From 1831dc4b8f01e9ffdab5f04863a4ff88ee0acc06 Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Tue, 15 Dec 2020 11:16:26 +0100
Subject: [PATCH] 2020: d15: ex2: add solution

---
 2020/d15/ex2/ex2.py | 40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)
 create mode 100755 2020/d15/ex2/ex2.py

diff --git a/2020/d15/ex2/ex2.py b/2020/d15/ex2/ex2.py
new file mode 100755
index 0000000..4d9077e
--- /dev/null
+++ b/2020/d15/ex2/ex2.py
@@ -0,0 +1,40 @@
+#!/usr/bin/env python
+import itertools
+import sys
+from collections import defaultdict
+from typing import Dict, Iterator, List
+
+
+def nth(iterable: Iterator[int], n: int) -> int:
+    return next(itertools.islice(iterable, n, None))
+
+
+def spoken(spoken: List[int]) -> Iterator[int]:
+    turn = 0
+    turns: Dict[int, List[int]] = defaultdict(list)
+    for last in spoken:
+        turn += 1
+        turns[last].append(turn)
+        yield last
+    while True:
+        if len(last_turn := turns[last]) < 2:
+            last = 0
+        else:
+            last = last_turn[-1] - last_turn[-2]
+        turn += 1
+        turns[last].append(turn)
+        yield last
+
+
+def solve(nums: List[int]) -> int:
+    return nth(spoken(nums), 30000000 - 1)
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin.readlines()]
+    assert len(input) == 1
+    print(solve([int(i) for i in input[0].split(",")]))
+
+
+if __name__ == "__main__":
+    main()