pop-n

(defmacro pop-n (place n &environment env)
  (multiple-value-bind (vars vals stores setter getter)
      (get-setf-expansion place env)
    (let ((old-head (gensym "OLDHEAD-"))
          (new-head (gensym "NEWHEAD-"))
          (num (gensym "N-")))
      `(let ,(mapcar #'list vars vals)
         (let* ((,old-head ,getter)
                (,num ,n)
                (,new-head (nthcdr ,num ,old-head))
                (,(first stores) ,new-head)
                ,@(rest stores))
           ,setter
           (when ,new-head
              (setf (cdr (nthcdr (1- ,num) ,old-head)) nil))
           ,old-head
           )))))


;; (defun test1 (list)
;;   (loop while list
;;         for sublist = (pop-n list 4)
;;         collect sublist))

;; (let ((inputs
;;        (list (list 1 2 3 4)
;;              (list 1 2 3 4 5 6 7 8)
;;              (list 1 2 3 4 5 6 7)
;;              (list 1)
;;              (list))))
;;   (dolist (input inputs)
;;     (print (test1 input))))

;; (defstruct container
;;   (list nil :type list))

;; (defun test2 (c)
;;   (loop while (container-list c)
;;         for sublist = (pop-n (container-list c) 4)
;;         collect sublist))

;; (let ((inputs
;;        (list (list 1 2 3 4)
;;              (list 1 2 3 4 5 6 7 8)
;;              (list 1 2 3 4 5 6 7)
;;              (list 1)
;;              (list))))
;;   (dolist (input inputs)
;;     (print (test2 (make-container :list input)))))