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#lang racket
(require advent-of-code
threading
graph)
(struct Point (x y) #:transparent)
(define data
(for/fold ([cells (hash)])
([row (in-lines (open-aoc-input (find-session) 2021 15 #:cache #true))] [x (in-naturals)])
(for/fold ([cells cells]) ([n (in-string row)] [y (in-naturals)])
(hash-set cells (Point x y) (~> n string string->number)))))
(define x-max (~>> data hash-keys (map Point-x) (apply max)))
(define y-max (~>> data hash-keys (map Point-y) (apply max)))
(define (neighbors pt d)
(match-define (Point x y) pt)
(~>> (list (Point (add1 x) y) (Point (sub1 x) y) (Point x (add1 y)) (Point x (sub1 y)))
(filter (curry hash-has-key? d))))
(define (grid-graph d)
(weighted-graph/directed (for*/list ([coord (in-list (hash-keys d))]
[neighbor (in-list (neighbors coord d))]
[weight (in-value (hash-ref d neighbor))])
(list weight coord neighbor))))
;; part 1
(define (find-path-weight d)
(define grid (grid-graph d))
(let-values ([(node-distances _) (dijkstra grid (Point 0 0))])
(define xm (~>> d hash-keys (map Point-x) (apply max)))
(define ym (~>> d hash-keys (map Point-y) (apply max)))
(hash-ref node-distances (Point xm ym))))
(~> data find-path-weight time)
;; part 2
(define nine-cycle (in-cycle (inclusive-range 1 9)))
(define (expand-data data)
(for/fold ([cells (hash)]) ([coord (in-list (hash-keys data))])
(match-define (Point x y) coord)
(for*/fold ([cells cells])
([n (in-range 5)] [m (in-range 5)] [val (in-value (hash-ref data coord))])
(hash-set cells
(Point (+ x (* n (add1 x-max))) (+ y (* m (add1 y-max))))
(sequence-ref nine-cycle (+ val n m -1))))))
(~> data expand-data find-path-weight time)
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