43 lines
1.2 KiB
Racket
43 lines
1.2 KiB
Racket
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#lang curly-fn racket
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(require "scripts/aoc.rkt")
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(require math/number-theory)
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;; solution for day 13
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(define (part1 input)
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(match-define (cons time buses) input)
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(define (find-bus x)
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(for/first ([bus (in-list buses)] #:unless (equal? 'x bus) #:when (zero? (modulo x bus)))
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bus))
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(for*/first ([x (in-range time +inf.0)] [bus (in-value (find-bus x))] #:when bus)
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(* bus (- x time))))
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(define (part2 input)
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(match-define (cons _ buses) input)
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(define eqns
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(for/fold ([eqns '()]) ([bus (in-list buses)] [i (in-naturals)] #:unless (equal? bus 'x))
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(cons (cons (- bus i) bus) eqns)))
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(solve-chinese (map car eqns) (map cdr eqns)))
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;; parse input file
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(define (parse fname)
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(define input (file->lines fname))
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(define time (string->number (first input)))
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(define buses (for/list ([x (in-list (string-split (second input) ","))])
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(match x
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["x" 'x]
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[_ (string->number x)])))
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(cons time buses))
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(module+ test
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(require rackunit)
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;; tests here
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(displayln "no tests :("))
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(module+ main
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(define input (parse "inputs/13"))
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(answer 13 1 (time (part1 input)))
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(answer 13 2 (time (part2 input)))
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(displayln "meow"))
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