+(defun ring-member (ring item)
+ "Return index of ITEM if on RING, else nil.
+Comparison is done via `equal'. The index is 0-based."
+ (catch 'found
+ (dotimes (ind (ring-length ring) nil)
+ (when (equal item (ring-ref ring ind))
+ (throw 'found ind)))))
+
+(defun ring-next (ring item)
+ "Return the next item in the RING, after ITEM.
+Raise error if ITEM is not in the RING."
+ (let ((curr-index (ring-member ring item)))
+ (unless curr-index (error "Item is not in the ring: `%s'" item))
+ (ring-ref ring (ring-plus1 curr-index (ring-length ring)))))
+
+(defun ring-previous (ring item)
+ "Return the previous item in the RING, before ITEM.
+Raise error if ITEM is not in the RING."
+ (let ((curr-index (ring-member ring item)))
+ (unless curr-index (error "Item is not in the ring: `%s'" item))
+ (ring-ref ring (ring-minus1 curr-index (ring-length ring)))))
+
+(defun ring-insert+extend (ring item &optional grow-p)
+ "Like `ring-insert', but if GROW-P is non-nil, then enlarge ring.
+Insert onto ring RING the item ITEM, as the newest (last) item.
+If the ring is full, behavior depends on GROW-P:
+ If GROW-P is non-nil, enlarge the ring to accommodate the new item.
+ If GROW-P is nil, dump the oldest item to make room for the new."
+ (let* ((vec (cddr ring))
+ (veclen (length vec))
+ (hd (car ring))
+ (ringlen (ring-length ring)))
+ (prog1
+ (cond ((and grow-p (= ringlen veclen)) ; Full ring. Enlarge it.
+ (setq veclen (1+ veclen))
+ (setcdr ring (cons (setq ringlen (1+ ringlen))
+ (setq vec (vconcat vec (vector item)))))
+ (setcar ring hd))
+ (t (aset vec (mod (+ hd ringlen) veclen) item)))
+ (if (= ringlen veclen)
+ (setcar ring (ring-plus1 hd veclen))
+ (setcar (cdr ring) (1+ ringlen))))))
+
+(defun ring-remove+insert+extend (ring item &optional grow-p)
+ "`ring-remove' ITEM from RING, then `ring-insert+extend' it.
+This ensures that there is only one ITEM on RING.
+
+If the RING is full, behavior depends on GROW-P:
+ If GROW-P is non-nil, enlarge the ring to accommodate the new ITEM.
+ If GROW-P is nil, dump the oldest item to make room for the new."
+ (let (ind)
+ (while (setq ind (ring-member ring item))
+ (ring-remove ring ind)))
+ (ring-insert+extend ring item grow-p))
+
+(defun ring-convert-sequence-to-ring (seq)
+ "Convert sequence SEQ to a ring. Return the ring.
+If SEQ is already a ring, return it."
+ (if (ring-p seq)
+ seq
+ (let* ((size (length seq))
+ (ring (make-ring size)))
+ (dotimes (count size)
+ (when (or (ring-empty-p ring)
+ (not (equal (ring-ref ring 0) (elt seq count))))
+ (ring-insert-at-beginning ring (elt seq count))))
+ ring)))
+