From 172c89a6eb08d28b5bc01797f9d1230fceae51c2 Mon Sep 17 00:00:00 2001
From: Bauke <me@bauke.xyz>
Date: Thu, 9 Dec 2021 13:12:49 +0100
Subject: [PATCH] Solve day 9!

---
 source/day_09/mod.rs | 158 +++++++++++++++++++++++++++++++++++++++++++
 source/main.rs       |   2 +
 2 files changed, 160 insertions(+)
 create mode 100644 source/day_09/mod.rs

diff --git a/source/day_09/mod.rs b/source/day_09/mod.rs
new file mode 100644
index 0000000..ed8454a
--- /dev/null
+++ b/source/day_09/mod.rs
@@ -0,0 +1,158 @@
+use std::collections::HashMap;
+
+use color_eyre::{eyre::eyre, Result};
+use itertools::Itertools;
+
+pub fn solve() -> Result<()> {
+  let input_data = include_str!("../../data/day_09.txt").trim();
+  println!("Day 09 Part 1: {}", part_1(input_data)?);
+  println!("Day 09 Part 2: {}", part_2(input_data)?);
+  Ok(())
+}
+
+type Coordinate = (isize, isize);
+
+type HeightMap = HashMap<Coordinate, isize>;
+
+const ADJACENT_OFFSETS: &[Coordinate] = &[
+  (-1, 00), // Left
+  (01, 00), // Right
+  (00, -1), // Up
+  (00, 01), // Down
+];
+
+fn parse_heightmap(input: &str) -> Result<HeightMap> {
+  let mut height_map = HeightMap::new();
+
+  for (y, line) in input.lines().enumerate() {
+    for (x, height) in line.char_indices() {
+      let height = height
+        .to_digit(10)
+        .ok_or(eyre!("Invalid input: {}", line))?
+        .try_into()?;
+
+      height_map.insert((x.try_into()?, y.try_into()?), height);
+    }
+  }
+
+  Ok(height_map)
+}
+
+fn parse_map_bounds(input: &str) -> Result<Coordinate> {
+  Ok((
+    input
+      .lines()
+      .next()
+      .map(|line| line.chars().count())
+      .ok_or(eyre!("Invalid input: {}", input))?
+      .try_into()?,
+    input.lines().count().try_into()?,
+  ))
+}
+
+fn discover_basins(
+  coordinate: Coordinate,
+  discovered_coordinates: &mut Vec<Coordinate>,
+  height_map: &HeightMap,
+) -> Vec<Coordinate> {
+  let mut coordinates = vec![];
+
+  for (x_offset, y_offset) in ADJACENT_OFFSETS {
+    let neighbor = (x_offset + coordinate.0, y_offset + coordinate.1);
+    if discovered_coordinates.contains(&neighbor) {
+      continue;
+    }
+
+    discovered_coordinates.push(neighbor);
+    match height_map.get(&neighbor) {
+      Some(9) | None => continue,
+      Some(_) => {
+        coordinates.push(neighbor);
+        coordinates.append(&mut discover_basins(
+          neighbor,
+          discovered_coordinates,
+          height_map,
+        ));
+      }
+    }
+  }
+
+  coordinates
+}
+
+fn part_1(input: &str) -> Result<isize> {
+  let height_map = parse_heightmap(input)?;
+  let map_bounds = parse_map_bounds(input)?;
+  let mut risk_level = 0;
+
+  for y in 0..map_bounds.1 {
+    'x_loop: for x in 0..map_bounds.0 {
+      let coordinate = (x, y);
+      let height = height_map
+        .get(&coordinate)
+        .ok_or(eyre!("Coordinate {:?} not found", coordinate))?;
+
+      for (x_offset, y_offset) in ADJACENT_OFFSETS {
+        let neighbor = (x_offset + x, y_offset + y);
+
+        if let Some(neighbor_height) = height_map.get(&neighbor) {
+          if neighbor_height <= height {
+            continue 'x_loop;
+          }
+        }
+      }
+
+      risk_level += 1 + height;
+    }
+  }
+
+  Ok(risk_level)
+}
+
+fn part_2(input: &str) -> Result<usize> {
+  let height_map = parse_heightmap(input)?;
+  let map_bounds = parse_map_bounds(input)?;
+  let mut basins = vec![];
+  let mut discovered_coordinates = vec![];
+  for y in 0..map_bounds.1 {
+    'x_loop: for x in 0..map_bounds.0 {
+      let coordinate = (x, y);
+      if discovered_coordinates.contains(&coordinate) {
+        continue 'x_loop;
+      }
+
+      let height = height_map
+        .get(&coordinate)
+        .ok_or(eyre!("Coordinate {:?} not found", coordinate))?;
+      if height == &9 {
+        continue 'x_loop;
+      }
+
+      let mut basin = vec![coordinate];
+      discovered_coordinates.push(coordinate);
+      basin.append(&mut discover_basins(
+        coordinate,
+        &mut discovered_coordinates,
+        &height_map,
+      ));
+      basins.push(basin.len());
+      discovered_coordinates.append(&mut basin);
+    }
+  }
+
+  Ok(
+    basins
+      .into_iter()
+      .sorted_by(|a, b| b.cmp(a))
+      .take(3)
+      .product(),
+  )
+}
+
+#[test]
+fn test_sample() -> Result<()> {
+  let sample = "2199943210\n3987894921\n9856789892\n8767896789\n9899965678";
+  assert_eq!(part_1(sample)?, 15);
+  assert_eq!(part_2(sample)?, 1134);
+  Ok(())
+}
diff --git a/source/main.rs b/source/main.rs
index d44f3b3..fecf887 100644
--- a/source/main.rs
+++ b/source/main.rs
@@ -10,6 +10,7 @@ mod day_05;
 mod day_06;
 mod day_07;
 mod day_08;
+mod day_09;
 
 fn main() -> Result<()> {
   color_eyre::install()?;
@@ -25,6 +26,7 @@ fn main() -> Result<()> {
     day_06::solve,
     day_07::solve,
     day_08::solve,
+    day_09::solve,
   ];
 
   for day in days {