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#lang racket
(define (mode xs)
(define ht (make-hash))
(define max-key '0)
(for ([x xs])
(define new-val (+ 1 (hash-ref ht x 0)))
(hash-set! ht x new-val)
(if (> new-val (hash-ref ht max-key 0)) (set! max-key x) 3))
max-key)
(define (transpose xss)
(apply map list xss))
(define (flip x)
(cond [(string=? x "0") "1"]
[else "0"]))
(define input (open-input-file "input"))
(define bit-positions
(transpose (map (lambda (xs)
(map (curry make-string 1) (string->list xs)))
(port->lines input))))
(define gamma-str (foldr string-append "" (map mode bit-positions)))
(define epsilon-str (foldr string-append "" (map (compose flip mode) bit-positions)))
(define gamma (string->number gamma-str 2))
(define epsilon (string->number epsilon-str 2))
(* gamma epsilon)
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