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#lang racket
(require advent-of-code
threading)
(define data
(~> (open-aoc-input (find-session) 2021 3 #:cache (string->path "./cache"))
port->lines
(map string->list _)))
;; part 1
(define most-common-bits
(for*/list ([row (in-list (apply map list data))]
[len (in-value (length data))])
(if (> (count (λ (c) (char=? #\1 c)) row) (/ len 2))
#\1
#\0)))
(define (bit-list->number lst)
(~> lst
(apply string _)
(string->number _ 2)))
(define gamma
(bit-list->number most-common-bits))
(define epsilon
(~> most-common-bits
(map (λ (c) (if (char=? c #\1) #\0 #\1)) _)
bit-list->number))
(* gamma epsilon)
;; part 2
(define (rating-search data comparison)
(for/fold ([candidates data]
#:result (bit-list->number (first candidates)))
([bit (in-list most-common-bits)]
[bit-index (in-range 0 (length most-common-bits))])
#:break (= 1 (length candidates))
(define keep-bit
(~> candidates
(apply map list _)
(list-ref _ bit-index)
(count (λ (c) (char=? #\1 c)) _)
(comparison _ (/ (length candidates) 2))
(if _ #\1 #\0)))
(filter (λ (row) (char=? keep-bit (list-ref row bit-index))) candidates)))
(define oxygen-rating (rating-search data >=))
(define scrubber-rating (rating-search data <))
(* oxygen-rating scrubber-rating)
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