#     Function LIST-LENGTH

Syntax:

list-length list => length

Arguments and Values:

list---a proper list or a circular list.

length---a non-negative integer, or nil.

Description:

Returns the length of list if list is a proper list. Returns nil if list is a circular list.

Examples:

``` (list-length '(a b c d)) =>  4
(list-length '(a (b c) d)) =>  3
(list-length '()) =>  0
(list-length nil) =>  0
(defun circular-list (&rest elements)
(let ((cycle (copy-list elements)))
(nconc cycle cycle)))
(list-length (circular-list 'a 'b)) =>  NIL
(list-length (circular-list 'a)) =>  NIL
(list-length (circular-list)) =>  0
```

Side Effects: None.

Affected By: None.

Exceptional Situations:

Should signal an error of type type-error if list is not a proper list or a circular list.

Notes:

list-length could be implemented as follows:

``` (defun list-length (x)
(do ((n 0 (+ n 2))           ;Counter.
(fast x (cddr fast))    ;Fast pointer: leaps by 2.
(slow x (cdr slow)))    ;Slow pointer: leaps by 1.
(nil)
;; If fast pointer hits the end, return the count.
(when (endp fast) (return n))
(when (endp (cdr fast)) (return (+ n 1)))
;; If fast pointer eventually equals slow pointer,
;;  then we must be stuck in a circular list.
;; (A deeper property is the converse: if we are
;;  stuck in a circular list, then eventually the
;;  fast pointer will equal the slow pointer.
;;  That fact justifies this implementation.)
(when (and (eq fast slow) (> n 0)) (return nil))))

```      