1 files changed, 51 insertions, 0 deletions
diff --git a/aoc2023-racket/day-24/day-24a.rkt b/aoc2023-racket/day-24/day-24a.rkt
new file mode 100644
index 0000000..31f526d
--- /dev/null
+++ b/aoc2023-racket/day-24/day-24a.rkt
@@ -0,0 +1,51 @@
+#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