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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, Inc., 59 Temple Place - Suite 330,
;; Boston, MA 02111-1307, USA.
;;; Commentary:
;; This code defines a ring data structure. A ring is a
;; (hd-index length . 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.
;;
;; hd-index = index of the newest item on the ring.
;; length = number of ring items.
;;
;; 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 0 (cons 0 (make-vector size nil))))
(defun ring-insert-at-beginning (ring item)
"Add to RING the item ITEM. Add it at the front (the early end)."
(let* ((vec (cdr (cdr ring)))
(veclen (length vec))
(hd (car ring))
(ln (car (cdr ring))))
(setq ln (min veclen (1+ ln))
hd (ring-minus1 hd veclen))
(aset vec hd item)
(setcar ring hd)
(setcar (cdr ring) ln)))
(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."
(car (cdr ring)))
(defun ring-empty-p (ring)
(= 0 (car (cdr ring))))
(defun ring-index (index head ringlen veclen)
(setq index (mod index ringlen))
(mod (1- (+ head (- ringlen index))) veclen))
(defun ring-insert (ring item)
"Insert onto ring RING the item ITEM, as the newest (last) item.
If the ring is full, dump the oldest item to make room."
(let* ((vec (cdr (cdr ring)))
(veclen (length vec))
(hd (car ring))
(ln (car (cdr ring))))
(prog1
(aset vec (mod (+ hd ln) veclen) item)
(if (= ln veclen)
(setcar ring (ring-plus1 hd veclen))
(setcar (cdr ring) (1+ ln))))))
(defun ring-remove (ring &optional index)
"Remove an item from the RING. Return the removed item.
If optional INDEX is nil, remove the oldest item. If it's
numeric, remove the element indexed."
(if (ring-empty-p ring)
(error "Ring empty")
(let* ((hd (car ring))
(ln (car (cdr ring)))
(vec (cdr (cdr ring)))
(veclen (length vec))
(tl (mod (1- (+ hd ln)) veclen))
oldelt)
(if (null index)
(setq index (1- ln)))
(setq index (ring-index index hd ln veclen))
(setq oldelt (aref vec index))
(while (/= index tl)
(aset vec index (aref vec (ring-plus1 index veclen)))
(setq index (ring-plus1 index veclen)))
(aset vec tl nil)
(setcar (cdr ring) (1- ln))
oldelt)))
(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."
(if (ring-empty-p ring)
(error "indexed empty ring")
(let* ((hd (car ring)) (ln (car (cdr ring))) (vec (cdr (cdr ring))))
(aref vec (ring-index index hd ln (length vec))))))
(provide 'ring)
;;; ring.el ends here
|
