Added 2015 day20

2015/day20.py

@@ -1,8 +1,15 @@
-for i in range(1, 500001):
-  presents = 0
-  for n in range(1, i+1):
-    if i%n == 0:
-      presents += 10*n
-  if presents >= 33100000:
-    print(i, presents)
-    break
+import numpy as np
+max_it = 813960  # Somewhat cheaty - this value was obtained from an earlier factor-based brute force. Just use an arbitrarily large number if it fails.
+input = 33100000
+
+houses = np.ones([max_it], dtype=np.int64)
+pres10 = input//10
+for i in range(2, max_it):
+  houses[i::i] += i
+print(np.argmax(houses > pres10))  # Part 1
+
+houses_2 = np.ones([max_it], dtype=np.int64)
+pres11 = input//11
+for i in range(2, max_it):
+  houses_2[i:i*51:i] += i
+print(np.argmax(houses_2 > pres11))  # Part 2