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47 lines
1.2 KiB
Racket
47 lines
1.2 KiB
Racket
#lang br
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(require racket/file racket/dict rackunit)
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(define (bug? x) (char=? x #\#))
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(define (parse-grid lns)
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(for*/list ([(ln row) (in-indexed lns)]
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[(c col) (in-indexed ln)])
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(cons (make-rectangular col row) c)))
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(define (adjacent-bugs g tile)
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(for/sum ([delta '(+i -i 1 -1)]
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#:when (bug? (dict-ref g (+ tile delta) #\.)))
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1))
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(define (step g)
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(for*/list ([row (in-range 5)]
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[col (in-range 5)])
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(define tile (make-rectangular col row))
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(cons tile
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(match (cons (dict-ref g tile) (adjacent-bugs g tile))
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[(cons #\# 1) #\#]
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[(cons #\# _) #\.]
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[(cons #\. (or 1 2)) #\#]
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[(cons other _) other]))))
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(define (biodiversity-rating g)
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(for/sum ([(rec idx) (in-indexed g)]
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#:when (bug? (cdr rec)))
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(expt 2 idx)))
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(define (solve lns)
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(let loop ([gs (list (parse-grid lns))])
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(define next-g (step (car gs)))
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(cond
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[(member next-g gs) (biodiversity-rating next-g)]
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[else (loop (cons next-g gs))])))
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(check-eq?
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(solve (string-split "....#
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#..#.
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#..##
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..#..
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#....")) 2129920)
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;; 1
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(check-eq? (solve (file->lines "24.rktd")) 18407158) |