60 lines
1.6 KiB
Racket
60 lines
1.6 KiB
Racket
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#lang racket
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(require "../lib.rkt")
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(define input
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(problem-input 10))
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(define width (string-length (car input)))
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(define height (length input))
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(define (coprime? ij)
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(= 1 (gcd (first ij)
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(second ij))))
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(define (offsets x y)
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(define is
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(range (negate x) (- width x)))
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(define js
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(range (negate y) (- height y)))
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(filter coprime? (cartesian-product is js)))
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(define (asteroid? xy)
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(define row (list-ref input (second xy)))
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(eq? #\# (string-ref row (first xy))))
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(define (asteroid-offset? x y ij)
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(define i (first ij))
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(define j (second ij))
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(define max-multiple
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(let ([max-x
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(cond
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[(zero? i) width]
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[(positive? i) (truncate (/ (- (sub1 width) x) i))]
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[else (abs (truncate (/ x i)))])]
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[max-y
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(cond
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[(zero? j) height]
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[(positive? j) (truncate (/ (- (sub1 height) y) j))]
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[else (abs (truncate (/ y j)))])])
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(min max-x max-y)))
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(ormap (λ (m) (asteroid? `(,(+ x (* m i))
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,(+ y (* m j)))))
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(range 1 (add1 max-multiple))))
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(define (asteroids xy)
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(define x (first xy))
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(define y (second xy))
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(count (curry asteroid-offset? x y) (offsets x y)))
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(define-values (part1 location)
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(let* ([cols (range width)]
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[rows (range height)]
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[locations (filter asteroid? (cartesian-product cols rows))]
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[asteroid-counts (map asteroids locations)]
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[maximum (apply max asteroid-counts)]
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[index (index-of asteroid-counts maximum)]
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[location (list-ref locations index)])
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(values maximum location)))
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(show-solution part1 #f)
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