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;; Copyright (C) 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994,
;; 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
;; 2006, 2007 Massachusetts Institute of Technology
;; This file is part of MIT/GNU Scheme.
;; MIT/GNU Scheme 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 2 of the License, or (at
;; your option) any later version.
;; MIT/GNU Scheme 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 MIT/GNU Scheme; if not, write to the Free Software
;; Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301,
;; USA.
;;;; Simple Queue Abstraction
(define (make-queue)
(cons '() '()))
(define (queue-empty? queue)
(not (pair? (car queue))))
(define (queued? queue item)
(memq item (car queue)))
(define (enqueue! queue object)
(let ((next (cons object '())))
(if (pair? (cdr queue))
(set-cdr! (cdr queue) next)
(set-car! queue next))
(set-cdr! queue next)))
(define (dequeue! queue)
(let ((next (car queue)))
(if (not (pair? next))
(error "Attempt to dequeue from empty queue"))
(if (pair? (cdr next))
(set-car! queue (cdr next))
(begin
(set-car! queue '())
(set-cdr! queue '())))
(car next)))
(define (queue-map! queue procedure)
(let loop ()
(if (not (queue-empty? queue))
(begin
(procedure (dequeue! queue))
(loop)))))
(define (queue->list queue)
(car queue))
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