day 23

README.md

@@ -60,8 +60,9 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 20](./src/bin/20.rs) | `4.3ms` | `18.2ms` |
 | [Day 21](./src/bin/21.rs) | `1.5ms` | `70.1ms` |
 | [Day 22](./src/bin/22.rs) | `9.5ms` | `17.6ms` |
+| [Day 23](./src/bin/23.rs) | `2.6ms` | `3.1s` |
 
-**Total: 46608.91ms**
+**Total: 49711.51ms**
 <!--- benchmarking table --->
 
 ---

src/bin/23.rs

@@ -0,0 +1,157 @@
+use std::cmp::max;
+use std::collections::{HashSet, HashMap};
+
+advent_of_code::solution!(23);
+
+#[derive(PartialEq)]
+enum Tile {
+    Path,
+    Forest,
+    SlopeUp,
+    SlopeLeft,
+    SlopeDown,
+    SlopeRight
+}
+
+type Map = Vec<Vec<Tile>>;
+
+#[derive(Debug, PartialEq, Eq, Hash, Clone, Copy)]
+struct Coord(isize, isize);
+
+type Graph = HashMap<Coord, HashMap<Coord, usize>>;
+
+impl Coord {
+    fn neighbours(&self, map: &Map) -> Vec<Coord> {
+        let Coord(x, y) = *self;
+
+        let ns: Vec<(isize, isize)> = match map[y as usize][x as usize] {
+            Tile::Path => vec![(x, y - 1), (x - 1, y), (x, y + 1), (x + 1, y)],
+            Tile::Forest => vec![],
+            Tile::SlopeUp => vec![(x, y - 1)],
+            Tile::SlopeLeft => vec![(x - 1, y)],
+            Tile::SlopeDown => vec![(x, y + 1)],
+            Tile::SlopeRight => vec![(x + 1, y)],
+        };
+
+        ns.into_iter()
+          .filter(|&(x,y)| x >= 0 && y >= 0 && x < map[0].len() as isize && y < map.len() as isize)
+          .filter(|&(x,y)| map[y as usize][x as usize] != Tile::Forest)
+          .map(|(x, y)| Coord(x, y))
+          .collect()
+    }
+}
+
+fn parse(input: &str) -> Map {
+    input.lines()
+         .map(|l| l.chars().map(|c| match c {
+             '.' => Tile::Path,
+             '#' => Tile::Forest,
+             '^' => Tile::SlopeUp,
+             '<' => Tile::SlopeLeft,
+             'v' => Tile::SlopeDown,
+             '>' => Tile::SlopeRight,
+             _   => panic!("Invalid char")
+         }).collect())
+         .collect()
+}
+
+fn build_graph(start: Coord, end: Coord, map: &Map) -> Graph {
+    let mut graph: Graph = HashMap::new();
+
+    let crossroads: Vec<Coord> = (0..map.len()).flat_map(|r|
+                                                         (0..map[0].len()).map(move |c| Coord(c as isize, r as isize)))
+                                               .filter(|c| c.neighbours(map).len() >= 3)
+                                               .chain([start, end])
+                                               .collect();
+
+    for c in crossroads.iter().cloned() {
+        graph.insert(c, HashMap::new());
+    }
+
+    for c in crossroads.iter().cloned() {
+        let mut stack: Vec<(usize, Coord)> = vec![(0, c)];
+        let mut seen: HashSet<Coord> = HashSet::new();
+        seen.insert(c);
+
+        while let Some((distance, coords)) = stack.pop() {
+            if distance != 0 && crossroads.contains(&coords) {
+                graph.get_mut(&c).unwrap().insert(coords, distance);
+                continue;
+            }
+
+            for n in coords.neighbours(map) {
+                if !seen.contains(&n) {
+                    stack.push((distance + 1, n));
+                    seen.insert(n);
+                }
+            }
+        }
+    }
+
+    graph
+}
+
+fn longest_path(start: Coord, end: Coord, visited: &mut Vec<Coord>, graph: &Graph) -> Option<usize> {
+    if start == end {
+        return Some(0);
+    }
+
+    let mut longest: Option<usize> = None;
+
+    visited.push(start);
+
+    for (coord, distance) in &graph[&start] {
+        if !visited.contains(coord) {
+            longest = max(longest, longest_path(*coord, end, visited, graph).map(|v| v + distance));
+        }
+    }
+
+    visited.pop();
+
+    longest
+}
+
+fn solve(input: &str) -> Option<u32> {
+    let data = parse(input);
+
+    let starting_col = data[0].iter().position(|t| *t == Tile::Path).unwrap();
+    let ending_col = data[data.len() - 1].iter().position(|t| *t == Tile::Path).unwrap();
+
+    let graph = build_graph(
+        Coord(starting_col as isize, 0),
+        Coord(ending_col as isize, data.len() as isize - 1),
+        &data);
+
+    longest_path(
+        Coord(starting_col as isize, 0),
+        Coord(ending_col as isize, data.len() as isize - 1),
+        &mut vec![],
+        &graph
+    ).map(|v| v as u32)
+}
+
+pub fn part_one(input: &str) -> Option<u32> {
+    solve(input)
+}
+
+pub fn part_two(input: &str) -> Option<u32> {
+    let new_input = input.replace("^", ".").replace("<", ".").replace("v", ".").replace(">", ".");
+    solve(&new_input)
+}
+
+#[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(94));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(154));
+    }
+}