(in-package #:cl-user)

(defpackage #:group
  (:use #:cl)
  (:export :group :test))

(in-package #:group)


(defun group (x y)
  (if (numberp y)
      (if (numberp x)
          (error "I need at least one list argument!")
          (group y x))
      (let ((n      x)
            (source y))
        (check-type n (integer 1))
        (labels ((inner (down up)
                   (if (null down)
                       (nreverse up)
                       (inner (nthcdr n down)
                              (push (subseq down 0 n) up)))))
          (let ((len (length source)))
            (if (= 0 (mod len n))
                (inner source nil)
                (error "Wrong length ~S of source list for groups of size ~S"
                       len n)))))))

(defun test ()
  (let ((res (list (equal (group 2 '(1 2)) '((1 2)))
                   (equal (group '(1 2) 2) '((1 2)))
                   (GROUP '(1 2) (+ 1 1)))))
    (if (every #'identity res)
        (format t "Group does pass tests. Results: ~S" res)
        (error "Group does not pass tests. Results: ~S" res))))