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36 lines
1.2 KiB
Racket
36 lines
1.2 KiB
Racket
3 years ago
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#lang br
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(require racket/file rackunit)
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(define depths
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(for*/hasheqv ([(line lidx) (in-indexed (file->lines "09.rktd"))]
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[(depth cidx) (in-indexed line)])
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(values (make-rectangular cidx lidx) (string->number (string depth)))))
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(define (neighbors loc)
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(for*/list ([delta '(1 -1)]
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[ifactor '(1 +i)])
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(+ loc (* delta ifactor))))
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(define (low? loc)
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(< (hash-ref depths loc)
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(apply min (filter-map (λ (li) (hash-ref depths li #f)) (neighbors loc)))))
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(define low-locs (for/list ([(loc depth) (in-hash depths)]
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#:when (low? loc))
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loc))
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(check-equal? (apply + (map (λ (lp) (add1 (hash-ref depths lp))) low-locs)) 539)
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(define (higher-adjacent loc)
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(for/list ([neighbor (neighbors loc)]
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#:when (and
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(hash-has-key? depths neighbor)
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(not (= (hash-ref depths neighbor) 9))
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(< (hash-ref depths loc) (hash-ref depths neighbor))))
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neighbor))
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(define (basin-at loc)
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(remove-duplicates (cons loc (append-map basin-at (higher-adjacent loc)))))
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(define basin-sizes (map length (map basin-at low-locs)))
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(check-equal? (apply * (take (sort basin-sizes >) 3)) 736920)
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