From f0bb869336a93e5f9405a47d4a4fee734d869e05 Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Mon, 19 May 2025 22:05:45 +0100
Subject: [PATCH] 2016: d17: ex2: add solution

---
 2016/d17/ex2/ex2.py | 83 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)
 create mode 100755 2016/d17/ex2/ex2.py

diff --git a/2016/d17/ex2/ex2.py b/2016/d17/ex2/ex2.py
new file mode 100755
index 0000000..e136018
--- /dev/null
+++ b/2016/d17/ex2/ex2.py
@@ -0,0 +1,83 @@
+#!/usr/bin/env python
+
+import enum
+import functools
+import hashlib
+import sys
+from collections.abc import Callable, Iterator
+from typing import NamedTuple
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+
+
+# Iteration order of `Direction` is relevant to the solution
+class Direction(enum.StrEnum):
+    UP = "U"
+    DOWN = "D"
+    LEFT = "L"
+    RIGHT = "R"
+
+    def apply(self, p: Point) -> Point:
+        match self:
+            case Direction.UP:
+                dx, dy = -1, 0
+            case Direction.DOWN:
+                dx, dy = 1, 0
+            case Direction.LEFT:
+                dx, dy = 0, -1
+            case Direction.RIGHT:
+                dx, dy = 0, 1
+        return Point(p.x + dx, p.y + dy)
+
+
+Path = tuple[Direction, ...]
+
+
+def solve(input: str) -> int:
+    def doors_at_passcode(path: Path, passcode: str) -> Iterator[Direction]:
+        hashed = hashlib.md5((passcode + "".join(path)).encode()).hexdigest()[:4]
+        for c, dir in zip(hashed, Direction):
+            if c.isdigit() or c == "a":
+                continue
+            yield Direction(dir)  # Satisfy Mypy to use the correct return type...
+
+    def bfs(
+        start: Point,
+        end: Point,
+        doors_at: Callable[[Path], Iterator[Direction]],
+    ) -> int:
+        queue: list[tuple[Point, Path]] = [(start, ())]
+        max_dist = -1
+
+        while queue:
+            new_queue: list[tuple[Point, Path]] = []
+            for p, path in queue:
+                if p == end:
+                    max_dist = max(max_dist, len(path))
+                    continue
+                for dir in doors_at(path):
+                    new_pos = dir.apply(p)
+                    if new_pos.x < 0 or new_pos.x > end.x:
+                        continue
+                    if new_pos.y < 0 or new_pos.y > end.y:
+                        continue
+                    new_queue.append((dir.apply(p), path + (dir,)))
+            queue = new_queue
+
+        return max_dist
+
+    passcode = input.strip()
+    doors_at = functools.partial(doors_at_passcode, passcode=passcode)
+    return bfs(Point(0, 0), Point(3, 3), doors_at)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()