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Day 3 part 1.

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Bill Thiede 2020-12-03 17:31:27 -08:00
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//! --- Day 3: Toboggan Trajectory ---
//! With the toboggan login problems resolved, you set off toward the airport. While travel by toboggan might be easy, it's certainly not safe: there's very minimal steering and the area is covered in trees. You'll need to see which angles will take you near the fewest trees.
//!
//! 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:
//!
//! ..##.........##.........##.........##.........##.........##....... --->
//! #...#...#..#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..
//! .#....#..#..#....#..#..#....#..#..#....#..#..#....#..#..#....#..#.
//! ..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#
//! .#...##..#..#...##..#..#...##..#..#...##..#..#...##..#..#...##..#.
//! ..#.##.......#.##.......#.##.......#.##.......#.##.......#.##..... --->
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//! #.##...#...#.##...#...#.##...#...#.##...#...#.##...#...#.##...#...
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//! .#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.# --->
//! 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?
use std::ops::Index;
use aoc_runner_derive::{aoc, aoc_generator};
#[derive(Debug, PartialEq)]
struct Bitmap {
width: usize,
height: usize,
bits: Vec<bool>,
}
impl Index<(usize, usize)> for Bitmap {
type Output = bool;
// Perform 2-dimensional indexing of bits, wrapping the X coordinates if it is larger than the
// width of the Bitmap.
fn index(&self, (x, y): (usize, usize)) -> &Self::Output {
&self.bits[x % self.width + y * self.width]
}
}
#[aoc_generator(day3)]
fn parse(input: &str) -> Bitmap {
let mut height = 0;
let bits: Vec<bool> = input
.chars()
.filter_map(|c| match c {
'.' => Some(false),
'#' => Some(true),
'\n' => {
height += 1;
None
}
_ => panic!("Unexpected character"),
})
.collect();
let width = bits.len() / height;
Bitmap {
width,
height,
bits,
}
}
#[aoc(day3, part1)]
fn answer(map: &Bitmap) -> usize {
(0..map.height).filter(|y| map[(*y * 3, *y)]).count()
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn make_bitmap() {
assert_eq!(
parse("..##\n##..\n"),
Bitmap {
width: 4,
height: 2,
bits: vec![false, false, true, true, true, true, false, false]
}
);
}
}

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2020/src/lib.rs
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mod day1; mod day1;
mod day2; mod day2;
mod day3;
use aoc_runner_derive::aoc_lib; use aoc_runner_derive::aoc_lib;