34 lines
1.3 KiB
Racket
34 lines
1.3 KiB
Racket
#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* ([`((,template*) ,rules) (problem-input-grouped-lines 14)]
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[(and template `(,left-end ,@_ ,right-end)) (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 ([`(,_ ,left ,right ,insert) (regexp-match #px"(\\w)(\\w) -> (\\w)" rule)])
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(values (cons (string-ref left 0)
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(string-ref right 0))
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(string-ref insert 0))))
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left-end right-end)))
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(define (step template)
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(for/fold ([polymer (hash)])
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([(k v) template])
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(define insert (hash-ref rules k))
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(~> polymer
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(hash-update (cons (car k) insert) #{+ % v} 0)
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(hash-update (cons insert (cdr k)) #{+ % v} 0))))
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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 (#{/ (- (maximum %) (minimum %)) 2} (hash-values counts)))
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([(k v) ((iterate step n) template)])
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(~> counts
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(hash-update (car k) #{+ % v} 0)
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(hash-update (cdr k) #{+ % v} 0))))
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(show-solution (count 10) (count 40)) |