3 files changed, 156 insertions(+), 0 deletions(-)

A input/day10/example
A src/day10/mod.rs
M src/main.rs

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

@@ 0,0 1,140 @@
+use std::{
+    collections::HashMap,
+    fs::File,
+    io::{BufRead, BufReader},
+    path::Path,
+};
+
+pub fn solve(input: &Path) -> anyhow::Result<()> {
+    println!("part one: {}", part_one(input)?);
+    println!("part two: {}", part_two(input)?);
+
+    Ok(())
+}
+
+fn navigate_one(
+    map: &[Vec<u8>],
+    h: u8,
+    p: (&usize, &usize),
+    nines: &mut HashMap<(usize, usize), ()>,
+) {
+    let mut around_us = vec![];
+    if *p.0 > 0 {
+        let (y, x) = (p.0 - 1, *p.1);
+        around_us.push((map[y][x], y, x));
+    }
+    if let Some(row) = map.get(p.0 + 1) {
+        let (y, x) = (p.0 + 1, *p.1);
+        around_us.push((row[x], y, x));
+    }
+    if *p.1 > 0 {
+        let (y, x) = (*p.0, p.1 - 1);
+        around_us.push((map[y][x], y, x));
+    }
+    if let Some(height) = map[*p.0].get(p.1 + 1) {
+        let (y, x) = (*p.0, p.1 + 1);
+        around_us.push((*height, y, x));
+    }
+
+    for (n, y, x) in around_us {
+        if h == 8 && n == 9 {
+            nines.insert((y, x), ());
+        } else if n == h + 1 {
+            navigate_one(map, h + 1, (&y, &x), nines);
+        }
+    }
+}
+
+fn part_one(input: &Path) -> anyhow::Result<usize> {
+    let reader = BufReader::new(File::open(input)?);
+
+    let mut map: Vec<Vec<u8>> = vec![];
+    for line in reader.lines() {
+        map.push(
+            line?
+                .chars()
+                .map(|c| -> u8 { c.to_string().parse().unwrap() })
+                .collect(),
+        );
+    }
+    let trailheads: Vec<(usize, usize)> = map
+        .iter()
+        .enumerate()
+        .flat_map(|(y, row)| -> Vec<(usize, usize)> {
+            row.iter()
+                .enumerate()
+                .filter_map(|(x, n)| if *n == 0 { Some((y, x)) } else { None })
+                .collect()
+        })
+        .collect();
+
+    let mut answer = 0;
+    for (head_y, head_x) in &trailheads {
+        let mut nines: HashMap<(usize, usize), ()> = HashMap::new();
+        navigate_one(&map, 0, (head_y, head_x), &mut nines);
+        answer += nines.len();
+    }
+
+    Ok(answer)
+}
+
+fn navigate_two(map: &[Vec<u8>], h: u8, p: (&usize, &usize)) -> i32 {
+    let mut nines = 0;
+    let mut around_us = vec![];
+    if *p.0 > 0 {
+        let (y, x) = (p.0 - 1, *p.1);
+        around_us.push((map[y][x], y, x));
+    }
+    if let Some(row) = map.get(p.0 + 1) {
+        let (y, x) = (p.0 + 1, *p.1);
+        around_us.push((row[x], y, x));
+    }
+    if *p.1 > 0 {
+        let (y, x) = (*p.0, p.1 - 1);
+        around_us.push((map[y][x], y, x));
+    }
+    if let Some(height) = map[*p.0].get(p.1 + 1) {
+        let (y, x) = (*p.0, p.1 + 1);
+        around_us.push((*height, y, x));
+    }
+
+    for (n, y, x) in around_us {
+        if h == 8 && n == 9 {
+            nines += 1;
+        } else if n == h + 1 {
+            nines += navigate_two(map, h + 1, (&y, &x));
+        }
+    }
+    nines
+}
+
+fn part_two(input: &Path) -> anyhow::Result<i32> {
+    let reader = BufReader::new(File::open(input)?);
+
+    let mut map: Vec<Vec<u8>> = vec![];
+    for line in reader.lines() {
+        map.push(
+            line?
+                .chars()
+                .map(|c| -> u8 { c.to_string().parse().unwrap() })
+                .collect(),
+        );
+    }
+    let trailheads: Vec<(usize, usize)> = map
+        .iter()
+        .enumerate()
+        .flat_map(|(y, row)| -> Vec<(usize, usize)> {
+            row.iter()
+                .enumerate()
+                .filter_map(|(x, n)| if *n == 0 { Some((y, x)) } else { None })
+                .collect()
+        })
+        .collect();
+
+    let mut answer = 0;
+    for (head_y, head_x) in &trailheads {
+        answer += navigate_two(&map, 0, (head_y, head_x));
+    }
+
+    Ok(answer)
+}

@@ 3,6 3,7 @@ use std::path::PathBuf;
 use clap::{Parser, Subcommand};
 
 mod day1;
+mod day10;
 mod day2;
 mod day3;
 mod day4;


@@ 56,6 57,10 @@ enum DayArgs {
         #[arg(short)]
         input: PathBuf,
     },
+    Day10 {
+        #[arg(short)]
+        input: PathBuf,
+    },
 }
 
 fn main() -> anyhow::Result<()> {


@@ 89,6 94,9 @@ fn main() -> anyhow::Result<()> {
         DayArgs::Day9 { input } => {
             day9::solve(&input)?;
         }
+        DayArgs::Day10 { input } => {
+            day10::solve(&input)?;
+        }
     }
 
     Ok(())