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#lang racket
(require advent-of-code
threading
(only-in relation ->string ->list ->number)
(only-in algorithms chunks-of))
(define data (~> (fetch-aoc-input (find-session) 2022 5) (string-split "\n")))
(struct instruction (n from to))
(define crates-list
(~>> data
(take _ 8)
(map (λ~>> ->list))
(apply map list _)
rest
(chunks-of _ 4)
(map (λ~> first ->string string-trim ->list) _)))
(define crates
(for/hash ([c (in-list crates-list)] [i (in-naturals 1)])
(values i c)))
(define (parse-instruction str)
(match str
[(regexp #px"move (\\d+) from (\\d) to (\\d)" (list _ n from to))
(instruction (->number n) (->number from) (->number to))]))
(define instructions (~>> data (drop _ 10) (map parse-instruction)))
(define (find-crate-message cs [reverse-function reverse])
(for/fold ([current-crates cs]
#:result (~>> (hash-values current-crates) (map first) (apply string)))
([i (in-list instructions)])
(match-define (instruction n from to) i)
(define taken (~> (hash-ref current-crates from) (take _ n) reverse-function))
(~> current-crates
(hash-update _ from (λ (v) (drop v n)))
(hash-update _ to (λ (v) (append taken v))))))
;; part 1
(find-crate-message crates)
;; part 2
(find-crate-message crates identity)
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