1 files changed, 161 insertions, 0 deletions
diff --git a/src/bin/day24.rs b/src/bin/day24.rs
new file mode 100644
index 0000000..109b7b5
--- /dev/null
+++ b/src/bin/day24.rs
@@ -0,0 +1,161 @@
+use std::collections::{HashMap, HashSet};
+
+static DAY: u8 = 24;
+
+fn main() {
+    let input = advent::read_lines(DAY);
+    println!("{DAY}a: {}", travel_time(&input));
+    println!("{DAY}b: {}", travel_time_3(&input));
+}
+
+#[derive(Clone,Copy)]
+enum Direction {
+    Up,
+    Down,
+    Left,
+    Right,
+}
+
+#[derive(Clone,Copy)]
+enum Object {
+    Wall,
+    Blizzard { direction: Direction },
+}
+
+impl Object {
+    fn new(input: char) -> Object {
+        match input {
+            '^' => Object::Blizzard { direction: Direction::Up },
+            'v' => Object::Blizzard { direction: Direction::Down },
+            '<' => Object::Blizzard { direction: Direction::Left },
+            '>' => Object::Blizzard { direction: Direction::Right },
+            '#' => Object::Wall,
+            _ => unimplemented!(),
+        }
+    }
+}
+
+struct Map {
+    map: HashMap<(isize,isize),Vec<Object>>,
+    dimensions: (isize, isize),
+    start: (isize, isize),
+    end: (isize, isize),
+}
+
+impl Map {
+    fn new(input: &[String]) -> Map {
+        let mut map = HashMap::new();
+        let mut dimensions = (0, 0);
+        let mut start = (0, 0);
+        let mut end = (0, 0);
+        for (y, line) in input.iter().enumerate() {
+            for (x, c) in line.chars().enumerate() {
+                let (x, y) = (x as isize, y as isize);
+                if c == '.' {
+                    if y == 0 {
+                        start = (x, y);
+                    } else {
+                        end = (x, y);
+                    }
+                } else {
+                    map.insert((x,y), vec![Object::new(c)]);
+                }
+                dimensions = (x, y);
+            }
+        }
+        Map { map, dimensions, start, end }
+    }
+
+    fn next_map(&self) -> HashMap<(isize,isize),Vec<Object>> {
+        let mut new_map = HashMap::<(isize,isize),Vec<Object>>::new();
+        for (&(x,y), objects) in &self.map {
+            for object in objects {
+                let new_pos = match object {
+                    Object::Wall => (x,y),
+                    Object::Blizzard { direction: Direction::Up } => {
+                        if y - 1 == 0 { (x, self.dimensions.1 - 1) } else { (x, y - 1) }
+                    },
+                    Object::Blizzard { direction: Direction::Down } => {
+                        if y + 1 == self.dimensions.1 { (x, 1) } else { (x, y + 1) }
+                    },
+                    Object::Blizzard { direction: Direction::Left } => {
+                        if x - 1 == 0 { (self.dimensions.0 - 1, y) } else { (x - 1, y) }
+                    },
+                    Object::Blizzard { direction: Direction::Right } => {
+                        if x + 1 == self.dimensions.0 { (1, y) } else { (x + 1, y) }
+                    },
+                };
+                new_map.entry(new_pos).or_default().push(*object);
+            }
+        }
+        new_map
+    }
+
+    fn travel_time(&mut self) -> usize {
+        let mut visited = HashSet::from([self.start]);
+        let mut time = 0;
+        loop {
+            let next_map = self.next_map();
+            let mut next_visited = HashSet::new();
+            for &(x,y) in &visited {
+                if !next_map.contains_key(&(x,y)) {
+                    next_visited.insert((x,y));
+                }
+                if x < self.dimensions.0 && !next_map.contains_key(&(x+1,y)) {
+                    next_visited.insert((x+1,y));
+                }
+                if x > 0 && !next_map.contains_key(&(x-1,y)) {
+                    next_visited.insert((x-1,y));
+                }
+                if y < self.dimensions.1 && !next_map.contains_key(&(x,y+1)) {
+                    next_visited.insert((x,y+1));
+                }
+                if y > 0 && !next_map.contains_key(&(x,y-1)) {
+                    next_visited.insert((x,y-1));
+                }
+            }
+            visited = next_visited;
+            self.map = next_map;
+            time += 1;
+            if visited.contains(&self.end) {
+                return time;
+            }
+        }
+    }
+}
+
+fn travel_time(input: &[String]) -> usize {
+    let mut map = Map::new(input);
+    map.travel_time()
+}
+
+fn travel_time_3(input: &[String]) -> usize {
+    let mut map = Map::new(input);
+    let mut time = map.travel_time();
+    (map.start, map.end) = (map.end, map.start);
+    time += map.travel_time();
+    (map.start, map.end) = (map.end, map.start);
+    time += map.travel_time();
+
+    time
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let input = [
+            "#.######",
+            "#>>.<^<#",
+            "#.<..<<#",
+            "#>v.><>#",
+            "#<^v^^>#",
+            "######.#",
+        ].iter().map(|&x| String::from(x)).collect::<Vec<_>>();
+
+        assert_eq!(travel_time(&input), 18);
+        assert_eq!(travel_time_3(&input), 54);
+    }
+}