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#lang racket
(require advent-of-code
threading)
(define ids (port->lines (open-aoc-input (find-session) 2018 2 #:cache #true)))
;; part 1
(define (make-baskets str)
(for/fold ([baskets (hash)]) ([ch (in-string str)])
(hash-update baskets ch add1 0)))
(define (has-count n ht)
(member n (hash-values ht)))
(for/fold ([two 0] [three 0] #:result (* two three)) ([id (in-list ids)])
(define baskets (make-baskets id))
(values (if (has-count 2 baskets) (add1 two) two) (if (has-count 3 baskets) (add1 three) three)))
;; part 2
(define (string-difference str1 str2)
(for/sum ([ch1 (in-string str1)] [ch2 (in-string str2)]) (if (equal? ch1 ch2) 0 1)))
(for*/first ([id1 (in-list ids)] [id2 (in-list ids)] #:when (= 1 (string-difference id1 id2)))
(~>> (for/list ([ch1 (in-string id1)] [ch2 (in-string id2)] #:when (equal? ch1 ch2))
ch1)
(apply string)))
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