From efd1fa7e5754361b6c5a83362b0cac15fc34bfbd Mon Sep 17 00:00:00 2001
From: Joseph Montanaro <jfmontanaro@modg.org>
Date: Thu, 13 Dec 2018 09:29:39 -0800
Subject: [PATCH] what.

---
 2018/12.py | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/2018/12.py b/2018/12.py
index 12c2a9c..4eb2567 100644
--- a/2018/12.py
+++ b/2018/12.py
@@ -57,6 +57,23 @@ with open('data/12.txt') as f:
 pots = grow(initial, 20)
 print(f'Part 1: {sum(pots)}\n')
 
-pots = grow(initial, 5 * 10 ** 10, True)
-print(f'Part 2: {pots}\n')
 
+# find pattern and exploit
+first_stable = 0
+prev_gen = []
+for i in range(1, 1000):
+    current_gen = sorted(grow(initial, i))
+    if current_gen == [x + 1 for x in prev_gen]:
+        first_stable = i - 1
+        break
+    else:
+        prev_gen = current_gen
+magic_number = sum(prev_gen)
+
+
+def cheat(num_gens):
+    if num_gens < first_stable:
+        raise RuntimeError("Can't cheat.")
+    return magic_number + (num_gens - first_stable) * len(prev_gen)
+
+print(f'Part 2: {cheat(5 * 10 ** 10)}\n')