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| ANSI Common Lisp 14 Conses 14.2 Dictionary of Conses
14.2.16 list-length | Function
  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
  Exceptional Situations: Should signal an error of type type-error if list is not a proper list or a circular list. See Also: length 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))))
Allegro CL Implementation Details: None.

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