6.8 Managing a Fixed-Size Ring of Objects

A ring is a fixed-size data structure that supports insertion, deletion, rotation, and modulo-indexed reference and traversal. An efficient ring data structure is implemented by the ring package. It provides the functions listed in this section.

Note that several rings in Emacs, like the kill ring and the mark ring, are actually implemented as simple lists, not using the ring package; thus the following functions won’t work on them.

Function: make-ring size

This returns a new ring capable of holding size objects. size should be an integer.

Function: ring-p object

This returns t if object is a ring, nil otherwise.

Function: ring-size ring

This returns the maximum capacity of the ring.

Function: ring-length ring

This returns the number of objects that ring currently contains. The value will never exceed that returned by ring-size.

Function: ring-elements ring

This returns a list of the objects in ring, in order, newest first.

Function: ring-copy ring

This returns a new ring which is a copy of ring. The new ring contains the same (eq) objects as ring.

Function: ring-empty-p ring

This returns t if ring is empty, nil otherwise.

The newest element in the ring always has index 0. Higher indices correspond to older elements. Indices are computed modulo the ring length. Index −1 corresponds to the oldest element, −2 to the next-oldest, and so forth.

Function: ring-ref ring index

This returns the object in ring found at index index. index may be negative or greater than the ring length. If ring is empty, ring-ref signals an error.

Function: ring-insert ring object

This inserts object into ring, making it the newest element, and returns object.

If the ring is full, insertion removes the oldest element to make room for the new element.

Function: ring-remove ring &optional index

Remove an object from ring, and return that object. The argument index specifies which item to remove; if it is nil, that means to remove the oldest item. If ring is empty, ring-remove signals an error.

Function: ring-insert-at-beginning ring object

This inserts object into ring, treating it as the oldest element. The return value is not significant.

If the ring is full, this function removes the newest element to make room for the inserted element.

If you are careful not to exceed the ring size, you can use the ring as a first-in-first-out queue. For example:

(let ((fifo (make-ring 5)))
  (mapc (lambda (obj) (ring-insert fifo obj))
        '(0 one "two"))
  (list (ring-remove fifo) t
        (ring-remove fifo) t
        (ring-remove fifo)))
     ⇒ (0 t one t "two")