Advent of code, day 17

(defun new-circ (value)
  (let ((head (cons value nil)))
    (setf (cdr head) head)
    head))

(defun ninsert-after-and-move (list i)
  (let ((new-cons (cons i (cdr list))))
    (setf (cdr list) new-cons)
    new-cons))

(defconstant +N-ITERATIONS-A+ 2017)
(defconstant +N-ITERATIONS-B+ 50000000)
(defconstant +N-STEPS+ 345)

(defun solve (n)
  (let ((list (new-circ 0)))
    (loop for i from 1 to n do
         (loop for j from 1 to +N-STEPS+ do
              (setf list (cdr list)))
         (setf list (ninsert-after-and-move list i))
         (if (= (mod i 200000) 0) (format t "~a~%" i)))
    list))

(defun cons-with-elem (list elem)
  (loop
     (if (= (car list) elem) (return list)
         (setf list (cdr list)))))

(format t "~a~%" (cadr (solve +N-ITERATIONS-A+)))
(format t "~a~%" (cadr (cons-with-elem (solve +N-ITERATIONS-B+) 0)))