From e1c56b22578a1e530152e8722c65ead4866a6990 Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Sat, 10 May 2025 12:50:24 +0100
Subject: [PATCH] 2017: d16: ex2: add solution

---
 2017/d16/ex2/ex2.py | 82 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 82 insertions(+)
 create mode 100755 2017/d16/ex2/ex2.py

diff --git a/2017/d16/ex2/ex2.py b/2017/d16/ex2/ex2.py
new file mode 100755
index 0000000..d65972d
--- /dev/null
+++ b/2017/d16/ex2/ex2.py
@@ -0,0 +1,82 @@
+#!/usr/bin/env python
+
+import sys
+from typing import NamedTuple
+
+
+class Spin(NamedTuple):
+    n: int
+
+
+class Exchange(NamedTuple):
+    a: int
+    b: int
+
+
+class Partner(NamedTuple):
+    a: str
+    b: str
+
+
+Move = Spin | Exchange | Partner
+
+Programs = tuple[str, ...]
+
+
+def solve(input: str) -> str:
+    def parse_move(input: str) -> Move:
+        if input[0] == "s":
+            return Spin(int(input[1:]))
+        if input[0] == "x":
+            return Exchange(*map(int, input[1:].split("/")))
+        if input[0] == "p":
+            return Partner(*input[1:].split("/"))
+        assert False  # Sanity check
+
+    def parse(input: str) -> list[Move]:
+        return [parse_move(move) for move in input.strip().split(",")]
+
+    def apply_move(programs: Programs, move: Move) -> Programs:
+        match move:
+            case Spin(n):
+                return programs[-n:] + programs[:-n]
+            case Exchange(a, b):
+                tmp = list(programs)
+                tmp[a], tmp[b] = tmp[b], tmp[a]
+                return tuple(tmp)
+            case Partner(a, b):
+                ia, ib = programs.index(a), programs.index(b)
+                tmp = list(programs)
+                tmp[ia], tmp[ib] = tmp[ib], tmp[ia]
+                return tuple(tmp)
+
+    def do_dance(programs: Programs, moves: list[Move]) -> Programs:
+        t = 0
+        cache = {(programs, 0): 0}
+        TOTAL_MOVES = 1000000000 * len(moves)
+        while t < TOTAL_MOVES:
+            programs = apply_move(programs, moves[t % len(moves)])
+            t += 1
+            key = (programs, t % len(moves))
+            if (previous_t := cache.get(key)) is not None:
+                cycle_length = t - previous_t
+                num_cycles = (TOTAL_MOVES - t) // cycle_length
+                t += num_cycles * cycle_length
+            else:
+                cache[key] = t
+
+        return programs
+
+    moves = parse(input)
+    programs = tuple(chr(ord("a") + i) for i in range(16))
+    programs = do_dance(programs, moves)
+    return "".join(programs)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()