🌲

y2019/p16.py

+import sys
+from itertools import accumulate, cycle
+from multiprocessing import Pool
+
+
+def _next(ns, i):
+    g = cycle([n for n in [0, 1, 0, -1] for _ in range(i + 1)])
+    next(g)  # throw away one
+    return abs(sum(a * b for a, b in zip(ns, g))) % 10
+
+
+text = sys.stdin.read().strip()
+
+ns = [int(n) for n in text]
+with Pool() as pool:
+    for _ in range(100):
+        ns = pool.starmap(_next, [(ns, i) for i in range(len(ns))])
+print(''.join(str(c) for c in ns[:8]))
+
+offset = int(text[:7])
+pending = [int(n) for n in (text * 10000)[offset:]][::-1]
+for _ in range(100):
+    pending = [n % 10 for n in accumulate(pending)]
+print(''.join(str(c) for c in pending[::-1][:8]))