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#lang racket
(require advent-of-code
threading)
(struct posn (x y) #:transparent)
(define input
(~> (fetch-aoc-input (find-session) 2023 11 #:cache #true)
(string-split "\n")
(map string->list _)))
(define (get-empty-ranks grid)
(for/list ([(rank n) (in-indexed grid)] #:when (equal? '(#\.) (remove-duplicates rank)))
n))
(define (count-prior-empty-ranks rank empty-ranks)
(~> empty-ranks
(takef (curryr < rank))
length))
(define empty-rows (get-empty-ranks input))
(define empty-columns (get-empty-ranks (apply map list input))) ;; transpose
(define (sum-of-star-distances in expand-by)
(define stars
(for*/list ([(row x) (in-indexed in)]
[(col y) (in-indexed row)]
#:when (equal? col #\#))
(posn (+ x (* (sub1 expand-by) (count-prior-empty-ranks x empty-rows)))
(+ y (* (sub1 expand-by) (count-prior-empty-ranks y empty-columns))))))
(for/sum ([star-pair (in-combinations stars 2)])
(match-define (list (posn x1 y1) (posn x2 y2)) star-pair)
(+ (abs (- x1 x2)) (abs (- y1 y2)))))
;; part 1
(sum-of-star-distances input 2)
;; part 2
(sum-of-star-distances input 1000000)
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