🎄⌨️ Advent of Code 2018

                  [else (cons ch lst)])))))

;; Is point p inside grid g? Film at 11
(define (inside-grid? g p)
  (match-define (posn px py) p)
  (and (>= px 0)
       (>= py 0)
       (< px (grid-rows g))
       (< py (grid-cols g))))
       (< px (grid-cols g))
       (< py (grid-rows g))))

;; Get a list of a positions neighboring points, ensuring none are out of bounds
(define (neighbor-coords g pos)
  (match-define (posn x y) pos)
  (filter (curry inside-grid? g)
          (map (lambda (lst) (apply posn lst))
               `((,(- x 1) ,y)
                 (,x ,(+ y 1))
                 (,(+ x 1) ,y)
                 (,x ,(- y 1))))))
          (list (posn (- x 1) y)
                (posn x (+ y 1))
                (posn (+ x 1) y)
                (posn x (- y 1)))))

;; Get all the EMPTY neighboring points of a given spot OR list of spots.
;; If a (listof posn?) is passed, ensures the returned list does not include
;; any of the original positions.
(define (free-neighbors-at world pos)
  (cond [(posn? pos)
         (~> (neighbor-coords world pos)