Day 8: Resonant Collinearity
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Rust
Proper Point and Vector types made this pretty simple, part 2 was just a tiny change (basically while instead of if), but left with a lot of copy-pasted code.
Solution
use euclid::default::*;
const N_ANTENNAS: usize = (b'z' - b'0') as usize + 1;
// For each frequency (from b'0' to b'z') the list of antenna positions
type Antennas = Box<[Vec<Point2D<i32>>]>;
fn parse(input: String) -> (Antennas, Rect<i32>) {
let mut antennas = vec![Vec::new(); N_ANTENNAS].into_boxed_slice();
let mut width = 0;
let mut height = 0;
for (y, l) in input.lines().enumerate() {
height = y + 1;
if width == 0 {
width = l.len()
} else {
assert!(width == l.len())
}
for (x, b) in l.bytes().enumerate().filter(|(_, b)| *b != b'.') {
antennas[(b - b'0') as usize].push(Point2D::new(x, y).to_i32())
}
}
let bounds = Rect::new(Point2D::origin(), Size2D::new(width, height).to_i32());
(antennas, bounds)
}
fn part1(input: String) {
let (antennas, bounds) = parse(input);
let mut antinodes = vec![vec![false; bounds.width() as usize]; bounds.height() as usize];
for list in antennas.iter().filter(|l| !l.is_empty()) {
for (i, &a) in list.iter().enumerate().skip(1) {
for &b in list.iter().take(i) {
let diff = b - a;
let ax = a - diff;
if bounds.contains(ax) {
antinodes[ax.y as usize][ax.x as usize] = true;
}
let bx = b + diff;
if bounds.contains(bx) {
antinodes[bx.y as usize][bx.x as usize] = true;
}
}
}
}
let sum = antinodes
.iter()
.map(|row| row.iter().map(|b| u32::from(*b)).sum::<u32>())
.sum::<u32>();
println!("{sum}");
}
fn part2(input: String) {
let (antennas, bounds) = parse(input);
let mut antinodes = vec![vec![false; bounds.width() as usize]; bounds.height() as usize];
for list in antennas.iter().filter(|l| !l.is_empty()) {
for (i, &a) in list.iter().enumerate().skip(1) {
for &b in list.iter().take(i) {
let diff = b - a;
// Start at antenna a, keep going until hitting bounds
let mut ax = a;
while bounds.contains(ax) {
antinodes[ax.y as usize][ax.x as usize] = true;
ax -= diff;
}
let mut bx = b;
while bounds.contains(bx) {
antinodes[bx.y as usize][bx.x as usize] = true;
bx += diff;
}
}
}
}
let sum = antinodes
.iter()
.map(|row| row.iter().map(|b| u32::from(*b)).sum::<u32>())
.sum::<u32>();
println!("{sum}");
}
util::aoc_main!();
also on github
I also did it in Rust, though mine is quite a bit more verbose than yours :). https://gitlab.com/bricka/advent-of-code-2024-rust/-/blob/main/src/days/day08.rs?ref_type=heads
Have you considered using a HashSet<(usize, usize)> to store the visited locations instead of a 2d vector?
I try to use Vecs instead of HashSets and maps whenever the key domain is reasonably small (just the grid in this case), simply because in the end direct memory access is a lot faster than always hashing values.
But looking at this case again, it is certainly a lot easier to have just antinodes.len() at the end instead of counting all true values. This datastructure is also not really performance-critical, so a HashSet is probably the cleaner choice here.
