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#lang racket
(require advent-of-code
(only-in relation ->number)
threading)
(define (process-ids str)
(~> str (string-split ",") (filter-map (λ (s) (string->number s 10 'number-or-false)) _)))
(match-define (regexp #px"(\\d+)\n(.+)" (list _ (app ->number timestamp) raw-bus-ids))
(fetch-aoc-input (find-session) 2020 13))
(define bus-ids (process-ids raw-bus-ids))
;; part 1
(for/first ([minute (in-naturals timestamp)]
#:do [(define departing-bus
(for/first ([b bus-ids] #:when (= 0 (remainder minute b)))
b))]
#:when departing-bus)
(* departing-bus (- minute timestamp)))
;; part 2
(for/fold ([step 1] [current-timestamp 1] #:result current-timestamp)
([b* (in-list (string-split (string-trim raw-bus-ids) ","))]
[offset (in-naturals)]
#:unless (equal? b* "x")
#:do [(define bus (->number b*))])
(values
(* step bus)
(for/first ([n (in-range current-timestamp +inf.0 step)] #:when (= 0 (remainder (+ n offset) bus)))
n)))
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