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### 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 âˆ’1 corresponds to the oldest element, âˆ’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")

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