Day 11: Cosmic Expansion

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  • capitalpb@programming.dev
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    1 year ago

    That was a fun one. Especially after yesterday. As soon as I saw that star 1 was expanding each gap by 1, I just had a feeling that star 2 would be doing the same calculation with a larger expansion, so I wrote my code in a way that would make that quite simple to modify. When I saw the factor of 1,000,000 I was scared that it was going to be one of those processor-destroying AoC challenges where you either wait for 2 hours to get an answer, or have to come up with a fancy mathematical way of solving things, but after changing my i32 distance to an i64, it calculated just fine and instantly. I guess only storing the locations of galaxies and not dealing with the entire grid was good enough to keep the performance down.

    https://github.com/capitalpb/advent_of_code_2023/blob/main/src/solvers/day11.rs

    use crate::Solver;
    use itertools::Itertools;
    use num::abs;
    
    #[derive(Debug)]
    struct Point {
        x: usize,
        y: usize,
    }
    
    struct GalaxyMap {
        locations: Vec,
    }
    
    impl GalaxyMap {
        fn from(input: &str) -> GalaxyMap {
            let locations = input
                .lines()
                .rev()
                .enumerate()
                .map(|(x, row)| {
                    row.chars()
                        .enumerate()
                        .filter_map(|(y, digit)| {
                            if digit == '#' {
                                Some(Point { x, y })
                            } else {
                                None
                            }
                        })
                        .collect::>()
                })
                .flatten()
                .collect::>();
    
            GalaxyMap { locations }
        }
    
        fn empty_rows(&self) -> Vec {
            let occupied_rows = self
                .locations
                .iter()
                .map(|point| point.y)
                .unique()
                .collect::>();
            let max_y = *occupied_rows.iter().max().unwrap();
    
            (0..max_y)
                .filter(move |y| !occupied_rows.contains(&y))
                .collect()
        }
    
        fn empty_cols(&self) -> Vec {
            let occupied_cols = self
                .locations
                .iter()
                .map(|point| point.x)
                .unique()
                .collect::>();
            let max_x = *occupied_cols.iter().max().unwrap();
    
            (0..max_x)
                .filter(move |x| !occupied_cols.contains(&x))
                .collect()
        }
    
        fn expand(&mut self, factor: usize) {
            let delta = factor - 1;
    
            for y in self.empty_rows().iter().rev() {
                for galaxy in &mut self.locations {
                    if galaxy.y > *y {
                        galaxy.y += delta;
                    }
                }
            }
    
            for x in self.empty_cols().iter().rev() {
                for galaxy in &mut self.locations {
                    if galaxy.x > *x {
                        galaxy.x += delta;
                    }
                }
            }
        }
    
        fn galactic_distance(&self) -> i64 {
            self.locations
                .iter()
                .combinations(2)
                .map(|pair| {
                    abs(pair[0].x as i64 - pair[1].x as i64) + abs(pair[0].y as i64 - pair[1].y as i64)
                })
                .sum::()
        }
    }
    
    pub struct Day11;
    
    impl Solver for Day11 {
        fn star_one(&self, input: &str) -> String {
            let mut galaxy = GalaxyMap::from(input);
            galaxy.expand(2);
            galaxy.galactic_distance().to_string()
        }
    
        fn star_two(&self, input: &str) -> String {
            let mut galaxy = GalaxyMap::from(input);
            galaxy.expand(1_000_000);
            galaxy.galactic_distance().to_string()
        }
    }