31 lines
853 B
Racket
31 lines
853 B
Racket
#lang curly-fn racket
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(require "scripts/aoc.rkt" math/number-theory)
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;; solution for day 25
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(define *mod* 20201227)
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(define *subject* 7)
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(define (part1 input)
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(match-define (list you door) input)
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(with-modulus *mod*
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(for/fold ([val 1] [you-loops #f] [door-loops #f] #:result (modexpt you door-loops))
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([loops (in-range 0 +inf.0)] #:break (and you-loops door-loops))
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(define new-val (mod* val *subject*))
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(cond
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[(= val you) (values new-val loops door-loops)]
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[(= val door) (values new-val you-loops loops)]
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[else (values new-val you-loops door-loops)]))))
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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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(map string->number input))
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(module+ main
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(define input (parse "inputs/25"))
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(answer 25 1 (time (part1 input)))
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(aoc-finish)
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(displayln "meow"))
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