Day 12: Garden Groups
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2 points
Rust
Areas are found by flooding, in the meantime whenever the adjacent plot would be outside the region (or out of bounds) the edge (inside plot, outside plot) is saved in a perimeter list. Part 1 takes just the size of that list, in part 2 we remove fence parts and all entries directly next to it on both sides.
Solution
use std::collections::{HashSet, VecDeque};
use euclid::{default::*, point2, vec2};
type Fences = HashSet<(Point2D<i32>, Point2D<i32>)>;
const DIRS: [Vector2D<i32>; 4] = [vec2(0, -1), vec2(1, 0), vec2(0, 1), vec2(-1, 0)];
fn parse(input: &str) -> Vec<&[u8]> {
input.lines().map(|l| l.as_bytes()).collect()
}
fn price(field: &[&[u8]], start: (usize, usize), visited: &mut [Vec<bool>]) -> (u32, Fences) {
let crop = field[start.1][start.0];
let width = field[0].len();
let height = field.len();
let mut area_visited = vec![vec![false; width]; height];
let mut area = 0;
let mut fences: Fences = HashSet::new();
area_visited[start.1][start.0] = true;
visited[start.1][start.0] = true;
let start = point2(start.0 as i32, start.1 as i32);
let bounds = Rect::new(Point2D::origin(), Size2D::new(width, height).to_i32());
let mut frontier = VecDeque::from([start]);
while let Some(p) = frontier.pop_front() {
area += 1;
for dir in DIRS {
let next = p + dir;
if bounds.contains(next) {
let next_u = next.to_usize();
if area_visited[next_u.y][next_u.x] {
continue;
}
if field[next_u.y][next_u.x] == crop {
visited[next_u.y][next_u.x] = true;
area_visited[next_u.y][next_u.x] = true;
frontier.push_back(next);
continue;
}
}
fences.insert((p, next));
}
}
(area, fences)
}
fn part1(input: String) {
let field = parse(&input);
let width = field[0].len();
let height = field.len();
let mut visited = vec![vec![false; width]; height];
let mut total_price = 0;
for y in 0..height {
for x in 0..width {
if !visited[y][x] {
let (area, fences) = price(&field, (x, y), &mut visited);
total_price += area * fences.len() as u32;
}
}
}
println!("{total_price}");
}
fn count_perimeter(mut fences: Fences) -> u32 {
let list: Vec<_> = fences.iter().copied().collect();
let mut perimeter = 0;
for (v, w) in list {
if fences.contains(&(v, w)) {
perimeter += 1;
let dir = w - v;
let orth = dir.yx();
let mut next = v + orth;
while fences.remove(&(next, next + dir)) {
next += orth;
}
let mut next = v - orth;
while fences.remove(&(next, next + dir)) {
next -= orth;
}
}
}
perimeter
}
fn part2(input: String) {
let field = parse(&input);
let width = field[0].len();
let height = field.len();
let mut visited = vec![vec![false; width]; height];
let mut total_price = 0;
for y in 0..height {
for x in 0..width {
if !visited[y][x] {
let (area, fences) = price(&field, (x, y), &mut visited);
total_price += area * count_perimeter(fences);
}
}
}
println!("{total_price}");
}
util::aoc_main!();
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