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[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))))
;; 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))))))
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[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-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)
(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)
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