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18 lines
705 B
Racket
18 lines
705 B
Racket
4 years ago
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#lang br
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(require racket/file rackunit)
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(define (solve str max-turns)
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(define nums (map string->number (string-split str ",")))
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(define previously-seen (make-hasheq))
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(for ([(num turn) (in-indexed (drop-right nums 1))])
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(hash-set! previously-seen num (add1 turn)))
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(for/fold ([last-seen (last nums)])
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([turn (in-range (add1 (length nums)) (add1 max-turns))])
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(begin0
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(match (hash-ref previously-seen last-seen #false)
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[#false 0]
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[where-previously-seen (- (sub1 turn) where-previously-seen)])
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(hash-set! previously-seen last-seen (sub1 turn)))))
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(check-equal? (solve "11,0,1,10,5,19" 2020) 870)
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(check-equal? (solve "11,0,1,10,5,19" 30000000) 9136)
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