day 23

Cargo.lock

@@ -222,6 +222,13 @@ dependencies = [
  "itertools",
 ]
 
+[[package]]
+name = "day-23"
+version = "0.0.0"
+dependencies = [
+ "indoc",
+]
+
 [[package]]
 name = "deprecate-until"
 version = "0.1.1"

day-23/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "day-23"
+version = "0.0.0"
+edition = "2021"
+
+[dev-dependencies]
+indoc.workspace = true

day-23/src/bin/input.txt

@@ -0,0 +1,141 @@
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+#.#...#...#.###...#...#.....#...#.......#...#...#.#...#.>.>.#.#.....#.#.#.#.#.#.#.#.#.>.>.#...#...#...#...#...###...#.#.#...#.#...#.#.#.>.###
+#.#####.#.#.#####.#####.#####.#########.#.###.#.#.#.###.###.#.#####.#.#.#.#.#.#.#.#.#.###.###.#######.###.#.#.#####.#.#.#.###.#.###.#.###v###
+#...#...#.#.#.....#...#.....#...#.......#.....#.#.#...#...#.#.#.....#.#.#.#.#.#.#.#.#.###...#...#####.....#.#.......#.#.#.###.#.#...#.###...#
+###.#.###.#.#.#####.#.#####.###.#.#############.#.###.###.#.#.#.#####.#.#.#.#.#.#.#.#.#####.###.###########.#########.#.#.###.#.#.###.#####.#
+#...#.#...#.#.....#.#.#...#...#.#.............#.#...#...#.#.#.#.#...#.#.#.#.#.#.#.#.#.....#...#.....#.......#...#...#.#.#...#.#.#.#...#.....#
+#.###.#.###.#####.#.#.#.#.###.#.#############.#.###.###.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#####.###.#####.#.#######.#.#.#.#.#.###.#.#.#.#.###.#####
+#.....#.....#####...#...#.....#...............#.....###...#...#...#...#...#...#...#.......###.......#.........#...#...#.....#...#...###.....#
+###########################################################################################################################################.#

day-23/src/bin/part1.rs

@@ -0,0 +1,176 @@
+use std::collections::{HashSet, VecDeque};
+
+fn main() {
+    println!("{}", part1(include_str!("./input.txt")));
+}
+
+fn part1(input: &str) -> usize {
+    let map = input
+        .lines()
+        .map(|line| line.chars().map(Tile::from).collect())
+        .collect::<Vec<Vec<_>>>();
+
+    let start = map
+        .get(0)
+        .and_then(|row| row.iter().position(|tile| *tile == Tile::Path))
+        .map(|x| (x, 0_usize))
+        .expect("No start found");
+
+    let mut visited = HashSet::new();
+    visited.insert(start);
+
+    let mut max_steps = Vec::new();
+
+    let mut queue = VecDeque::from(vec![(start, visited, 0)]);
+
+    while let Some((coord, visited, steps)) = queue.pop_front() {
+        let next = successors(coord, &map, &visited);
+
+        if next.is_empty() {
+            max_steps.push(steps);
+        } else {
+            queue.extend(
+                next.into_iter()
+                    .map(|(coord, visited)| (coord, visited, steps + 1)),
+            );
+        }
+    }
+
+    max_steps.into_iter().max().unwrap_or_default()
+}
+
+type Coord = (usize, usize);
+
+fn successors(
+    (x, y): Coord,
+    map: &[Vec<Tile>],
+    visited: &HashSet<(usize, usize)>,
+) -> Vec<(Coord, HashSet<Coord>)> {
+    let above = y
+        .checked_sub(1)
+        .and_then(|yn| map.get(yn))
+        .and_then(|row| row.get(x));
+    let below = map.get(y + 1).and_then(|row| row.get(x));
+    let left = x
+        .checked_sub(1)
+        .and_then(|xn| map.get(y).and_then(|row| row.get(xn)));
+    let right = map.get(y).and_then(|row| row.get(x + 1));
+
+    let coords = match map
+        .get(y)
+        .and_then(|row| row.get(x))
+        .expect("Invalid position")
+    {
+        Tile::Forest => panic!("Forest"),
+        Tile::Path => {
+            let mut next = Vec::new();
+            match above {
+                Some(Tile::Forest) | None => {}
+                Some(_) => next.push((x, y - 1)),
+            }
+            match right {
+                Some(Tile::Forest) | None => {}
+                Some(_) => next.push((x + 1, y)),
+            }
+            match below {
+                Some(Tile::Forest) | None => {}
+                Some(_) => next.push((x, y + 1)),
+            }
+            match left {
+                Some(Tile::Forest) | None => {}
+                Some(_) => next.push((x - 1, y)),
+            }
+            next
+        }
+        Tile::SlopeUp => above
+            .is_some_and(|t| t != &Tile::Forest)
+            .then_some(vec![(x, y - 1)])
+            .unwrap_or_default(),
+        Tile::SlopeRight => right
+            .is_some_and(|t| t != &Tile::Forest)
+            .then_some(vec![(x + 1, y)])
+            .unwrap_or_default(),
+        Tile::SlopeDown => below
+            .is_some_and(|t| t != &Tile::Forest)
+            .then_some(vec![(x, y + 1)])
+            .unwrap_or_default(),
+        Tile::SlopeLeft => left
+            .is_some_and(|t| t != &Tile::Forest)
+            .then_some(vec![(x - 1, y)])
+            .unwrap_or_default(),
+    };
+
+    coords
+        .into_iter()
+        .filter(|coord| !visited.contains(coord))
+        .map(|coord| {
+            let mut visited = visited.clone();
+            visited.insert(coord);
+            (coord, visited)
+        })
+        .collect()
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum Tile {
+    Path,
+    Forest,
+    SlopeUp,
+    SlopeDown,
+    SlopeLeft,
+    SlopeRight,
+}
+
+impl From<char> for Tile {
+    fn from(value: char) -> Self {
+        match value {
+            '#' => Self::Forest,
+            '.' => Self::Path,
+            '>' => Self::SlopeRight,
+            '<' => Self::SlopeLeft,
+            '^' => Self::SlopeUp,
+            'v' => Self::SlopeDown,
+            _ => panic!("Invalid tile: {}", value),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use indoc::indoc;
+
+    #[test]
+    fn test_part1() {
+        assert_eq!(
+            part1(indoc!(
+                "
+                #.#####################
+                #.......#########...###
+                #######.#########.#.###
+                ###.....#.>.>.###.#.###
+                ###v#####.#v#.###.#.###
+                ###.>...#.#.#.....#...#
+                ###v###.#.#.#########.#
+                ###...#.#.#.......#...#
+                #####.#.#.#######.#.###
+                #.....#.#.#.......#...#
+                #.#####.#.#.#########v#
+                #.#...#...#...###...>.#
+                #.#.#v#######v###.###v#
+                #...#.>.#...>.>.#.###.#
+                #####v#.#.###v#.#.###.#
+                #.....#...#...#.#.#...#
+                #.#########.###.#.#.###
+                #...###...#...#...#.###
+                ###.###.#.###v#####v###
+                #...#...#.#.>.>.#.>.###
+                #.###.###.#.###.#.#v###
+                #.....###...###...#...#
+                #####################.#
+                "
+            )),
+            94
+        );
+    }
+}

day-23/src/bin/part2.rs

@@ -0,0 +1,233 @@
+fn main() {
+    println!("{}", part2(include_str!("./input.txt")));
+}
+
+fn part2(input: &str) -> usize {
+    let map = input
+        .lines()
+        .map(|line| line.chars().map(Tile::from).collect())
+        .collect::<Vec<Vec<_>>>();
+
+    let start_x = map
+        .first()
+        .and_then(|row| row.iter().position(|tile| *tile == Tile::Path))
+        .expect("No start found");
+    let end_x = map
+        .last()
+        .and_then(|row| row.iter().position(|tile| *tile == Tile::Path))
+        .expect("No end found");
+
+    let width = map[0].len();
+    let pos_to_index = move |(x, y)| x + y * width;
+    let index_to_pos = move |index| (index % width, index / width);
+
+    let mut node_map = vec![None; map.len() * map[0].len()];
+
+    *node_map.get_mut(start_x).unwrap() = Some(0);
+    *node_map
+        .get_mut(pos_to_index((end_x, map.len() - 1)))
+        .unwrap() = Some(1);
+
+    let start = (start_x, 0_usize);
+
+    let mut graph = vec![Node { edges: Vec::new() }; 2];
+    let mut directions_exited = vec![0; 2];
+    let mut open_set = vec![(0, start, Direction::Down)];
+    let mut next_steps = Vec::new();
+    let mut nearest_goal = 0;
+
+    while let Some((start_node, start_pos, start_direction)) = open_set.pop() {
+        // if already exited this node in this direction, skip it
+        if directions_exited[start_node] & start_direction.bit() != 0 {
+            continue;
+        }
+        let mut pos = start_direction.increment_position(start_pos);
+        let mut direction = start_direction;
+        let mut index = pos_to_index(pos);
+        let mut length = 1;
+
+        // walk until hitting a dead end or a node
+        loop {
+            if let Some(end_node) = node_map[index] {
+                if end_node == 1 {
+                    nearest_goal = start_node;
+                }
+                // save the new direction exited for the nodes
+                directions_exited[start_node] |= start_direction.bit();
+                directions_exited[end_node] |= direction.invert().bit();
+                // add the edge to the graph
+                graph[start_node].edges.push((end_node, length));
+                graph[end_node].edges.push((start_node, length));
+                break;
+            }
+
+            // check every direction
+            for new_direction in Direction::ALL {
+                // don't go back the same way
+                if new_direction.invert() == direction {
+                    continue;
+                }
+                // go one step in the new direction
+                let new_pos = new_direction.increment_position(pos);
+                let new_index = pos_to_index(new_pos);
+                let (new_index_x, new_index_y) = index_to_pos(new_index);
+                // if not a path, skip it
+                if map[new_index_y][new_index_x] == Tile::Forest {
+                    continue;
+                }
+                next_steps.push((new_index, new_pos, new_direction));
+            }
+
+            // if only one direction is available, continue walking
+            if next_steps.len() == 1 {
+                let (new_index, new_pos, new_direction) = next_steps.pop().unwrap();
+                index = new_index;
+                pos = new_pos;
+                direction = new_direction;
+                length += 1;
+            } else {
+                // otherwise, create a new node
+                let end_node = graph.len();
+                node_map[index] = Some(end_node);
+                graph.push(Node { edges: Vec::new() });
+                directions_exited.push(0);
+                directions_exited[start_node] |= start_direction.bit();
+                directions_exited[end_node] |= direction.invert().bit();
+                graph[start_node].edges.push((end_node, length));
+                graph[end_node].edges.push((start_node, length));
+
+                open_set.extend(
+                    next_steps
+                        .iter()
+                        .map(|(_, _, direction)| (end_node, pos, *direction)),
+                );
+                next_steps.clear();
+                break;
+            }
+        }
+    }
+
+    longest_path(&graph, 0, nearest_goal, 1, &mut Vec::new()).unwrap()
+}
+
+fn longest_path(
+    graph: &Vec<Node>,
+    current_node: usize,
+    nearest_goal: usize,
+    goal: usize,
+    visited: &mut Vec<usize>,
+) -> Option<usize> {
+    graph[current_node]
+        .edges
+        .iter()
+        .filter_map(|&(next_node, length)| {
+            if next_node == goal {
+                Some(length)
+            } else if current_node == nearest_goal && next_node != goal
+                || visited.contains(&next_node)
+            {
+                None
+            } else {
+                visited.push(next_node);
+                let result = longest_path(graph, next_node, nearest_goal, goal, visited);
+                visited.pop();
+                result.map(|x| x + length)
+            }
+        })
+        .max()
+}
+
+type Coord = (usize, usize);
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+enum Direction {
+    Up,
+    Right,
+    Down,
+    Left,
+}
+impl Direction {
+    const ALL: [Self; 4] = [Self::Up, Self::Right, Self::Down, Self::Left];
+
+    fn invert(self) -> Self {
+        match self {
+            Self::Up => Self::Down,
+            Self::Right => Self::Left,
+            Self::Down => Self::Up,
+            Self::Left => Self::Right,
+        }
+    }
+    fn increment_position(&self, (x, y): Coord) -> Coord {
+        match self {
+            Direction::Up => (x, y - 1),
+            Direction::Right => (x + 1, y),
+            Direction::Down => (x, y + 1),
+            Direction::Left => (x - 1, y),
+        }
+    }
+
+    fn bit(&self) -> u8 {
+        1 << (*self as u8)
+    }
+}
+
+#[derive(Debug, Clone)]
+struct Node {
+    edges: Vec<(usize, usize)>,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum Tile {
+    Path,
+    Forest,
+}
+
+impl From<char> for Tile {
+    fn from(value: char) -> Self {
+        match value {
+            '#' => Self::Forest,
+            '.' | '>' | '<' | '^' | 'v' => Self::Path,
+            _ => panic!("Invalid tile: {}", value),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use indoc::indoc;
+
+    #[test]
+    fn test_part2() {
+        assert_eq!(
+            part2(indoc!(
+                "
+                #.#####################
+                #.......#########...###
+                #######.#########.#.###
+                ###.....#.>.>.###.#.###
+                ###v#####.#v#.###.#.###
+                ###.>...#.#.#.....#...#
+                ###v###.#.#.#########.#
+                ###...#.#.#.......#...#
+                #####.#.#.#######.#.###
+                #.....#.#.#.......#...#
+                #.#####.#.#.#########v#
+                #.#...#...#...###...>.#
+                #.#.#v#######v###.###v#
+                #...#.>.#...>.>.#.###.#
+                #####v#.#.###v#.#.###.#
+                #.....#...#...#.#.#...#
+                #.#########.###.#.#.###
+                #...###...#...#...#.###
+                ###.###.#.###v#####v###
+                #...#...#.#.>.>.#.>.###
+                #.###.###.#.###.#.#v###
+                #.....###...###...#...#
+                #####################.#
+                "
+            )),
+            154
+        );
+    }
+}