73 lines
1.9 KiB
Racket
73 lines
1.9 KiB
Racket
#lang racket
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(require racket/sequence)
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(module+ test
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(require rackunit))
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(define (pattern n i)
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(define index (remainder (add1 i) (* 4 n)))
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(cond
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[(< index n) 0]
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[(< index (* 2 n)) 1]
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[(< index (* 3 n)) 0]
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[(< index (* 4 n)) -1]))
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(define (last-digit n)
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(abs (remainder n 10)))
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(define (phase signal)
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(for/list ([i (in-range (length signal))])
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(last-digit (for/sum ([d (in-list signal)]
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[j (in-naturals)]
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#:when (>= j i))
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(* d (pattern (add1 i) j))))))
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(define (number->digits n)
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(define (aux n)
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(define-values (q r) (quotient/remainder n 10))
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(if (> q 0)
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(cons r (aux q))
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(list r)))
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(reverse (aux n)))
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(define (digits->number lst)
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(for/sum ([d (in-list (reverse lst))]
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[i (in-naturals)])
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(* d (expt 10 i))))
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(define (apply-n-times f n x)
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(for/fold ([val x]) ([i (in-range n)])
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(f val)))
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(define (part1 n)
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(digits->number (take (apply-n-times phase 100 n) 8)))
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(module+ test
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(check-eq? (part1 (number->digits 80871224585914546619083218645595)) 24176176)
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(check-eq? (part1 (number->digits 19617804207202209144916044189917)) 73745418)
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(check-eq? (part1 (number->digits 69317163492948606335995924319873)) 52432133))
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(module+ main
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(define input (number->digits
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(string->number
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(string-trim
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(port->string (open-input-file "input") #:close? #t)))))
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(displayln (part1 input)))
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(define (phase2 signal)
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(define-values (s res)
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(for/fold ([s (apply + signal)]
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[res '()])
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([d (in-list signal)])
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(values (- s d) (cons (last-digit s) res))))
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(reverse res))
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(define (part2 input)
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(define offset (digits->number (take input 7)))
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(define n (drop input offset))
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(digits->number (take (apply-n-times phase2 100 n) 8)))
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(module+ main
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(define input2 (flatten (make-list 10000 input)))
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(displayln (part2 input2)))
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