(defun draw-line (buffer x1 y1 x2 y2 pixel)
  (declare (type rgb-pixel-buffer buffer))
  (declare (type integer x1 y1 x2 y2))
  (declare (type rgb-pixel pixel))
  (let* ((dist-x (abs (- x1 x2)))
         (dist-y (abs (- y1 y2)))
         (steep (> dist-y dist-x)))
    (when steep
      (psetf x1 y1 y1 x1
             x2 y2 y2 x2))
    (when (> x1 x2)
      (psetf x1 x2 x2 x1
             y1 y2 y2 y1))
    (let* ((delta-x (- x2 x1))
           (delta-y (abs (- y1 y2)))
           (error (floor delta-x 2))
           (y-step (if (< y1 y2) 1 -1))
           (y y1))
      (loop
        :for x :upfrom x1 :to x2
        :do (if steep
                (setf (rgb-pixel buffer x y) pixel)
                (setf (rgb-pixel buffer y x) pixel))
            (setf error (- error delta-y))
            (when (< error 0)
              (incf y y-step)
              (incf error delta-x))))
    buffer))