solution: day 6

data/examples/06.txt

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

src/bin/06.rs

@@ -0,0 +1,97 @@
+use hashbrown::HashSet;
+use itertools::Itertools;
+
+const DIRS: [(isize, isize); 4] = [(-1, 0), (0, 1), (1, 0), (0, -1)];
+
+fn count_steps(
+    grid: &[Vec<char>],
+    start: (usize, usize),
+    visited: &mut HashSet<(usize, usize, usize)>,
+) -> Option<usize> {
+    visited.clear();
+
+    let (mut i, mut j) = start;
+    let mut turn = 0;
+
+    loop {
+        if !visited.insert((i, j, turn % 4)) {
+            return None;
+        }
+        let (di, dj) = DIRS[turn % 4];
+        let (ni, nj) = (i.wrapping_add(di as usize), j.wrapping_add(dj as usize));
+
+        match grid.get(ni).and_then(|row| row.get(nj)) {
+            Some(g) => match g {
+                '.' => (i, j) = (ni, nj),
+                '#' => turn += 1,
+                _ => unreachable!(),
+            },
+            None => break,
+        }
+    }
+    visited.len().into()
+}
+
+fn parse_grid(input: &str) -> (Vec<Vec<char>>, (usize, usize)) {
+    let mut grid = input.lines().map(|x| x.chars().collect_vec()).collect_vec();
+
+    let (_, i, j) = grid
+        .iter()
+        .enumerate()
+        .flat_map(|(i, x)| x.iter().copied().enumerate().map(move |(j, y)| (y, i, j)))
+        .find(|x| x.0 == '^')
+        .unwrap();
+
+    grid[i][j] = '.';
+
+    (grid, (i, j))
+}
+
+pub fn part_one(input: &str) -> Option<usize> {
+    let (grid, start) = parse_grid(input);
+
+    let mut visited = HashSet::new();
+    count_steps(&grid, start, &mut visited);
+
+    HashSet::<(usize, usize)>::from_iter(visited.into_iter().map(|(x, y, _)| (x, y)))
+        .len()
+        .into()
+}
+
+pub fn part_two(input: &str) -> Option<usize> {
+    let (mut grid, start) = parse_grid(input);
+
+    let mut count = 0;
+    let mut visited = HashSet::new();
+
+    for oi in 0..grid.len() {
+        for oj in 0..grid[0].len() {
+            if (oi, oj) == start || grid[oi][oj] == '#' {
+                continue;
+            }
+
+            grid[oi][oj] = '#';
+            if count_steps(&grid, start, &mut visited).is_none() {
+                count += 1;
+            }
+            grid[oi][oj] = '.';
+        }
+    }
+
+    count.into()
+}
+
+aoc::solution!(6);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_part_one() {
+        assert_eq!(part_one(&aoc::template::read_file("examples", 6)), Some(41));
+    }
+    #[test]
+    fn test_part_two() {
+        assert_eq!(part_two(&aoc::template::read_file("examples", 6)), Some(6));
+    }
+}