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#lang rosette
(require advent-of-code
threading)
(struct hail (posn vel) #:transparent)
(struct posn (x y z) #:transparent)
(struct vel (x y z) #:transparent)
(define input (fetch-aoc-input (find-session) 2023 24 #:cache #true))
(define LOWER-BOUND 200000000000000)
(define UPPER-BOUND 400000000000000)
(define (->struct f str)
(~> str (string-split _ ",") (map (λ~> string-trim string->number) _) (apply f _)))
(define (parse-hail-record str)
(match-define (list p v) (string-split str " @ "))
(hail (->struct posn p)
(->struct vel v)))
(define hail-paths
(for/list ([hail (in-list (string-split input "\n"))])
(parse-hail-record hail)))
;; part 1
(define (valid-intersection? h1 h2)
(match-define (hail (posn x1 y1 _) (vel vx1 vy1 _)) h1)
(match-define (hail (posn x2 y2 _) (vel vx2 vy2 _)) h2)
(cond
[(= (* vy1 vx2) (* vx1 vy2)) #f]
[else
(define t1 (/ (- (* vy2 (- x1 x2)) (* vx2 (- y1 y2)))
(- (* vy1 vx2) (* vx1 vy2))))
(define t2 (/ (- (* vy1 (- x2 x1)) (* vx1 (- y2 y1)))
(- (* vy2 vx1) (* vx2 vy1))))
(define x (+ x1 (* t1 vx1)))
(define y (+ y1 (* t1 vy1)))
(and (<= LOWER-BOUND x UPPER-BOUND)
(<= LOWER-BOUND y UPPER-BOUND)
(<= 0 t1)
(<= 0 t2))]))
(for/sum ([(trial-paths) (in-combinations hail-paths 2)] ;
#:when (apply valid-intersection? trial-paths))
1)
;; part 2 - see day-24b.rkt
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