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#lang racket
(require advent-of-code
threading
algorithms)
(define data (fetch-aoc-input (find-session) 2022 14 #:cache #true))
(define (trace-line-between-points p1 p2)
(match* (p1 p2)
[((list x y1) (list x y2)) (map (λ (y) (cons x y)) (inclusive-range (min y1 y2) (max y1 y2)))]
[((list x1 y) (list x2 y)) (map (λ (x) (cons x y)) (inclusive-range (min x1 x2) (max x1 x2)))]))
(define (find-points-in-structure str)
(define endpoints
(for/list ([coord-pair (in-list (string-split str " -> "))])
(for/list ([coord (in-list (string-split coord-pair ","))])
(string->number coord))))
(~>> endpoints (adjacent-map trace-line-between-points) (apply append) (list->set)))
(define blocked-locations
(~> data (string-split "\n") (map find-points-in-structure _) (apply set-union _)))
(define max-vertical-distance (~>> blocked-locations (set->list) (argmax cdr) cdr add1))
(define (open? pts p)
(not (set-member? pts p)))
;; part 1
(define (trace-grain pts path #:at-limit do-at-limit)
(match-define (list* (and p (cons x y)) _) path)
(match-define (list dest-1 dest-2 dest-3) (map (λ (d) (cons (+ x d) (add1 y))) '(0 -1 1)))
(cond
[(>= y max-vertical-distance) (values (do-at-limit pts p) path)]
[(open? pts dest-1) (trace-grain pts (cons dest-1 path) #:at-limit do-at-limit)]
[(open? pts dest-2) (trace-grain pts (cons dest-2 path) #:at-limit do-at-limit)]
[(open? pts dest-3) (trace-grain pts (cons dest-3 path) #:at-limit do-at-limit)]
[else (values (set-add pts (car path)) path)]))
(time (for/fold ([pts blocked-locations] [path (list (cons 500 0))] [grains 0] #:result grains)
([_ (in-naturals 1)])
(define-values (pts* path*) (trace-grain pts path #:at-limit (const 'break)))
#:break (equal? pts* 'break)
(values pts* (cdr path*) (add1 grains))))
;; part 2
(time (for/fold ([pts blocked-locations] [path (list (cons 500 0))] [grains 0] #:result grains)
([_ (in-naturals 1)])
#:break (not (open? pts (cons 500 0)))
(define-values (pts* path*) (trace-grain pts path #:at-limit set-add))
(values pts* (cdr path*) (add1 grains))))
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