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;;; ring.el --- handle rings of items
;; Copyright (C) 1992 Free Software Foundation, Inc.
;; Maintainer: FSF
;; Keywords: extensions
;; This file is part of GNU 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 2, 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; see the file COPYING. If not, write to
;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
;;; Commentary:
;;; This code defines a ring data structure. A ring is a
;;; (hd-index tl-index . vector)
;;; list. You can insert to, remove from, and rotate a ring. When the ring
;;; fills up, insertions cause the oldest elts to be quietly dropped.
;;;
;;; In ring-ref, 0 is the index of the newest element. Higher indexes
;;; correspond to older elements until they wrap.
;;;
;;; HEAD = index of the newest item on the ring.
;;; TAIL = index of the oldest item on the ring.
;;;
;;; These functions are used by the input history mechanism, but they can
;;; be used for other purposes as well.
;;; Code:
;;;###autoload
(defun ring-p (x)
"Returns t if X is a ring; nil otherwise."
(and (consp x) (integerp (car x))
(consp (cdr x)) (integerp (car (cdr x)))
(vectorp (cdr (cdr x)))))
;;;###autoload
(defun make-ring (size)
"Make a ring that can contain SIZE elements."
(cons 1 (cons 0 (make-vector (+ size 1) nil))))
(defun ring-plus1 (index veclen)
"INDEX+1, with wraparound"
(let ((new-index (+ index 1)))
(if (= new-index veclen) 0 new-index)))
(defun ring-minus1 (index veclen)
"INDEX-1, with wraparound"
(- (if (= 0 index) veclen index) 1))
(defun ring-length (ring)
"Number of elements in the ring."
(let ((hd (car ring)) (tl (car (cdr ring))) (siz (length (cdr (cdr ring)))))
(let ((len (if (<= hd tl) (+ 1 (- tl hd)) (+ 1 tl (- siz hd)))))
(if (= len siz) 0 len))))
(defun ring-empty-p (ring)
(= 0 (ring-length ring)))
(defun ring-insert (ring item)
"Insert a new item onto the ring. If the ring is full, dump the oldest
item to make room."
(let* ((vec (cdr (cdr ring))) (len (length vec))
(new-hd (ring-minus1 (car ring) len)))
(setcar ring new-hd)
(aset vec new-hd item)
(if (ring-empty-p ring) ;overflow -- dump one off the tail.
(setcar (cdr ring) (ring-minus1 (car (cdr ring)) len)))))
(defun ring-remove (ring)
"Remove the oldest item retained on the ring."
(if (ring-empty-p ring) (error "Ring empty")
(let ((tl (car (cdr ring))) (vec (cdr (cdr ring))))
(setcar (cdr ring) (ring-minus1 tl (length vec)))
(aref vec tl))))
(defun ring-mod (n m)
"Returns N mod M. M is positive.
Answer is guaranteed to be non-negative, and less than m."
(let ((n (% n m)))
(if (>= n 0) n
(+ n
(if (>= m 0) m (- m)))))) ; (abs m)
(defun ring-ref (ring index)
"Returns RING's INDEX element.
INDEX need not be <= the ring length, the appropriate modulo operation
will be performed. Element 0 is the most recently inserted; higher indices
correspond to older elements until they wrap."
(let ((numelts (ring-length ring)))
(if (= numelts 0) (error "indexed empty ring")
(let* ((hd (car ring)) (tl (car (cdr ring))) (vec (cdr (cdr ring)))
(index (ring-mod index numelts))
(vec-index (ring-mod (+ index hd) (length vec))))
(aref vec vec-index)))))
(provide 'ring)
;;; ring.el ends here
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