day 23: bugfix: fix dfs

aoc23/src/main.rs

@@ -20,18 +20,33 @@ fn parse_input<T: AsRef<str>>(input: T) -> Vec<Vec<char>> {
         .collect()
 }
 
-fn dfs(pos: Coord, trails: &[Vec<char>], visited: &mut HashSet<Coord>) -> usize {
-    if pos.1 == trails.len() - 1 {
-        0
+fn dfs(
+    pos: Coord,
+    trails: &[Vec<char>],
+    visited: &mut HashSet<Coord>,
+    part2: bool,
+) -> Option<usize> {
+    if pos.0 == trails.len() - 1 {
+        if trails[pos.0][pos.1] == '.' {
+            Some(0)
+        } else {
+            None
+        }
     } else {
         let (x, y) = pos;
-        let possible = match trails[x][y] {
-            '.' => vec![(-1, 0), (1, 0), (0, -1), (0, 1)],
-            '>' => vec![(0, 1)],
-            '<' => vec![(0, -1)],
-            '^' => vec![(-1, 0)],
-            'v' => vec![(1, 0)],
-            _ => return 0,
+        let possible = if !part2 {
+            match trails[x][y] {
+                '.' => vec![(-1, 0), (1, 0), (0, -1), (0, 1)],
+                '>' => vec![(0, 1)],
+                '<' => vec![(0, -1)],
+                '^' => vec![(-1, 0)],
+                'v' => vec![(1, 0)],
+                _ => return None,
+            }
+        } else if trails[x][y] != '#' {
+            vec![(-1, 0), (1, 0), (0, -1), (0, 1)]
+        } else {
+            return None;
         };
         let mut count = 0;
         for (dx, dy) in possible {
@@ -43,19 +58,25 @@ fn dfs(pos: Coord, trails: &[Vec<char>], visited: &mut HashSet<Coord>) -> usize
             let nx = nx as usize;
             let ny = ny as usize;
             if trails[nx][ny] != '#' && visited.insert((nx, ny)) {
-                count = count.max(1 + dfs((nx, ny), trails, visited));
+                if let Some(remain) = dfs((nx, ny), trails, visited, part2) {
+                    count = count.max(1 + remain);
+                }
                 visited.remove(&(nx, ny));
             }
         }
-        count
+        if count == 0 {
+            None
+        } else {
+            Some(count)
+        }
     }
 }
 
 fn part1(trails: &[Vec<char>]) -> Result<usize> {
     let _start = Instant::now();
 
     let result = (0..trails[0].len())
-        .map(|y| dfs((0, y), trails, &mut HashSet::new()))
+        .filter_map(|y| dfs((0, y), trails, &mut HashSet::new(), false))
         .max()
         .unwrap();
 
@@ -64,13 +85,26 @@ fn part1(trails: &[Vec<char>]) -> Result<usize> {
     Ok(result)
 }
 
+fn part2(trails: &[Vec<char>]) -> Result<usize> {
+    let _start = Instant::now();
+
+    let result = (0..trails[0].len())
+        .filter_map(|y| dfs((0, y), trails, &mut HashSet::new(), true))
+        .max()
+        .unwrap();
+
+    writeln!(io::stdout(), "Part 2: {result}")?;
+    writeln!(io::stdout(), "> Time elapsed is: {:?}", _start.elapsed())?;
+    Ok(result)
+}
+
 fn main() -> Result<()> {
     let mut input = String::new();
     io::stdin().read_to_string(&mut input)?;
 
     let trails = parse_input(input);
     part1(&trails)?;
-    // part2()?;
+    part2(&trails)?;
     Ok(())
 }
 
@@ -101,12 +135,14 @@ fn example_input() {
 #####################.#";
     let trails = parse_input(input);
     assert_eq!(part1(&trails).unwrap(), 94);
+    assert_eq!(part2(&trails).unwrap(), 154);
 }
 
 #[test]
 fn real_input() {
     let input = std::fs::read_to_string("input/input.txt").unwrap();
     let trails = parse_input(input);
     assert_eq!(part1(&trails).unwrap(), 2394);
+    assert_eq!(part2(&trails).unwrap(), 2394);
     assert_eq!(2, 2);
 }