Previous: Advent of Code 2020 – Day 2: validating passwords
I decided to take a shot at Advent of Code 2020 to exercise my Rust knowledge. Here’s the task for Day 3:
Day 3: Password Philosophy
(…)
Due to the local geology, trees in this area only grow on exact
integer coordinates in a grid. You make a map (your puzzle input) of
the open squares (.) and trees (#) you can see. For example:
..##.......
#...#...#..
.#....#..#.
..#.#...#.#
.#...##..#.
..#.##.....
.#.#.#....#
.#........#
#.##...#...
#...##....#
.#..#...#.#
These aren’t the only trees, though; due to something you read about
once involving arboreal genetics and biome stability, the same pattern
repeats to the right many times:
..##.........##.........##.........##.........##.........##....... --->
#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..
.#....#..#..#....#..#..#....#..#..#....#..#..#....#..#..#....#..#.
..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#
.#...##..#..#...##..#..#...##..#..#...##..#..#...##..#..#...##..#.
..#.##.......#.##.......#.##.......#.##.......#.##.......#.##..... --->
.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#
.#........#.#........#.#........#.#........#.#........#.#........#
#.##...#...#.##...#...#.##...#...#.##...#...#.##...#...#.##...#...
#...##....##...##....##...##....##...##....##...##....##...##....#
.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.# --->
You start on the open square (.) in the top-left corner and need to
reach the bottom (below the bottom-most row on your map).
The toboggan can only follow a few specific slopes (you opted for a
cheaper model that prefers rational numbers); start by counting all
the trees you would encounter for the slope right 3, down 1:
From your starting position at the top-left, check the position that
is right 3 and down 1. Then, check the position that is right 3 and
down 1 from there, and so on until you go past the bottom of the map.
The locations you’d check in the above example are marked here with
O where there was an open square and X where there was a tree:
..##.........##.........##.........##.........##.........##....... --->
#..O#...#..#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..
.#....X..#..#....#..#..#....#..#..#....#..#..#....#..#..#....#..#.
..#.#...#O#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#
.#...##..#..X...##..#..#...##..#..#...##..#..#...##..#..#...##..#.
..#.##.......#.X#.......#.##.......#.##.......#.##.......#.##..... --->
.#.#.#....#.#.#.#.O..#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#
.#........#.#........X.#........#.#........#.#........#.#........#
#.##...#...#.##...#...#.X#...#...#.##...#...#.##...#...#.##...#...
#...##....##...##....##...#X....##...##....##...##....##...##....#
.#..#...#.#.#..#...#.#.#..#...X.#.#..#...#.#.#..#...#.#.#..#...#.# --->
In this example, traversing the map using this slope would cause you
to encounter 7 trees.
Starting at the top-left corner of your map and following a slope of
right 3 and down 1, how many trees would you encounter?
(…)
Part Two
Time to check the rest of the slopes – you need to minimize the
probability of a sudden arboreal stop, after all.
Determine the number of trees you would encounter if, for each of the
following slopes, you start at the top-left corner and traverse the
map all the way to the bottom:
- Right 1, down 1.
- Right 3, down 1. (This is the slope you already checked.)
- Right 5, down 1.
- Right 7, down 1.
- Right 1, down 2.
In the above example, these slopes would find 2, 7, 3, 4, and
2 tree(s) respectively; multiplied together, these produce the
answer 336.
What do you get if you multiply together the number of trees
encountered on each of the listed slopes?
The full story can be found on the website.
src/day_3.rs
use {
anyhow::{anyhow, bail, ensure, Result},
itertools::Itertools,
ndarray::prelude::*,
std::io::{self, prelude::*},
};
pub const PATH: &str = "./data/day_3/input";
#(derive(Clone, Copy, Debug, Eq, Hash, PartialEq))
pub enum Pixel {
Empty,
Tree,
}
impl Pixel {
pub fn from_char(c: char) -> Result<Self> {
match c {
'.' => Ok(Self::Empty),
'#' => Ok(Self::Tree),
_ => bail!("invalid pixel"),
}
}
}
#(derive(Clone, Debug, Eq, Hash, PartialEq))
pub struct Terrain {
pixels: Array2<Pixel>,
}
impl Terrain {
pub fn parse_from<R: BufRead>(reader: R) -> Result<Self> {
TerrainParser::parse(reader.lines())
}
pub fn slope_count(&self, delta_x: usize, delta_y: usize) -> usize {
assert!(delta_y != 0, "delta_y is zero");
let pixels = &self.pixels;
(0..pixels.nrows())
.step_by(delta_y)
.zip((0..).step_by(delta_x).map(|x| x % pixels.ncols()))
.filter(|pos| pixels(*pos) == Pixel::Tree)
.count()
}
}
#(derive(Debug))
struct TerrainParser {
pixels: Vec<Pixel>,
width: usize,
height: usize,
}
impl TerrainParser {
fn parse<R: BufRead>(mut lines: io::Lines<R>) -> Result<Terrain> {
let first_line =
lines.next().ok_or_else(|| anyhow!("empty terrain"))??;
let mut parser = Self::parse_first_line(&first_line)?;
for line in lines {
parser = parser.parse_line(&line?)?;
}
let TerrainParser {
pixels,
width,
height,
} = parser;
Ok(Terrain {
pixels: Array2::from_shape_vec((height, width), pixels)?,
})
}
fn parse_first_line(line: &str) -> Result<Self> {
let pixels: Vec<_> =
line.chars().map(Pixel::from_char).try_collect()?;
let width = pixels.len();
ensure!(width != 0, "zero-width terrain");
Ok(Self {
pixels,
width,
height: 1,
})
}
fn parse_line(mut self, line: &str) -> Result<Self> {
let expected_len = self.pixels.len() + self.width;
self.pixels.reserve_exact(self.width);
itertools::process_results(
line.chars().map(Pixel::from_char),
|pixels| self.pixels.extend(pixels),
)?;
ensure!(self.pixels.len() == expected_len, "jagged terrain");
self.height += 1;
Ok(self)
}
}
#(cfg(test))
mod tests {
use {super::*, std::io::BufReader};
#(test)
fn pixel_from_char() {
assert_eq!(Pixel::from_char('.').unwrap(), Pixel::Empty);
assert_eq!(Pixel::from_char('#').unwrap(), Pixel::Tree);
assert!(Pixel::from_char(' ').is_err());
}
#(test)
fn terrain_parse_from() -> anyhow::Result<()> {
fn parse(input: &str) -> Result<Terrain> {
Terrain::parse_from(BufReader::new(input.as_bytes()))
}
let expected = Array2::from_shape_vec(
(3, 3),
(Pixel::Empty, Pixel::Tree)
.iter()
.copied()
.cycle()
.take(9)
.collect(),
)?;
assert_eq!(parse(".#.n#.#n.#.n")?.pixels, expected);
assert!(parse("").is_err());
assert!(parse(". #").is_err());
assert!(parse(".n##").is_err());
Ok(())
}
#(test)
fn terrain_slope_count() -> anyhow::Result<()> {
// .#.
// #.#
// .#.
// #.#
let pixels = Array2::from_shape_vec(
(4, 3),
(Pixel::Empty, Pixel::Tree)
.iter()
.copied()
.cycle()
.take(12)
.collect(),
)?;
let terrain = Terrain { pixels };
assert_eq!(terrain.slope_count(1, 1), 1);
assert_eq!(terrain.slope_count(2, 1), 3);
assert_eq!(terrain.slope_count(3, 1), 2);
assert_eq!(terrain.slope_count(1, 2), 1);
Ok(())
}
}
src/bin/day_3_1.rs
use {
anyhow::Result,
aoc_2020::day_3::{self as lib, Terrain},
std::{fs::File, io::BufReader},
};
fn main() -> Result<()> {
let file = BufReader::new(File::open(lib::PATH)?);
let count = Terrain::parse_from(file)?.slope_count(3, 1);
println!("{}", count);
Ok(())
}
src/bin/day_3_2.rs
use {
anyhow::Result,
aoc_2020::day_3::{self as lib, Terrain},
std::{fs::File, io::BufReader},
};
const SLOPES: &((usize; 2)) = &((1, 1), (3, 1), (5, 1), (7, 1), (1, 2));
fn main() -> Result<()> {
let file = BufReader::new(File::open(lib::PATH)?);
let terrain = Terrain::parse_from(file)?;
let product: usize = SLOPES
.iter()
.map(|&(delta_x, delta_y)| terrain.slope_count(delta_x, delta_y))
.product();
println!("{}", product);
Ok(())
}
Crates used: anyhow 1.0.37 itertools 0.10.0 ndarray 0.14.0
cargo fmt and cargo clippy have been applied.
