From d14c0cc1e8c25dba67259a8bff463007a155f140 Mon Sep 17 00:00:00 2001
From: Jonathan Chan <jonathanhwchan@gmail.com>
Date: Wed, 11 Dec 2019 23:28:53 -0800
Subject: [PATCH] Day 12: Part 1, with a Part 2 that is too slow :(

---
 src/12.rkt | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)
 create mode 100644 src/12.rkt

diff --git a/src/12.rkt b/src/12.rkt
new file mode 100644
index 0000000..d007253
--- /dev/null
+++ b/src/12.rkt
@@ -0,0 +1,84 @@
+#lang racket
+
+(require racket/set
+         "../lib.rkt")
+
+(define moons-pos
+  '((-9 -1  -1)
+    ( 2  9   5)
+    (10 18 -12)
+    (-6 15  -7)))
+
+(define moons-vel
+  '((0 0 0)
+    (0 0 0)
+    (0 0 0)
+    (0 0 0)))
+
+(define example
+  '((-1   0  2)
+    ( 2 -10 -7)
+    ( 4  -8  8)
+    ( 3   5 -1)))
+
+(define positions (mutable-set))
+
+(define (less-than->number n1 n2)
+  (cond
+    [(< n1 n2)  1]
+    [(= n1 n2)  0]
+    [(> n1 n2) -1]))
+
+;; Calculate the velocity on the first moon
+;; caused by the second moon
+(define (velocity moon-on moon-by)
+  (list (less-than->number (first  moon-on) (first  moon-by))
+        (less-than->number (second moon-on) (second moon-by))
+        (less-than->number (third  moon-on) (third  moon-by))))
+
+(define (<+> v1 v2)
+  (zip + v1 v2))
+
+;; Calculate velocities for all moons
+(define (velocities moons vels)
+  (zip <+> vels
+       (map (λ (moon)
+              (foldl (λ (other v)
+                       (<+> v (velocity moon other)))
+                     '(0 0 0) moons))
+            moons)))
+
+(define (energy moons vels)
+  (sum (zip (λ (moon vel)
+              (* (sum (map abs moon))
+                 (sum (map abs vel))))
+            moons vels)))
+
+(define (step moons vels)
+  (let* ([vels (velocities moons vels)]
+         [moons (zip <+> moons vels)])
+    (values moons vels)))
+
+(define (step-n n moons vels)
+  (if (zero? n)
+      (values moons vels)
+      (let-values ([(moons vels) (step moons vels)])
+        (step-n (sub1 n) moons vels))))
+
+(define part1
+  (let-values ([(moons vels)
+                (step-n 1000 moons-pos moons-vel)])
+    (energy moons vels)))
+
+(define (part2-nogood)
+  (let loop ([steps 0]
+             [moons moons-pos]
+             [vels  moons-vel])
+    (if (set-member? positions moons)
+        steps
+        (let*-values ([(moons* vels*)
+                       (step moons vels)])
+          (set-add! positions moons)
+          (loop (add1 steps) moons* vels*)))))
+
+(show-solution part1 #f)
\ No newline at end of file