---
title: "Advent of Code 2024: Day 6"
url: https://perrotta.dev/2024/12/advent-of-code-2024-day-6/
last_updated: 2026-01-03
---


Refer to the [previous post]({{< ref "2022-01-28-advent-of-code" >}}) about AoC,
and to the [git repository](https://github.com/thiagowfx/adventofcode) with my
solutions in Python 3.

Link to [Day #6](https://adventofcode.com/2024/day/6) puzzle.

Part one is a classic 2D matrix (graph) traversal problem.

To store state I created a `visited` set with the `(x, y)` coordinates.
Alternatively I could have changed the input inplace, but I didn't want to deal
with the immutability of python strings, i.e. given:

```
l = ["....."]
```

...you can't simply do `l[0][0] = 'X'`, because python strings are immutable. We
could define a new string and assign it to `l[0]`, or we could change the input
to:

```
l = ['.', '.', '.', '.', '.']
```

...so that replacing characters becomes trivial.

The full solution:

```python
#!/usr/bin/env python3
import sys

def find(lines, c):
    for i, line in enumerate(lines):
        if c in line:
            return i, line.index(c)
    raise ValueError(f'Could not find {c} in lines')

def move(pos, dir, lines, visited):
    dirs_clockwise = ((1, 0), (0, -1), (-1, 0), (0, 1))

    while True:
        next_pos = pos[0] + dir[0], pos[1] + dir[1]

        if next_pos[0] < 0 or next_pos[0] >= len(lines) or next_pos[1] < 0 or next_pos[1] >= len(lines[0]):
            break

        if lines[next_pos[0]][next_pos[1]] in '.^':
            pos = next_pos
            visited.add(pos)
        elif lines[next_pos[0]][next_pos[1]] == '#':
            dir = dirs_clockwise[(dirs_clockwise.index(dir) + 1) % 4]

def main():
    with open(sys.argv[1]) as input:
        lines = input.read().splitlines()

    pos = find(lines, '^')
    dir = (-1, 0)  # up

    visited = set((pos,))

    move(pos, dir, lines, visited)

    # part one
    print(len(visited))

if __name__ == '__main__':
    main()
```

I did not solve part two yet. I know how to do it, but my initial approach is
too brute force to my taste. Perhaps I'll come up with something clever later
on.