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1 files changed, 43 insertions, 0 deletions

diff --git a/racket/aoc2021/day-17/day-17.rkt b/racket/aoc2021/day-17/day-17.rkt
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+++ b/racket/aoc2021/day-17/day-17.rkt
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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)