split-list-if.lisp

;; thanks, beach!

(defun split-list-if (predicate list)
  "Destructively splits the provided list into two lists: one contains the
elements of the original list that satisfy the predicate, the other contains
elements that do not satisfy the predicate. The order of the original elements
is preserved."
  (loop with result1 = '()
        with result2 = '()
        with rest = list
        until (null rest)
        do (let ((temp rest))
             (pop rest)
             (if (funcall predicate (car temp))
                 (setf (cdr temp) result1
                       result1 temp)
                 (setf (cdr temp) result2
                       result2 temp)))
        finally (return (values (nreverse result1)
                                (nreverse result2)))))