From 92ab44f92a734e37b8f6c2c9c4983c9890346911 Mon Sep 17 00:00:00 2001
From: Matthew Butterick <mb@mbtype.com>
Date: Thu, 12 Dec 2019 06:45:38 -0800
Subject: [PATCH] d12 p2 (stuck)

---
 2019/12.rkt | 45 ++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 40 insertions(+), 5 deletions(-)

diff --git a/2019/12.rkt b/2019/12.rkt
index 4e74251..0abf18b 100644
--- a/2019/12.rkt
+++ b/2019/12.rkt
@@ -37,7 +37,7 @@
         (update-pair-velocity! m0 m))
       (loop ms))))
 
-(define (step moons count)
+(define (step! moons [count 1])
   (for ([i (in-range count)])
     (update-gravity! moons)
     (for-each update-position! moons))
@@ -50,14 +50,14 @@
                  (abs val))))))
 
 (check-eq?
- (total-energy (step (str->moons "<x=-1, y=0, z=2>
+ (total-energy (step! (str->moons "<x=-1, y=0, z=2>
 <x=2, y=-10, z=-7>
 <x=4, y=-8, z=8>
 <x=3, y=5, z=-1>") 10))
  179)
 
 (check-eq?
- (total-energy (step (str->moons "<x=-8, y=-10, z=0>
+ (total-energy (step! (str->moons "<x=-8, y=-10, z=0>
 <x=5, y=5, z=10>
 <x=2, y=-7, z=3>
 <x=9, y=-8, z=-3>") 100))
@@ -65,5 +65,40 @@
 
 ;; 1
 (check-eq?
- (total-energy (step (str->moons (file->string "12.rktd")) 1000))
- 9876)
\ No newline at end of file
+ (total-energy (step! (str->moons (file->string "12.rktd")) 1000))
+ 9876)
+
+;; 2
+(define (shift-origin! moons pos)
+  (for ([moon (in-list moons)])
+    (set-$moon-pos! moon (map - ($moon-pos moon) pos))))
+        
+(define (period str)
+  (for*/list ([which-moon (list first second third fourth)]
+              [which-dim (list first second third)])
+    (define moons (str->moons str))
+    (shift-origin! moons ($moon-pos (which-moon moons)))
+    (let loop ([count 1])
+      (step! moons)
+      (if (and (= 0 (which-dim ($moon-pos (which-moon moons))))
+               (= 0 (which-dim ($moon-vel (which-moon moons)))))
+          count
+          (loop (add1 count))))))
+
+(check-eq?
+ (apply lcm (period "<x=-1, y=0, z=2>
+<x=2, y=-10, z=-7>
+<x=4, y=-8, z=8>
+<x=3, y=5, z=-1>"))
+ 2772)
+
+(check-eq?
+ (apply lcm
+   (period "<x=-8, y=-10, z=0>
+<x=5, y=5, z=10>
+<x=2, y=-7, z=3>
+<x=9, y=-8, z=-3>"))
+   4686774924)
+
+
+