common-features.lisp

;;; Compare descriptions of two objects and report the number of common features. Descriptions are lists of features
;;; separated by the symbol -VS- (e.g. (large red shiny cube -vs- small shiny red four-sided pyramid)).

(defun right-side (descriptions)
  "The features of the object to the right of the separator."
  (cdr (member '-vs- descriptions)))

(defun left-side (descriptions)
  "The features of the object to the left of the separator."
  (right-side (reverse descriptions)))

(defun count-common (descriptions)
  "The number of features the two objects have in common."
  (length (intersection (left-side descriptions)
                        (right-side descriptions))))

(defun compare (descriptions)
  "Report the number of features that the two objects have in common."
  `(,(count-common descriptions) common features))

From Common Lisp: A Gentle Introduction to Symbolic Computing - the mini keyboard exercise 6.26.