From d4c9aa4c5e71c5f7c04a5298febef387e23b0dac Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Wed, 23 Dec 2020 13:11:04 +0100
Subject: [PATCH] 2020: d23: ex1: add solution

---
 2020/d23/ex1/ex1.py | 58 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 58 insertions(+)
 create mode 100755 2020/d23/ex1/ex1.py

diff --git a/2020/d23/ex1/ex1.py b/2020/d23/ex1/ex1.py
new file mode 100755
index 0000000..faae133
--- /dev/null
+++ b/2020/d23/ex1/ex1.py
@@ -0,0 +1,58 @@
+#!/usr/bin/env python
+
+import itertools
+import sys
+from typing import List, Tuple
+
+
+def solve(circle: List[int]) -> int:
+    def to_links(circle: List[int]) -> List[int]:
+        links = [-1 for __ in range(len(circle) + 1)]
+        cyclic = itertools.cycle(circle)
+        next(cyclic)  # Advance it by one
+        for prev, cur in zip(circle, cyclic):
+            links[prev] = cur
+        return links
+
+    def step(links: List[int], current: int) -> int:
+        cup0 = links[current]
+        cup1 = links[cup0]
+        cup2 = links[cup1]
+
+        links[current] = links[cup2]  # Remove 3-tuple from the linked-list
+
+        # Find destination
+        dest = (current - 1) if current > 1 else (len(links) - 1)
+        while dest in (cup0, cup1, cup2):
+            dest = (dest - 1) if dest > 1 else (len(links) - 1)
+
+        # Update our links
+        links[cup2], links[dest] = links[dest], cup0
+
+        return links[current]  # What's the next value in the cycle ?
+
+    def to_answer(links: List[int]) -> int:
+        next = links[1]
+        res = 0
+        while next != 1:
+            res = res * 10 + next
+            next = links[next]
+        return res
+
+    current = circle[0]
+    links = to_links(circle)
+
+    for __ in range(100):
+        current = step(links, current)
+
+    return to_answer(links)
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin.readlines()]
+    assert len(input) == 1
+    print(solve([int(c) for c in input[0]]))
+
+
+if __name__ == "__main__":
+    main()