Solve Day 18, Part 2

Author: intrepidcoder

src/bin/day18_part2.rs

@@ -0,0 +1,83 @@
+use adventofcode::grid::Grid;
+use itertools::Itertools;
+use std::{
+    collections::VecDeque,
+    io::{self, BufRead},
+};
+
+fn main() {
+    let (x, y) = solve(io::stdin().lock(), 71, 1024);
+    println!("{x},{y}");
+}
+
+fn solve(input: impl BufRead, size: usize, bytes: usize) -> (usize, usize) {
+    let mut grid = Grid::new(size, size, '.');
+    input
+        .lines()
+        .map(|line| -> (usize, usize) {
+            line.expect("IO error")
+                .split(',')
+                .flat_map(|s| s.parse())
+                .collect_tuple()
+                .unwrap()
+        })
+        .enumerate()
+        .find(|&(i, (x, y))| {
+            let pos = grid.get_coords(x, y).unwrap();
+            grid.set(pos, '#');
+
+            i > bytes && !is_reachable(&grid)
+        })
+        .unwrap()
+        .1
+}
+
+fn is_reachable(grid: &Grid) -> bool {
+    let mut queue = VecDeque::new();
+    let mut dist = vec![usize::MAX; grid.len()];
+    dist[0] = 0;
+    queue.push_back(grid.pos(0));
+    while let Some(pos) = queue.pop_front() {
+        queue.extend(
+            pos.neighbors(grid)
+                .filter(|&next| next.value() == '.')
+                .filter(|&next| {
+                    let d = dist[next.index()];
+                    if d == usize::MAX {
+                        dist[next.index()] = dist[pos.index()] + 1;
+                        true
+                    } else {
+                        false
+                    }
+                }),
+        );
+    }
+
+    dist[grid.len() - 1] < usize::MAX
+}
+
+#[cfg(test)]
+mod test {
+    use super::solve;
+    use std::{
+        fs::File,
+        io::{self, BufReader},
+    };
+
+    fn open(path: &str) -> io::Result<BufReader<File>> {
+        let file = File::open(path)?;
+        Ok(BufReader::new(file))
+    }
+
+    #[test]
+    fn test_example() {
+        let input = open("input/day18/example.txt").expect("Missing file");
+        assert_eq!((6, 1), solve(input, 7, 12));
+    }
+
+    #[test]
+    fn test_input() {
+        let input = open("input/day18/input.txt").expect("Missing file");
+        assert_eq!((62, 6), solve(input, 71, 1024));
+    }
+}