Add solution for day 10

Author: slgeay

README.md

@@ -34,8 +34,9 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 7](./src/bin/07.rs) | `2.3ms` | `928.1ms` |
 | [Day 8](./src/bin/08.rs) | `39.3µs` | `85.2µs` |
 | [Day 9](./src/bin/09.rs) | `8.1ms` | `9.3ms` |
+| [Day 10](./src/bin/10.rs) | `1.6ms` | `1.4ms` |
 
-**Total: 1044.82ms**
+**Total: 1047.82ms**
 <!--- benchmarking table --->
 
 ---

data/examples/10.txt

@@ -0,0 +1,8 @@
+89010123
+78121874
+87430965
+96549874
+45678903
+32019012
+01329801
+10456732

src/bin/10.rs

@@ -0,0 +1,70 @@
+use std::collections::HashSet;
+
+advent_of_code::solution!(10);
+
+pub fn solution<T>(input: &str) -> Option<u64>
+    where T: FromIterator<(usize, usize)> + IntoIterator<Item = (usize, usize)> + Clone,
+
+{
+    let w = input.find('\n').unwrap() + 1;
+
+    let positions = input
+        .chars()
+        .enumerate()
+        .filter_map(|(i, c)| match c {
+            '.' | '\n' => None,
+            _ => Some((c.to_digit(10).unwrap() as u8, (i % w, i / w))),
+        })
+        .fold(
+            std::collections::HashMap::new(),
+            |mut acc: std::collections::HashMap<u8, HashSet<_>>, (c, (x, y))| {
+                acc.entry(c).or_default().insert((x, y));
+                acc
+            },
+        );
+
+    let mut reachable: std::collections::HashMap<(usize, usize), T> = std::collections::HashMap::new();
+    positions[&9].iter().for_each(|&(x, y)| {
+        reachable.insert((x, y), T::from_iter([(x, y)]));
+    });
+
+    for i in (0..=8).rev() {
+        for &(x, y) in positions[&i].iter() {
+            reachable.insert(
+                (x, y),
+                positions[&(i + 1)]
+                    .iter()
+                    .filter(|&&(nx, ny)| x.abs_diff(nx) + y.abs_diff(ny) == 1)
+                    .flat_map(|&(nx, ny)| reachable[&(nx, ny)].clone().into_iter())
+                    .collect(),
+            );
+        }
+    }
+
+    Some(positions[&0].iter().map(|&(x, y)| reachable[&(x, y)].clone().into_iter().count() as u64).sum())
+}
+
+pub fn part_one(input: &str) -> Option<u64> {
+    solution::<HashSet<_>>(input)
+}
+
+pub fn part_two(input: &str) -> Option<u64> {
+    solution::<Vec<_>>(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(36));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(81));
+    }
+}