From 6c64d1b62e15cf2c788f005a88fdf7429f911d5c Mon Sep 17 00:00:00 2001
From: Sebastian Mark <smark@posteo.net>
Date: Fri, 6 Dec 2024 13:05:45 +0100
Subject: [PATCH] add 2024/day6

---
 2024/06/main.py | 109 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 109 insertions(+)
 create mode 100644 2024/06/main.py

diff --git a/2024/06/main.py b/2024/06/main.py
new file mode 100644
index 0000000..2f1a807
--- /dev/null
+++ b/2024/06/main.py
@@ -0,0 +1,109 @@
+#!/usr/bin/env python
+# -*- encoding: utf-8; py-indent-offset: 4 -*-
+
+# Author: Sebastian Mark
+# CC-BY-SA (https://creativecommons.org/licenses/by-sa/4.0/deed.de)
+
+# pylint: disable=missing-module-docstring,missing-function-docstring,consider-using-f-string
+
+
+from copy import deepcopy
+
+
+def readinput() -> str:
+    with open("input", "r", encoding="utf-8") as file:
+        lines = [list(line.strip()) for line in file]
+    return lines
+
+
+def find_guard(maze: list) -> tuple:
+    for idx, row in enumerate(maze):
+        try:
+            x = row.index("^")
+            y = idx
+        except ValueError:
+            pass
+        else:
+            return (x, y)
+    return tuple(-1, -1)
+
+
+def move_guard(maze: list, guard: tuple) -> tuple[int, bool]:
+    width = len(maze[0])
+    height = len(maze)
+
+    path_history = {}
+
+    # remember the start position as visited
+    visited = set()
+    visited.add(guard)
+
+    # start upwards
+    direction = (0, -1)
+
+    while True:
+        # get next position
+        nxt = (
+            guard[0] + direction[0],
+            guard[1] + direction[1],
+        )
+
+        # check borders
+        if not (0 <= nxt[0] < width and 0 <= nxt[1] < height):
+            visited.add(guard)
+            break
+
+        # check for obstacle and rotate clockwise
+        if maze[nxt[1]][nxt[0]] == "#":
+            direction = (-direction[1], direction[0])
+            continue
+
+        # loop detection
+        # (has the guard been here and walking in the same direction?)
+        if nxt not in path_history:
+            path_history[nxt] = (guard, direction)
+        elif path_history[nxt] == (guard, direction):
+            return 0, True
+
+        # mark spot as visited and move guard
+        visited.add(guard)
+        guard = nxt
+
+    return len(visited), False
+
+
+def print_maze(maze: list):
+    for row in maze:
+        print("".join(row))
+    print("-")
+
+
+def main():
+    # part 1
+    maze = readinput()
+    guard = find_guard(maze)
+    count, _ = move_guard(maze, guard)
+    print("Guard positions: %d" % count)
+
+    # part 2
+    count = 0
+    maze = readinput()
+    guard = find_guard(maze)
+
+    # brute force:
+    # place an obstacle everywhere (except the guards position)
+    for row, _ in enumerate(maze):
+        for col, _ in enumerate(maze[0]):
+            if (col, row) == guard:
+                continue
+            new_maze = deepcopy(maze)
+            new_maze[row][col] = "#"
+            _, loop = move_guard(new_maze, guard)
+            if loop:
+                count += 1
+
+    print("Loop options: %d" % count)
+
+
+if __name__ == "__main__":
+    main()