From ec3d1e40f306261fdcb2a69a07d24cd30bea1c7e Mon Sep 17 00:00:00 2001
From: Dustin Swan <dustin@dustinswan.com>
Date: Mon, 20 Dec 2021 22:03:36 -0500
Subject: [PATCH] day 17

---
 day17.rkt | 79 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 79 insertions(+)
 create mode 100644 day17.rkt

diff --git a/day17.rkt b/day17.rkt
new file mode 100644
index 0000000..edad495
--- /dev/null
+++ b/day17.rkt
@@ -0,0 +1,79 @@
+#lang racket
+
+(define target
+  (let* (;[input "target area: x=20..30, y=-10..-5"]
+         [input "target area: x=144..178, y=-100..-76"]
+         [regex #rx"x=([^\\.]+)\\.\\.([^\\.]+), y=([^\\.]+)\\.\\.([^\\.]+)"]
+         [matches (regexp-match regex input)]
+         [coords (drop matches 1)])
+    (map string->number coords)))
+
+(define (point-in-target? point)
+  (match-let* ([(list x y) point]
+               [(list x1 x2 y1 y2) target])
+    (and (>= x x1) (<= x x2)
+         (>= y y1) (<= y y2))))
+
+(define (point-past-target? point)
+  (match-let* ([(list x y) point]
+               [(list x1 x2 y1 y2) target])
+    (or (> x x2) (< y y1))))
+
+(define (step point velocity)
+  (match-let ([(list x y) point]
+              [(list dx dy) velocity])
+    (let* ([new-point (list (+ x dx) (+ y dy))]
+           [dx-op (if (= dx 0) identity (if (> dx 0) sub1 add1))]
+           [new-velocity (list (dx-op dx) (sub1 dy))])
+      (values new-point new-velocity))))
+
+(define (good-velocity? point velocity)
+  (if (point-in-target? point)
+      #t
+      (if (point-past-target? point)
+          #f
+          (let-values ([(p2 v2) (step point velocity)])
+            (good-velocity? p2 v2)))))
+
+; shoot it as high as you can
+; such that it never goes *below* the bottom of the target
+; bottom of target: -10 in the example
+; by the time it gets back to 0, dy is equal to -initial dy
+; so.. initial dy can't be > -10? set it to 9 and find a valid x
+
+(define (find-max-velocity)
+  (let* ([dy (sub1 (* -1 (third target)))])
+    (let loop ([dx 0])
+      (if (good-velocity? '(0 0) (list dx dy))
+          (list dx dy)
+          (loop (add1 dx))))))
+
+(define (find-max-height velocity)
+  (let loop ([p '(0 0)]
+             [v velocity])
+    (let-values ([(p2 v2) (step p v)])
+      (if (= 0 (second v2))
+          (second p2)
+          (loop p2 v2)))))
+    
+(define part1
+  (let* ([v (find-max-velocity)]
+         [height (find-max-height v)])
+    height))
+
+part1
+
+; PART 2
+
+(define part2
+  (let* ([dy-max (* -1 (third target))]
+         [dy-min (* -1 dy-max)]
+         [dx-max (* 2 (second target))]
+         [all (apply append
+                     (for/list ([dy (range dy-min dy-max)])
+                       (for/list ([dx (range 0 dx-max)]) 
+                         (list dx dy (good-velocity? '(0 0) (list dx dy))))))]
+         [goods (filter (λ (x) (third x)) all)])
+    (length goods)))
+
+part2
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