41dc743ded
(ring-rotate): Nuked. It was (a) unused, and (b) totally broken (as in, any attempt to use it died with a type error, and when I patched it to fix that I found its algorithm was broken). (ring-ref): Added doc string.
111 lines
3.7 KiB
EmacsLisp
111 lines
3.7 KiB
EmacsLisp
;;; ring.el --- handle rings of items
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;; Copyright (C) 1992 Free Software Foundation, Inc.
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;; Maintainer: FSF
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;; Keywords: extensions
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;; This file is part of GNU Emacs.
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;; GNU Emacs is free software; you can redistribute it and/or modify
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;; it under the terms of the GNU General Public License as published by
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;; the Free Software Foundation; either version 2, or (at your option)
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;; any later version.
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;; GNU Emacs is distributed in the hope that it will be useful,
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;; but WITHOUT ANY WARRANTY; without even the implied warranty of
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;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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;; GNU General Public License for more details.
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;; You should have received a copy of the GNU General Public License
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;; along with GNU Emacs; see the file COPYING. If not, write to
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;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
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;;; Commentary:
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;;; This code defines a ring data structure. A ring is a
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;;; (hd-index tl-index . vector)
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;;; list. You can insert to, remove from, and rotate a ring. When the ring
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;;; fills up, insertions cause the oldest elts to be quietly dropped.
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;;;
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;;; In ring-ref, 0 is the index of the newest element. Higher indexes
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;;; correspond to older elements until they wrap.
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;;;
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;;; HEAD = index of the newest item on the ring.
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;;; TAIL = index of the oldest item on the ring.
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;;;
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;;; These functions are used by the input history mechanism, but they can
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;;; be used for other purposes as well.
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;;; Code:
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;;;###autoload
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(defun ring-p (x)
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"Returns t if X is a ring; nil otherwise."
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(and (consp x) (integerp (car x))
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(consp (cdr x)) (integerp (car (cdr x)))
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(vectorp (cdr (cdr x)))))
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;;;###autoload
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(defun make-ring (size)
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"Make a ring that can contain SIZE elements."
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(cons 1 (cons 0 (make-vector (+ size 1) nil))))
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(defun ring-plus1 (index veclen)
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"INDEX+1, with wraparound"
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(let ((new-index (+ index 1)))
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(if (= new-index veclen) 0 new-index)))
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(defun ring-minus1 (index veclen)
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"INDEX-1, with wraparound"
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(- (if (= 0 index) veclen index) 1))
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(defun ring-length (ring)
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"Number of elements in the ring."
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(let ((hd (car ring)) (tl (car (cdr ring))) (siz (length (cdr (cdr ring)))))
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(let ((len (if (<= hd tl) (+ 1 (- tl hd)) (+ 1 tl (- siz hd)))))
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(if (= len siz) 0 len))))
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(defun ring-empty-p (ring)
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(= 0 (ring-length ring)))
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(defun ring-insert (ring item)
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"Insert a new item onto the ring. If the ring is full, dump the oldest
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item to make room."
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(let* ((vec (cdr (cdr ring))) (len (length vec))
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(new-hd (ring-minus1 (car ring) len)))
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(setcar ring new-hd)
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(aset vec new-hd item)
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(if (ring-empty-p ring) ;overflow -- dump one off the tail.
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(setcar (cdr ring) (ring-minus1 (car (cdr ring)) len)))))
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(defun ring-remove (ring)
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"Remove the oldest item retained on the ring."
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(if (ring-empty-p ring) (error "Ring empty")
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(let ((tl (car (cdr ring))) (vec (cdr (cdr ring))))
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(setcar (cdr ring) (ring-minus1 tl (length vec)))
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(aref vec tl))))
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(defun ring-mod (n m)
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"Returns N mod M. M is positive.
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Answer is guaranteed to be non-negative, and less than m."
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(let ((n (% n m)))
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(if (>= n 0) n
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(+ n
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(if (>= m 0) m (- m)))))) ; (abs m)
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(defun ring-ref (ring index)
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"Returns RING's INDEX element.
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INDEX need not be <= the ring length, the appropriate modulo operation
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will be performed. Element 0 is the most recently inserted; higher indices
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correspond to older elements until they wrap."
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(let ((numelts (ring-length ring)))
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(if (= numelts 0) (error "indexed empty ring")
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(let* ((hd (car ring)) (tl (car (cdr ring))) (vec (cdr (cdr ring)))
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(index (ring-mod index numelts))
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(vec-index (ring-mod (+ index hd) (length vec))))
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(aref vec vec-index)))))
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(provide 'ring)
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;;; ring.el ends here
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