#| -*-Scheme-*-

$Id: ring.scm,v 1.18 2008/01/30 20:02:05 cph Exp $

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    1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
    2006, 2007, 2008 Massachusetts Institute of Technology

This file is part of MIT/GNU Scheme.

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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
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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
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USA.

|#

;;;; Rings


(define (ring-list ring)
  (list-copy (vector-ref ring 2)))

(define (list-ref l i)
  (cond ((null? l) (error "Index too large" 'LIST-REF))
	((zero? i) (car l))
	(else (list-ref (cdr l) (-1+ i)))))

(define (list-set! l i o)
  (let loop ((l l) (i i))
    (cond ((null? l) (error "index too large" i))
	  ((zero? i) (set-car! l o))
	  (else (list-ref (cdr l) (-1+ i)))))
  unspecific)

(define (list-truncate! l i)
  (cond ((null? l) unspecific)
	((= i 1) (set-cdr! l '()))
	(else (list-truncate! (cdr l) (-1+ i))))
  unspecific)

(define (make-ring size)
  (if (< size 1)
      (error "Ring size too small" size)
      (vector "Ring" size '())))

(define (ring-size ring)
  (length (vector-ref ring 2)))

(define (ring-clear! ring)
  (vector-set! ring 2 '())
  unspecific)

(define (ring-empty? ring)
  (null? (vector-ref ring 2)))


(define (ring-push! ring object)
  (vector-set! ring 2 (cons object (vector-ref ring 2)))
  (list-truncate! (vector-ref ring 2) (vector-ref ring 1)))

(define (ring-pop! ring)
  (let ((l (vector-ref ring 2)))
    (if (null? l)
	(error "Ring empty" ring)
	(let ((object (car l)))
	  (vector-set! ring 2 (append! (cdr l) (list object)))
	  object))))

(define (ring-ref ring index)
  (list-ref (vector-ref ring 2) (modulo index (ring-size ring))))

(define (ring-set! ring index object)
  (list-set! (vector-ref ring 2) (modulo index (ring-size ring)) object))
