Finally solve 2019 day 12 and others

2019/d12.py

@@ -1,4 +1,5 @@
 from lib import get_data, str_to_ints
+from math import lcm
 
 
 def freeze(coords, velos):
@@ -40,7 +41,40 @@ def part_1(data):
 
 
 def part_2(data):
-    raise Exception("TBD")
+    steps = []
+    for i in range(3):
+        coords = []
+        velos = []
+
+        for line in data.splitlines():
+            coords.append(str_to_ints(line)[i])
+            velos.append(0)
+
+        seen = {}
+        for step in range(1_000_000):
+            state = tuple(coords) + tuple(velos)
+            if state in seen:
+                steps.append(step)
+                break
+            else:
+                seen[state] = step
+
+            for i in range(len(coords)):
+                for j in range(i + 1, len(coords)):
+                    if coords[i] > coords[j]:
+                        velos[i] -= 1
+                        velos[j] += 1
+                    elif coords[i] < coords[j]:
+                        velos[i] += 1
+                        velos[j] -= 1
+
+            # update coords
+            for i in range(len(coords)):
+                coords[i] += velos[i]
+
+    # Intuition: Find when position repeats on each axis and then find lowest
+    # common multiple for all three values.
+    print(lcm(*steps))
 
 
 def main():