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59 lines
2.0 KiB
Racket
59 lines
2.0 KiB
Racket
#lang debug br
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(require racket/file)
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#|
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This puzzle seemed annoying at first but turned out to be educational.
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An epic difference between the naive solution (= make a list and iterate from the start)
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and using a double-linked list, which minimizes traversal.
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|#
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(struct dll (val prev next) #:mutable)
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(define (move-by dll n)
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(define iterator
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(match n
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[(? positive?) dll-next]
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[(? negative?) dll-prev]
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[_ values]))
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(for/fold ([dll dll])
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([i (in-range (abs n))])
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(iterator dll)))
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(define (remove-marble! marble)
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(set-dll-next! (dll-prev marble) (dll-next marble))
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(set-dll-prev! (dll-next marble) (dll-prev marble)))
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(define (find-winner player-count max-marbles)
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(define scores (make-hasheqv))
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(define first-marble (dll 0 #f #f))
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(set-dll-prev! first-marble first-marble)
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(set-dll-next! first-marble first-marble)
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(for/fold ([current-marble first-marble]
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#:result (cdr (argmax cdr (hash->list scores))))
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([marble (in-range 1 max-marbles)])
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(cond
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[(zero? (modulo marble 23))
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(define marble-to-remove (move-by current-marble -7))
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(remove-marble! marble-to-remove)
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(define player (modulo marble player-count))
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(hash-update! scores player (λ (sc) (+ (dll-val marble-to-remove) marble sc)) 0)
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(dll-next marble-to-remove)]
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[else
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(define left-marble (move-by current-marble 1))
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(define right-marble (dll-next left-marble))
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(define new-marble (dll marble left-marble right-marble))
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(set-dll-next! left-marble new-marble)
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(set-dll-prev! right-marble new-marble)
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new-marble])))
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(match-define (list player-count max-marbles)
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(map string->number (regexp-match* #px"\\d+" (file->string "09.txt"))))
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(define (★) (find-winner player-count max-marbles))
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(define (★★) (find-winner player-count (* 100 max-marbles)))
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(module+ test
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(require rackunit)
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(check-equal? (time (★)) 437654)
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(check-equal? (time (★★)) 3689913905)) |