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74 lines
1.9 KiB
Racket
74 lines
1.9 KiB
Racket
3 years ago
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#lang racket
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; utility functions
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(define (char->number char)
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(if (equal? char #\0) 0 1))
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(define (number->char num)
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(if (= num 0) #\0 #\1))
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(define (string->numbers str)
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(map char->number (string->list str)))
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(define (transpose xss)
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(apply map list xss))
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; IO
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(define lines
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(file->lines "day3.txt"))
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(define chars
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(map string->numbers lines))
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; part 1
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(define (count-ones-in-list xss)
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(foldl + 0 (filter positive? xss)))
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(define counts
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(map count-ones-in-list (transpose chars)))
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(define gamma-bin-list
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(map (λ (count)
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(if (> count (/ (length chars) 2)) 1 0))
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counts))
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(define (flip-bits num)
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(map (λ (n) (modulo (+ 1 n) 2)) num))
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(define epsilon-bin-list (flip-bits gamma-bin-list))
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(define (binary-list->decimal num)
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(string->number (list->string (map number->char num)) 2))
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(define gamma-dec (binary-list->decimal gamma-bin-list))
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(define epsilon-dec (binary-list->decimal epsilon-bin-list))
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(define part1 (* gamma-dec epsilon-dec))
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; part 2
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; sounds like a fold to me
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(define (most-common-digit column xs type)
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(define col-values (map (λ (x) (list-ref x column)) xs))
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(define ones (count-ones-in-list col-values))
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(define op (if (equal? type 'o2) >= <)) ; 'o2 uses >=, 'co2 uses <
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(if (op ones (/ (length xs) 2)) 1 0))
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(define (get-rating xs type)
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(flatten
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(foldl (λ (column remaining)
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(define most-common (most-common-digit column remaining type))
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(if (= (length remaining) 1) ; if there's only one left
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remaining ; return it
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(filter (λ (ys)
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(= (list-ref ys column) most-common))
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remaining)))
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xs
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(range (length (first xs))))))
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(define o2-rating (binary-list->decimal (get-rating chars 'o2)))
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(define co2-rating (binary-list->decimal (get-rating chars 'co2)))
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(define part2 (* o2-rating co2-rating))
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