67 lines
1.9 KiB
Racket
67 lines
1.9 KiB
Racket
#lang racket
|
|
|
|
(require "../lib.rkt")
|
|
|
|
(define input
|
|
(problem-input 10))
|
|
|
|
(define width (string-length (car input)))
|
|
(define height (length input))
|
|
|
|
(define (out-of-range? x y)
|
|
(or (< x 0)
|
|
(< y 0)
|
|
(>= x width)
|
|
(>= y height)))
|
|
|
|
(define (coprime? i j)
|
|
(= 1 (gcd i j)))
|
|
|
|
(define (offsets x y)
|
|
(let ([is (range (negate x) (- width x))]
|
|
[js (range (negate y) (- height y))])
|
|
(filter (uncurry coprime?) (cartesian-product is js))))
|
|
|
|
(define (asteroid? x y)
|
|
(define row (list-ref input y))
|
|
(eq? #\# (string-ref row x)))
|
|
|
|
(define (asteroid-offset x y i j)
|
|
(let loop ([m 1])
|
|
(let ([x* (+ x (* i m))]
|
|
[y* (+ y (* j m))])
|
|
(cond
|
|
[(out-of-range? x* y*) #f]
|
|
[(asteroid? x* y*) (list x* y*)]
|
|
[else (loop (add1 m))]))))
|
|
|
|
(define (asteroids x y)
|
|
(filter-map (uncurry (curry asteroid-offset x y)) (offsets x y)))
|
|
|
|
(define-values (part1 location in-view)
|
|
(let* ([cols (range width)]
|
|
[rows (range height)]
|
|
[locations (filter (uncurry asteroid?) (cartesian-product cols rows))]
|
|
[in-views (map (uncurry asteroids) locations)]
|
|
[counts (map length in-views)]
|
|
[maximum (apply max counts)]
|
|
[index (index-of counts maximum)])
|
|
(values maximum
|
|
(list-ref locations index)
|
|
(list-ref in-views index))))
|
|
|
|
(define (offset<? xy1 xy2)
|
|
(let* ([x1 (first xy1)] [y1 (second xy1)]
|
|
[x2 (first xy2)] [y2 (second xy2)]
|
|
[θ1 (atan y1 x1)] [θ2 (atan y2 x2)]
|
|
[θ1 (+ θ1 (if (< θ1 (/ pi -2)) (* 2 pi) 0))]
|
|
[θ2 (+ θ2 (if (< θ2 (/ pi -2)) (* 2 pi) 0))])
|
|
(< θ1 θ2)))
|
|
|
|
(define part2
|
|
(let* ([offsets (map (λ (ast) (zip - ast location)) in-view)]
|
|
[offsets (sort offsets offset<?)]
|
|
[200th (zip + location (list-ref offsets (sub1 200)))])
|
|
(+ (* (first 200th) 100) (second 200th))))
|
|
|
|
(show-solution part1 part2) |