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#lang br
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(require racket/file rackunit racket/math )
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(define (string->asteroids str)
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(for*/list ([(row ridx) (in-indexed (string-split str))]
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[(col cidx) (in-indexed row)]
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#:when (char=? #\# col))
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(make-rectangular cidx ridx)))
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(define ((count-visible roids) roid-origin)
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(length (remove-duplicates (for/list ([roid (in-list roids)]
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#:unless (= roid-origin roid))
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(angle (- roid roid-origin))))))
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(define (find-best roids)
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(argmax (count-visible roids) roids))
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(check-equal? (find-best (string->asteroids "......#.#.
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#..#.#....
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..#######.
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.#.#.###..
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.#..#.....
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..#....#.#
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#..#....#.
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.##.#..###
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##...#..#.
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.#....####")) 5+8i)
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(check-equal? (find-best (string->asteroids "#.#...#.#.
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.###....#.
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.#....#...
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##.#.#.#.#
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....#.#.#.
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.##..###.#
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..#...##..
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..##....##
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......#...
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.####.###.")) 1+2i)
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(check-equal? (find-best (string->asteroids ".#..#..###
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####.###.#
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....###.#.
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..###.##.#
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##.##.#.#.
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....###..#
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..#.#..#.#
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#..#.#.###
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.##...##.#
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.....#.#..")) 6+3i)
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(check-equal? (find-best (string->asteroids ".#..##.###...#######
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##.############..##.
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.#.######.########.#
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.###.#######.####.#.
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#####.##.#.##.###.##
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..#####..#.#########
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####################
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#.####....###.#.#.##
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##.#################
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#####.##.###..####..
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..######..##.#######
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####.##.####...##..#
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.#####..#.######.###
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##...#.##########...
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#.##########.#######
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.####.#.###.###.#.##
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....##.##.###..#####
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.#.#.###########.###
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#.#.#.#####.####.###
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###.##.####.##.#..##")) 11+13i)
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;; 1
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(define roids (string->asteroids (file->string "10.rktd")))
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(define best-roid (find-best roids))
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(check-eq? ((count-visible roids) best-roid) 214)
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(define (radians roid)
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(match (+ pi (angle (* -i (- roid best-roid))))
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[(== (* pi 2)) 0]
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[res res]))
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(check-= (radians 8) 0 0.1)
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(check-= (radians 16+16i) (/ pi 2) 0.1)
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(check-= (radians 8+24i) pi 0.1)
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(check-= (radians +16i) (* 1.5 pi) 0.1)
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;; 2
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(define roids-by-dist (sort (remove best-roid roids) < #:key magnitude))
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(define radian-groups (group-by radians roids-by-dist))
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(define radian-groups-by-angle
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(sort radian-groups < #:key (compose1 radians car)))
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(check-eq?
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(match (list-ref radian-groups-by-angle (sub1 200))
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[(cons c _) (+ (* 100 (real-part c)) (imag-part c))])
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502)
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