Add 'ring-resize' function

* lisp/emacs-lisp/ring.el (ring-resize): New function.  (Bug#32849)
* doc/lispref/sequences.texi (Rings): Document new function 'ring-resize'.
* etc/NEWS: Document new function 'ring-resize'.
* test/lisp/emacs-lisp/ring-tests.el (ring-test-ring-resize): New tests.

1 files changed, 22 insertions, 11 deletions

diff --git a/lisp/emacs-lisp/ring.el b/lisp/emacs-lisp/ring.el
index 312df6b2de3..1b36811f9e5 100644
--- a/lisp/emacs-lisp/ring.el
+++ b/lisp/emacs-lisp/ring.el
@@ -189,17 +189,28 @@ Raise error if ITEM is not in the RING."
 (defun ring-extend (ring x)
   "Increase the size of RING by X."
   (when (and (integerp x) (> x 0))
-    (let* ((hd       (car ring))
-           (length   (ring-length ring))
-           (size     (ring-size ring))
-           (old-vec  (cddr ring))
-           (new-vec  (make-vector (+ size x) nil)))
-      (setcdr ring (cons length new-vec))
-      ;; If the ring is wrapped, the existing elements must be written
-      ;; out in the right order.
-      (dotimes (j length)
-        (aset new-vec j (aref old-vec (mod (+ hd j) size))))
-      (setcar ring 0))))
+    (ring-resize ring (+ x (ring-size ring)))))
+
+(defun ring-resize (ring size)
+  "Set the size of RING to SIZE.
+If the new size is smaller, then the oldest items in the ring are
+discarded."
+  (when (integerp size)
+    (let ((length (ring-length ring))
+          (new-vec (make-vector size nil)))
+      (if (= length 0)
+          (setcdr ring (cons 0 new-vec))
+        (let* ((hd (car ring))
+               (old-size (ring-size ring))
+               (old-vec (cddr ring))
+               (copy-length (min size length))
+               (copy-hd (mod (+ hd (- length copy-length)) length)))
+          (setcdr ring (cons copy-length new-vec))
+          ;; If the ring is wrapped, the existing elements must be written
+          ;; out in the right order.
+          (dotimes (j copy-length)
+            (aset new-vec j (aref old-vec (mod (+ copy-hd j) old-size))))
+          (setcar ring 0))))))
 
 (defun ring-insert+extend (ring item &optional grow-p)
   "Like `ring-insert', but if GROW-P is non-nil, then enlarge ring.