Added Solution for 2021 day 09

2021/day09_smoke_basin/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "day09_smoke_basin"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

2021/day09_smoke_basin/challenge.txt

@@ -0,0 +1,91 @@
+These caves seem to be [lava tubes](https://en.wikipedia.org/wiki/Lava_tube). Parts are even still volcanically active; small hydrothermal vents release smoke into the caves that slowly settles like rain.
+
+If you can model how the smoke flows through the caves, you might be able to avoid it and be that much safer. The submarine generates a heightmap of the floor of the nearby caves for you (your puzzle input).
+
+Smoke flows to the lowest point of the area it's in. For example, consider the following heightmap:
+
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+
+```
+
+Each number corresponds to the height of a particular location, where `9` is the highest and `0` is the lowest a location can be.
+
+Your first goal is to find the *low points* - the locations that are lower than any of its adjacent locations. Most locations have four adjacent locations (up, down, left, and right); locations on the edge or corner of the map have three or two adjacent locations, respectively. (Diagonal locations do not count as adjacent.)
+
+In the above example, there are *four* low points, all highlighted: two are in the first row (a `1` and a `0`), one is in the third row (a `5`), and one is in the bottom row (also a `5`). All other locations on the heightmap have some lower adjacent location, and so are not low points.
+
+The *risk level* of a low point is *1 plus its height*. In the above example, the risk levels of the low points are `2`, `1`, `6`, and `6`. The sum of the risk levels of all low points in the heightmap is therefore `*15*`.
+
+Find all of the low points on your heightmap. *What is the sum of the risk levels of all low points on your heightmap?*
+
+Your puzzle answer was `524`.
+
+\--- Part Two ---
+----------
+
+Next, you need to find the largest basins so you know what areas are most important to avoid.
+
+A *basin* is all locations that eventually flow downward to a single low point. Therefore, every low point has a basin, although some basins are very small. Locations of height `9` do not count as being in any basin, and all other locations will always be part of exactly one basin.
+
+The *size* of a basin is the number of locations within the basin, including the low point. The example above has four basins.
+
+The top-left basin, size `3`:
+
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+
+```
+
+The top-right basin, size `9`:
+
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+
+```
+
+The middle basin, size `14`:
+
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+
+```
+
+The bottom-right basin, size `9`:
+
+```
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+
+```
+
+Find the three largest basins and multiply their sizes together. In the above example, this is `9 * 14 * 9 = *1134*`.
+
+*What do you get if you multiply together the sizes of the three largest basins?*
+
+Your puzzle answer was `1235430`.
+
+Both parts of this puzzle are complete! They provide two gold stars: \*\*
+
+At this point, you should [return to your Advent calendar](/2021) and try another puzzle.
+
+If you still want to see it, you can [get your puzzle input](9/input).

2021/day09_smoke_basin/src/lib.rs

@@ -0,0 +1,113 @@
+use core::fmt::Display;
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum ParseError {
+    ParseIntError(char),
+}
+
+impl Display for ParseError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::ParseIntError(c) => write!(f, "Unable to parse {c} into integer"),
+        }
+    }
+}
+
+struct Map {
+    heights: Vec<Vec<usize>>,
+}
+
+impl TryFrom<&str> for Map {
+    type Error = ParseError;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        Ok(Self {
+            heights: value.lines().map(|line| line.chars().map(|c| c.to_digit(10).ok_or(Self::Error::ParseIntError(c)).map(|i| i as usize)).collect::<Result<Vec<_>, _>>()).collect::<Result<Vec<_>, _>>()?,
+        })
+    }
+}
+
+impl Map {
+    fn neighbours(&self, (x, y): (usize, usize)) -> Vec<((usize, usize), usize)> {
+        [(1, 0), (0, 1), (2, 1), (1, 2)].iter().filter(|(dx, dy)| (1..=self.heights[0].len()).contains(&(x+dx)) && (1..=self.heights.len()).contains(&(y+dy))).map(|(dx, dy)| ((x+dx-1, y+dy-1), self.heights[y+dy-1][x+dx-1])).collect()
+    }
+
+    fn is_local_low_point(&self, (x, y): (usize, usize)) -> bool {
+        let this_height = self.heights[y][x];
+
+        self.neighbours((x, y)).iter().all(|(_coords, height)| height > &this_height)
+    }
+
+    fn get_local_low_points(&self) -> Vec<usize> {
+        self.heights.iter()
+                    .enumerate()
+                    .flat_map(|(y, row)| row.iter()
+                                            .enumerate()
+                                            .filter(|(x, _height)| self.is_local_low_point((*x, y)))
+                                            .map(|(_x, height)| *height)
+                                            .collect::<Vec<_>>())
+                    .collect()
+    }
+}
+
+pub fn run(input: &str) -> Result<(usize, usize), ParseError> {
+    let mut map = Map::try_from(input)?;
+    let first = map.get_local_low_points().iter().map(|height| height+1).sum();
+    let second = get_largest_basin(&mut map);
+    Ok((first, second))
+}
+
+fn get_largest_basin(map: &mut Map) -> usize {
+    let height = map.heights.len();
+    let width = map.heights[0].len();
+
+    let mut basins = Vec::new();
+
+    // While it's tempting to only iterate over the local lows from step 1, we cannot garranty that
+    // every basin contains a local low point (there may be multiple consecutive points forming a
+    // valley of equal height). Therefore we iterate over the entire array.
+    (0..height).for_each(|y| {
+        (0..width).for_each(|x| {
+            let mut this_basin = 0;
+
+            let mut open_set = vec![(x, y)];
+            while let Some((x, y)) = open_set.pop() {
+                if map.heights[y][x] == 9 {
+                    continue;
+                }
+                this_basin += 1;
+                map.heights[y][x] = 9;
+
+                for neighbour in map.neighbours((x, y)) {
+                    open_set.push(neighbour.0);
+                }
+            }
+            basins.push(this_basin);
+        });
+    });
+
+    basins.sort_by(|a, b| b.cmp(&a));
+    basins.iter().take(3).product()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs::read_to_string;
+
+    fn read_file(name: &str) -> String {
+        read_to_string(name).expect(&format!("Unable to read file: {name}")[..]).trim().to_string()
+    }
+
+    #[test]
+    fn test_sample() {
+        let sample_input = read_file("tests/sample_input");
+        assert_eq!(run(&sample_input), Ok((15, 1134)));
+    }
+
+    #[test]
+    fn test_challenge() {
+        let challenge_input = read_file("tests/challenge_input");
+        assert_eq!(run(&challenge_input), Ok((524, 1235430)));
+    }
+}

2021/day09_smoke_basin/tests/challenge_input

@@ -0,0 +1,100 @@
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2021/day09_smoke_basin/tests/sample_input

@@ -0,0 +1,5 @@
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678