Day 18: Ram Run

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FAQ

  • @[email protected]
    2•6 months ago

    Rust

    Naive approach running BFS after every dropped byte after 1024. Still runs in 50ms. This could be much optimized by using binary search to find the first blocked round and using A* instead of BFS, but I didn’t feel like doing more today.

    Solution 
    use std::collections::VecDeque;

use euclid::{default::*, vec2};

fn parse(input: &str) -> Vec<Point2D<i32>> {
    input
        .lines()
        .map(|l| {
            let (x, y) = l.split_once(',').unwrap();
            Point2D::new(x.parse().unwrap(), y.parse().unwrap())
        })
        .collect()
}

const BOUNDS: Rect<i32> = Rect::new(Point2D::new(0, 0), Size2D::new(71, 71));
const START: Point2D<i32> = Point2D::new(0, 0);
const TARGET: Point2D<i32> = Point2D::new(70, 70);
const N_BYTES: usize = 1024;
const DIRS: [Vector2D<i32>; 4] = [vec2(1, 0), vec2(0, 1), vec2(-1, 0), vec2(0, -1)];

fn adj(
    field: &[[bool; BOUNDS.size.width as usize]],
    v: Point2D<i32>,
) -> impl Iterator<Item = Point2D<i32>> + use<'_> {
    DIRS.iter()
        .map(move |&d| v + d)
        .filter(|&next| BOUNDS.contains(next) && !field[next.y as usize][next.x as usize])
}

fn find_path(field: &[[bool; BOUNDS.size.width as usize]]) -> Option<u32> {
    let mut seen = [[false; BOUNDS.size.width as usize]; BOUNDS.size.height as usize];
    let mut q = VecDeque::from([(START, 0)]);
    seen[START.y as usize][START.x as usize] = true;
    while let Some((v, dist)) = q.pop_front() {
        for w in adj(field, v) {
            if w == TARGET {
                return Some(dist + 1);
            }
            if !seen[w.y as usize][w.x as usize] {
                seen[w.y as usize][w.x as usize] = true;
                q.push_back((w, dist + 1));
            }
        }
    }
    None
}

fn part1(input: String) {
    let bytes = parse(&input);
    let mut field = [[false; BOUNDS.size.width as usize]; BOUNDS.size.height as usize];
    for b in &bytes[..N_BYTES] {
        field[b.y as usize][b.x as usize] = true;
    }
    println!("{}", find_path(&field).unwrap());
}

fn part2(input: String) {
    let bytes = parse(&input);
    let mut field = [[false; BOUNDS.size.width as usize]; BOUNDS.size.height as usize];
    for (i, b) in bytes.iter().enumerate() {
        field[b.y as usize][b.x as usize] = true;
        // We already know from part 1 that below N_BYTES there is a path
        if i > N_BYTES && find_path(&field).is_none() {
            println!("{},{}", b.x, b.y);
            break;
        }
    }
}

util::aoc_main!();

    Also on github