Day 19

(defparameter *stepdist* 367)
(defparameter *buf* '(0))
(rplacd *buf* *buf*)

(defun insert! (l s n)
  (let ((c (nthcdr s l)))
    (rplacd c (cons n (cdr c)))
    (cdr c)))

(defun fill! (l s lim)
  (loop for i from 1 to lim
     do (setf l (insert! l s i)))
  l)

(defun find-after (l n)
  (if (eq (car l) n)
      (cadr l)
      (find-after (cdr l) n)))
(format t "~a~%" (cadr (fill! *buf* *stepdist* 2017)))
(setf *buf* '(0))
(rplacd *buf* *buf*)
(format t "~a~%" (find-after (fill! *buf* *stepdist* 50000000) 0))