Group by in Common Lisp

group.lisp

(defun group-by (fn coll &key groupfn keyname valuename)
  "Groups the collection by the results of calling fn on each element.
   The results of calling groupfn on the elements are grouped together.
   Returned is a property list of the keyname to valuename properties."
  (labels ((get-item (item)
             (if groupfn
                 (funcall groupfn item)
                 item))

           (find-plist (grouped keyvalue)
             (first
              (remove-if-not (lambda (plist)
                               (equal (getf plist keyname) keyvalue))
                             grouped)))

           (group (grouped item)
             (let ((curr (find-plist grouped (funcall fn item))))
               (if curr
                   (progn
                     (setf (getf curr valuename) 
                           (cons (get-item item) (getf curr valuename)))
                     grouped)

                   (cons (list keyname (funcall fn item)
                               valuename (list (get-item item)))
                         grouped)))))

    (reduce #'group coll :initial-value '())))