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#lang racket
(require advent-of-code
threading)
(define input
(~> (fetch-aoc-input (find-session) 2023 15 #:cache #true) string-trim (string-split ",")))
(define (hash-algorithm str)
(for/fold ([acc 0]) ([c (in-string str)])
(~> c char->integer (+ acc) (* 17) (modulo _ 256))))
;; part 1
(for/sum ([code (in-list input)]) (hash-algorithm code))
;; part 2
(define (remove-lens boxes label)
(hash-update boxes
(hash-algorithm label)
(λ (lens-set) (remove label lens-set (λ (rem l) (equal? rem (car l)))))
'()))
(define (insert-lens boxes label focal)
(define new-lens (cons label focal))
(hash-update boxes
(hash-algorithm label)
(λ (lens-set)
(if (assoc label lens-set)
(map (λ (pair) (if (equal? (car pair) label) new-lens pair)) lens-set)
(append lens-set (list new-lens))))
(list new-lens)))
(define (focusing-power boxes)
(for*/sum ([(box-number lenses) (in-hash boxes)] [(lens order) (in-indexed lenses)])
(* (add1 box-number) (add1 order) (cdr lens))))
(for/fold ([boxes (hash)] #:result (focusing-power boxes)) ([code (in-list input)])
(match code
[(regexp #rx"(.*)=(.*)" (list _ label (app string->number focal)))
(insert-lens boxes label focal)]
[(regexp #rx"(.*)-" (list _ label)) (remove-lens boxes label)]))
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