;; 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.
+;; 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.
+;; 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; when the index equals the ring length,
+;; it wraps to the newest element again.
+;;
+;; hd-index = vector index of the oldest ring item.
+;; Newer items follow this item; at the end of the vector,
+;; they wrap around to the start of the vector.
+;; length = number of items currently in the ring.
+;; This never exceeds the length of the vector itself.
+;;
+;; These functions are used by the input history mechanism, but they can
+;; be used for other purposes as well.
;;; Code:
"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, as the oldest item."
+ (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"
+ "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"
+ "INDEX-1, with wraparound."
(- (if (= 0 index) veclen index) 1))
(defun ring-length (ring)
- "Number of elements in the ring."
+ "Number of elements in the ring RING."
(car (cdr ring)))
(defun ring-empty-p (ring)
(mod (1- (+ head (- ringlen index))) veclen))
(defun ring-insert (ring item)
- "Insert a new item onto the ring. If the ring is full, dump the oldest
-item to make room."
+ "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))
(defun ring-ref (ring index)
"Returns RING's INDEX element.
+INDEX = 0 is the most recently inserted; higher indices
+correspond to older elements.
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."
+will be performed."
(if (ring-empty-p ring)
- (error "indexed empty ring")
+ (error "Accessing an empty ring")
(let* ((hd (car ring)) (ln (car (cdr ring))) (vec (cdr (cdr ring))))
(aref vec (ring-index index hd ln (length vec))))))