Year 2024, Day 06

year2024/src/day06.rs

@@ -0,0 +1,104 @@
+use std::collections::HashSet;
+
+use common::day::Day;
+use util::grid2d::{Coords, Direction, Grid2D};
+
+pub struct Day06 {}
+
+impl Day for Day06 {
+    fn star1(&self, input: &str) -> String {
+        let grid = Grid2D::new(input).unwrap();
+        let visited = walk(grid).unwrap();
+        let visited: HashSet<_> = visited.into_iter().map(|g| g.pos).collect();
+        format!("{}", visited.len())
+    }
+
+    fn star2(&self, input: &str) -> String {
+        let grid = Grid2D::new(input).unwrap();
+        let empty_pos = grid.filter(&['.']);
+
+        let mut num_loop_pos = 0;
+        for add_pos in empty_pos {
+            let mut grid_add = grid.clone();
+            grid_add.set(&add_pos, '#');
+            if walk(grid_add).is_err() {
+                num_loop_pos += 1;
+            }
+        }
+        format!("{}", num_loop_pos)
+    }
+}
+
+#[derive(Copy, Clone, Hash, PartialEq, Eq, Debug)]
+struct Guard {
+    pos: Coords,
+    dir: Direction,
+}
+
+fn walk(mut grid: Grid2D<char>) -> Result<HashSet<Guard>, Guard> {
+    let pos = grid.find('^').unwrap();
+    let mut guard = Guard {
+        pos,
+        dir: Direction::N,
+    };
+
+    let mut visited = HashSet::new();
+    visited.insert(guard);
+    grid.set(&pos, '.');
+
+    loop {
+        let check_pos = guard.pos.mov(guard.dir);
+        if let Some(v) = grid.at(&check_pos) {
+            match v {
+                '#' => {
+                    guard.dir = match guard.dir {
+                        Direction::N => Direction::E,
+                        Direction::E => Direction::S,
+                        Direction::S => Direction::W,
+                        Direction::W => Direction::N,
+                        _ => unreachable!(),
+                    };
+                }
+                '.' => {
+                    guard.pos = check_pos;
+                    if !visited.insert(guard) {
+                        return Err(guard);
+                    }
+                }
+                _ => unreachable!(),
+            }
+        } else {
+            break;
+        }
+    }
+
+    Ok(visited)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const INPUT: &str = r#"....#.....
+.........#
+..........
+..#.......
+.......#..
+..........
+.#..^.....
+........#.
+#.........
+......#..."#;
+
+    #[test]
+    fn star1() {
+        let d = Day06 {};
+        assert_eq!(d.star1(INPUT), "41");
+    }
+
+    #[test]
+    fn star2() {
+        let d = Day06 {};
+        assert_eq!(d.star2(INPUT), "6");
+    }
+}

year2024/src/lib.rs

@@ -6,6 +6,7 @@ mod day02;
 mod day03;
 mod day04;
 mod day05;
+mod day06;
 
 pub struct Year2024 {}
 
@@ -17,6 +18,7 @@ impl Year for Year2024 {
             3 => Some(Box::new(day03::Day03 {})),
             4 => Some(Box::new(day04::Day04 {})),
             5 => Some(Box::new(day05::Day05 {})),
+            6 => Some(Box::new(day06::Day06 {})),
             _ => None,
         }
     }