38 lines
1.6 KiB
Racket
38 lines
1.6 KiB
Racket
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#lang curly-fn racket
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(require "../lib.rkt")
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(define-values (template rules left-end right-end)
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(match-let ([(list `(,template*) rules) (problem-input-grouped-lines 14)])
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(define template (map char->symbol (string->list template*)))
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(values (for/hash ([left template]
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[right (rest template)])
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(values (cons left right) 1))
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(for/hash ([rule rules])
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(match-let ([(list _ left right insert) (regexp-match #px"(\\w)(\\w) -> (\\w)" rule)])
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(values (cons (string->symbol left)
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(string->symbol right))
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(string->symbol insert))))
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(first template) (last template))))
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(define (step template)
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(for/fold ([polymer (hash)])
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([pair (hash-keys template)])
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(match-let* ([(cons left right) pair]
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[insert (hash-ref rules pair)]
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[count (hash-ref template pair)]
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[polymer (hash-update polymer (cons left insert) #{+ % count} 0)]
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[polymer (hash-update polymer (cons insert right) #{+ % count} 0)])
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polymer)))
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(define (count n)
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(for/fold ([counts (hash right-end 1 left-end 1)]
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#:result (let ([counts (hash-values counts)])
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(/ (- (maximum counts) (minimum counts)) 2)))
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([(k v) ((iterate step n) template)])
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(match-let* ([(cons left right) k]
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[counts (hash-update counts left #{+ % v} 0)]
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[counts (hash-update counts right #{+ % v} 0)])
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counts)))
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(show-solution (count 10) (count 40))
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