Day 18

Author: prevostc

README.md

@@ -28,8 +28,9 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 15](./src/bin/15.rs) | `119.7µs` | `591.0µs` |
 | [Day 16](./src/bin/16.rs) | `158.9µs` | `202.6µs` |
 | [Day 17](./src/bin/17.rs) | `1.7µs` | `361.3µs` |
+| [Day 18](./src/bin/18.rs) | `85.5µs` | `60.2µs` |
 
-**Total: 13.45ms**
+**Total: 13.60ms**
 <!--- benchmarking table --->
 
 ---

data/examples/18.txt

@@ -0,0 +1,25 @@
+5,4
+4,2
+4,5
+3,0
+2,1
+6,3
+2,4
+1,5
+0,6
+3,3
+2,6
+5,1
+1,2
+5,5
+2,5
+6,5
+1,4
+0,4
+6,4
+1,1
+6,1
+1,0
+0,5
+1,6
+2,0

src/bin/18.rs

@@ -0,0 +1,135 @@
+use std::{collections::VecDeque, u32};
+
+use mygrid::{direction::ORTHOGONAL, grid::Grid, point::Point};
+
+advent_of_code::solution!(18);
+
+fn parse_points(input: &str) -> Vec<Point> {
+    input
+        .lines()
+        .filter(|line| !line.is_empty())
+        .map(|line| {
+            let (col, line) = line.split_once(',').unwrap();
+            Point::new(line.parse().unwrap(), col.parse().unwrap())
+        })
+        .collect()
+}
+
+fn fill_min_dst_grid(grid: &Grid<char>, min_dst: &mut Grid<u32>, start: (Point, u32), end: Point) {
+    let mut q = VecDeque::new();
+    q.push_back(start);
+
+    while let Some((current, dst)) = q.pop_front() {
+        min_dst[current] = min_dst[current].min(dst);
+        if current == end {
+            continue;
+        }
+        ORTHOGONAL
+            .iter()
+            .map(|&dir| current + dir)
+            .filter(|&p| grid.is_in_bounds(p))
+            .filter(|&p| grid[p] == '.')
+            .for_each(|p| {
+                if min_dst[p] > min_dst[current] + 1 {
+                    min_dst[p] = min_dst[current] + 1;
+                    q.push_back((p, min_dst[p]));
+                }
+            })
+    }
+}
+
+fn find_shortest_path(input: &str, width: usize, height: usize, take: usize) -> Option<u32> {
+    let mut grid = Grid::new(width, height, '.');
+    let points = parse_points(input);
+    points.iter().take(take).for_each(|&p| {
+        grid[p] = '#';
+    });
+
+    let start = Point::new(0, 0);
+    let end = Point::new(width as isize - 1, height as isize - 1);
+    let mut min_dst = Grid::new(width, height, u32::MAX);
+    min_dst[start] = 0;
+
+    fill_min_dst_grid(&grid, &mut min_dst, (start, 0), end);
+
+    Some(min_dst[end])
+}
+
+pub fn part_one(input: &str) -> Option<u32> {
+    find_shortest_path(input, 71, 71, 1024)
+}
+
+// work in reverse, fill them all in and find the first point that makes it possible to reach the end
+fn find_cutoff(input: &str, width: usize, height: usize, _initial_take: usize) -> Option<Point> {
+    let mut grid = Grid::new(width, height, '.');
+    let points = parse_points(input);
+
+    points.iter().for_each(|&p| {
+        grid[p] = '#';
+    });
+
+    let start = Point::new(0, 0);
+    let end = Point::new(width as isize - 1, height as isize - 1);
+    let mut min_dst = Grid::new(width, height, u32::MAX);
+    min_dst[start] = 0;
+
+    fill_min_dst_grid(&grid, &mut min_dst, (start, 0), end);
+
+    let mut idx = points.len();
+    while min_dst[end] == u32::MAX {
+        idx -= 1;
+
+        let p = points[idx];
+        grid[p] = '.';
+
+        let min_neighbour_dst = ORTHOGONAL
+            .iter()
+            .map(|&dir| p + dir)
+            .filter(|&p| grid.is_in_bounds(p))
+            .filter(|&p| grid[p] == '.')
+            .map(|p| min_dst[p])
+            .min()
+            .unwrap_or(u32::MAX);
+
+        if min_neighbour_dst == u32::MAX {
+            continue;
+        }
+        let dst = min_neighbour_dst + 1;
+
+        fill_min_dst_grid(&grid, &mut min_dst, (p, dst), end);
+    }
+
+    Some(points[idx])
+}
+
+pub fn part_two(input: &str) -> Option<String> {
+    let cutoff = find_cutoff(input, 71, 71, 1024);
+    cutoff.map(|p| format!("{},{}", p.column, p.line))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part_one() {
+        let result = find_shortest_path(
+            &advent_of_code::template::read_file("examples", DAY),
+            7,
+            7,
+            12,
+        );
+        assert_eq!(result, Some(22));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = find_cutoff(
+            &advent_of_code::template::read_file("examples", DAY),
+            7,
+            7,
+            12,
+        );
+        assert_eq!(result, Some(Point::new(1, 6)));
+    }
+}