day3.el

(defun create-grid ()
  (let ((grid (make-vector aoc-size 0)))
    (dotimes (i aoc-size)
      (setf (aref grid i) (make-vector aoc-size 0)))
    (setf (aref (aref grid center)
                center) 1)
    grid))

(defun get-cell (x y)
  (cond
   ((or (< x 0)
        (< y 0)) 0)
   ((or (> x (1- aoc-size))
        (> y (1- aoc-size))) 0)
   (t (aref (aref aoc-grid y) x))))

(defun sum-neighbors ()
  (let ((x (car aoc-position))
        (y (cdr aoc-position)))
    (+ (get-cell (- x 1) (- y 1))
       (get-cell (- x 1)    y   )
       (get-cell (- x 1) (+ y 1))
       (get-cell    x    (- y 1))
       (get-cell    x       y   )
       (get-cell    x    (+ y 1))
       (get-cell (+ x 1) (- y 1))
       (get-cell (+ x 1)    y   )
       (get-cell (+ x 1) (+ y 1)))))

(defun spiral-step ()
    (setf aoc-position (cons (+ (car aoc-position) (car aoc-direction))
                             (+ (cdr aoc-position) (cdr aoc-direction))))
    (when (and (not found-answer)
               (< day3-input (sum-neighbors)))
      (print (sum-neighbors))
      (setf found-answer t))
    (setf (aref (aref aoc-grid (cdr aoc-position)) (car aoc-position)) (sum-neighbors))
    (cond
     ;; Hit the right limit -> move up
     ((> (car aoc-position) maxright)
      (incf maxright)
      (setf aoc-direction '(0 . -1)))
     ;; Hit top limit, move left
     ((< (cdr aoc-position) maxtop)
      (decf maxtop)
      (setf aoc-direction '(-1 . 0)))
     ;; Hit left limit, move down
     ((< (car aoc-position) maxleft)
      (decf maxleft)
      (setf aoc-direction '(0 . 1)))
     ;; Hit bottom limit, move right
     ((> (cdr aoc-position) maxbot)
      (incf maxbot)
      (setf aoc-direction '(1 . 0)))))

(defun aoc-day3-2 ()
  (setf day3-input 312051)
  (setf aoc-size 11)
  (setf center (/ aoc-size 2))
  (setf aoc-direction '(1 . 0))
  (setf aoc-position (cons center center))
  (setf maxleft center)
  (setf maxright center)
  (setf maxtop center)
  (setf maxbot center)
  (setf aoc-grid (create-grid))
  (setf found-answer nil)
  (dotimes (i (1- (* aoc-size aoc-size)))
    (spiral-step))
  aoc-grid)