
;;; The procedure
;;;
;;; (sync-q! q)
;;;
;;; recomputes and resets the <last-pair> component of a queue.
;;;
(define (sync-q! q)
(set-cdr! q (if (pair? (car q)) (last-pair (car q))
#f))
q)
;;; make-q
;;; return a new q.
;;;
(define (make-q) (cons '() #f))
;;; q? obj
;;; Return true if obj is a Q.
;;; An object is a queue if it is equal? to '(() . #f)
;;; or it is a pair P with (list? (car P))
;;; and (eq? (cdr P) (last-pair (car P))).
;;;
(define (q? obj)
(and (pair? obj)
(if (pair? (car obj))
(eq? (cdr obj) (last-pair (car obj)))
(and (null? (car obj))
(not (cdr obj))))))
;;; q-empty? obj
;;;
(define (q-empty? obj) (null? (car obj)))
;;; q-empty-check q
;;; Throw a q-empty exception if Q is empty.
(define (q-empty-check q) (if (q-empty? q) (throw 'q-empty q)))
;;; q-front q
;;; Return the first element of Q.
(define (q-front q) (q-empty-check q) (caar q))
;;; q-rear q
;;; Return the last element of Q.
(define (q-rear q) (q-empty-check q) (cadr q))
;;; q-remove! q obj
;;; Remove all occurences of obj from Q.
(define (q-remove! q obj)
(set-car! q (delq! obj (car q)))
(sync-q! q))
;;; q-push! q obj
;;; Add obj to the front of Q
(define (q-push! q obj)
(let ((h (cons obj (car q))))
(set-car! q h)
(or (cdr q) (set-cdr! q h)))
q)
;;; enq! q obj
;;; Add obj to the rear of Q
(define (enq! q obj)
(let ((h (cons obj '())))
(if (null? (car q))
(set-car! q h)
(set-cdr! (cdr q) h))
(set-cdr! q h))
q)
;;; q-pop! q
;;; Take the front of Q and return it.
(define (q-pop! q)
(q-empty-check q)
(let ((it (caar q))
(next (cdar q)))
(if (null? next)
(set-cdr! q #f))
(set-car! q next)
it))
;;; deq! q
;;; Take the front of Q and return it.
(define deq! q-pop!)
;;; q-length q
;;; Return the number of enqueued elements.
;;;
(define (q-length q) (length (car q)))
;;; q.scm ends here
|
