d12 p2

2019/12.rkt

@@ -69,36 +69,35 @@
  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)])
+  (apply lcm
+         (for*/list ([dim (list first second third)])
                     (define moons (str->moons str))
-    (shift-origin! moons ($moon-pos (which-moon moons)))
-    (let loop ([count 1])
+                    (define (get-dims) (for*/list ([moon (in-list moons)]
+                                                   [field (in-list (list $moon-pos $moon-vel))])
+                                                  (dim (field moon))))
+                    (define dims0 (get-dims))
+                    (let loop ([count 0])
                       (step! moons)
-      (if (and (= 0 (which-dim ($moon-pos (which-moon moons))))
-               (= 0 (which-dim ($moon-vel (which-moon moons)))))
-          count
-          (loop (add1 count))))))
+                      (if (andmap = dims0 (get-dims))
+                          (add1 count)
+                          (loop (add1 count)))))))
 
 (check-eq?
- (apply lcm (period "<x=-1, y=0, z=2>
+ (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>"))
+<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>"))
+<x=9, y=-8, z=-3>")
  4686774924)
 
+(check-eq? (period (file->string "12.rktd")) 307043147758488)
+