2019: d16: ex2: add solution

2019/d16/ex2/ex2.py

@@ -0,0 +1,27 @@
+#!/usr/bin/env python
+
+import sys
+from functools import reduce
+
+
+def main() -> None:
+    rep = 10000
+    signal = [int(d) for d in sys.stdin.read().strip()] * rep
+    offset = reduce(lambda a, b: a * 10 + b, signal[0:7])
+
+    assert offset >= len(signal) / 2  # Sanity check
+    # The trick is that the second half is only affected by itself (triangular matrix):
+    # For i > len(signal) / 2, new_signal[i] = sum(signal, i, len(signal))
+    # Therefore, we're only interested in numbers that start at the offset
+    signal = signal[offset:]  # Only take the end we need
+
+    for __ in range(100):
+        for i in range(len(signal) - 1, 0, -1):  # Do the sum from the end
+            signal[i - 1] += signal[i]
+            signal[i - 1] = signal[i - 1] % 10
+
+    print(reduce(lambda a, b: a * 10 + b, signal[:8]))
+
+
+if __name__ == "__main__":
+    main()