2016: d19: ex1: add solution

2016/d19/ex1/ex1.py

@@ -0,0 +1,25 @@
+#!/usr/bin/env python
+
+import sys
+
+
+def solve(input: str) -> int:
+    def msb(n: int) -> int:
+        assert n  # Sanity check
+        return n.bit_length() - 1
+
+    # https://en.wikipedia.org/wiki/Josephus_problem
+    def josephus(n: int) -> int:
+        return 2 * (n - (1 << msb(n))) + 1
+
+    num_elves = int(input.strip())
+    return josephus(num_elves)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()