day 17 complete

1 files changed, 53 insertions, 0 deletions

diff --git a/2021/day-17/day-17.rkt b/2021/day-17/day-17.rkt
new file mode 100644
index 0000000..b8563a5
--- /dev/null
+++ b/2021/day-17/day-17.rkt
@@ -0,0 +1,53 @@
+#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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