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#lang racket
(require "../../jj-aoc.rkt"
threading)
(define-values (x-min x-max y-min y-max)
(~> (open-day 17 2021)
port->string
(regexp-match #px"target area: x=(.*)\\.\\.(.*), y=(.*)\\.\\.(.*)\n" _)
rest
(map string->number _)
(apply values _)))
(define (hit? x y)
(and (x . >= . x-min)
(x . <= . x-max)
(y . >= . y-min)
(y . <= . y-max)))
(define (miss? x y)
(or (y . < . y-min)
(x . > . x-max)))
(define (drag dx i) (max (- dx i) 0))
(define (gravity dy i) (- dy i))
(define (find-trajectory-apex dx dy)
(for/fold ([x 0] [y 0] [y-apex 0] [result #f]
#:result (list y-apex result))
([i (in-naturals)]
#:break result)
(cond
[(hit? x y) (values dx dy y-apex 'hit)]
[(miss? x y) (values x y 'miss 'miss)]
[else
(values (+ x (drag dx i))
(+ y (gravity dy i))
(if (y . > . y-apex) y y-apex)
#f)])))
(define on-target
(for*/list ([dx (in-inclusive-range 1 x-max)]
[dy (in-inclusive-range y-min (abs (* 2 y-max)))]
[velocity (in-value (find-trajectory-apex dx dy))]
#:when (equal? (second velocity) 'hit))
(list dx dy (first velocity))))
;; part 1
(~>> on-target
(argmax third)
third)
;; part 2
(length on-target)
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