day 6

data/examples/06.txt

@@ -0,0 +1,10 @@
+....#.....
+.........#
+..........
+..#.......
+.......#..
+..........
+.#..^.....
+........#.
+#.........
+......#...

src/bin/06.rs

@@ -0,0 +1,127 @@
+use std::collections::HashSet;
+
+advent_of_code::solution!(6);
+
+pub fn part_one(input: &str) -> Option<u32> {
+    let map: Vec<Vec<char>> = input.lines().map(|line| line.chars().collect()).collect();
+    let directions = [
+        (0, -1), // up
+        (1, 0),  // right
+        (0, 1),  // down
+        (-1, 0), // left
+    ];
+
+    let (mut x, mut y, mut dir_idx) = find_guard(&map).expect("No guard found");
+
+    let mut visited = HashSet::new();
+    visited.insert((x, y));
+
+    loop {
+        let (dx, dy) = directions[dir_idx];
+        let (nx, ny) = (x as isize + dx, y as isize + dy);
+
+        if nx < 0 || ny < 0 || ny >= map.len() as isize || nx >= map[0].len() as isize {
+            break;
+        }
+
+        let (nx, ny) = (nx as usize, ny as usize);
+
+        if map[ny][nx] == '#' {
+            dir_idx = (dir_idx + 1) % 4;
+        } else {
+            x = nx;
+            y = ny;
+            visited.insert((x, y));
+        }
+    }
+
+    Some(visited.len() as u32)
+}
+
+fn find_guard(map: &[Vec<char>]) -> Option<(usize, usize, usize)> {
+    for (y, row) in map.iter().enumerate() {
+        for (x, &cell) in row.iter().enumerate() {
+            let dir_idx = match cell {
+                '^' => Some(0), // up
+                '>' => Some(1), // right
+                'v' => Some(2), // down
+                '<' => Some(3), // left
+                _ => None,
+            };
+            if let Some(dir_idx) = dir_idx {
+                return Some((x, y, dir_idx));
+            }
+        }
+    }
+    None
+}
+
+pub fn part_two(input: &str) -> Option<u32> {
+    let map: Vec<Vec<char>> = input.lines().map(|line| line.chars().collect()).collect();
+    let directions = [
+        (0, -1), // up
+        (1, 0),  // right
+        (0, 1),  // down
+        (-1, 0), // left
+    ];
+
+    let (start_x, start_y, _) = find_guard(&map).expect("No guard found");
+    let mut valid_pos_count = 0;
+
+    for y in 0..map.len() {
+        for x in 0..map[0].len() {
+            if map[y][x] == '#' || (x == start_x && y == start_y) {
+                continue;
+            }
+
+            let mut map = map.clone();
+            map[y][x] = '#';
+
+            if is_stuck(&map, &directions) {
+                valid_pos_count += 1;
+            }
+        }
+    }
+    Some(valid_pos_count)
+}
+
+fn is_stuck(map: &[Vec<char>], directions: &[(isize, isize)]) -> bool {
+    let (mut x, mut y, mut dir_idx) = find_guard(map).expect("No guard found");
+    let mut visited = HashSet::new();
+
+    while visited.insert((x, y, dir_idx)) {
+        let (dx, dy) = directions[dir_idx];
+        let (nx, ny) = (x as isize + dx, y as isize + dy);
+
+        if nx < 0 || ny < 0 || ny >= map.len() as isize || nx >= map[0].len() as isize {
+            return false;
+        }
+
+        let (nx, ny) = (nx as usize, ny as usize);
+
+        if map[ny][nx] == '#' {
+            dir_idx = (dir_idx + 1) % 4;
+        } else {
+            x = nx;
+            y = ny;
+        }
+    }
+    true
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part_one() {
+        let result = part_one(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(41));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(6));
+    }
+}