The first part was simple enough. Adding in the 3 remaining tilt methods for star 2 was also simple enough, and worked just how I figured it would. Tried the brute force solution first, but realized it was going to take a ridiculous amount of time and went back to figure out an algorithm. It was simple enough to guess that it would hit a point where it just repeats infinitely, but actually coding out the math to extrapolate that took way more time than I want to admit. Not sure why I struggled with it so much, but after some pen and paper mathing, I essentially got there. Ended up having to subract 1 from this calculation, and either I’m just missing something or am way too tired, because I don’t know why it’s one less than what I thought it would be, but it works so who am I to complain.

https://github.com/capitalpb/advent_of_code_2023/blob/main/src/solvers/day14.rs

use crate::Solver;

#[derive(Debug)]
struct PlatformMap {
    tiles: Vec>,
}

impl PlatformMap {
    fn from(input: &str) -> PlatformMap {
        PlatformMap {
            tiles: input.lines().map(|line| line.chars().collect()).collect(),
        }
    }

    fn load(&self) -> usize {
        self.tiles
            .iter()
            .enumerate()
            .map(|(row, tiles)| {
                tiles.iter().filter(|tile| *tile == &'O').count() * (self.tiles.len() - row)
            })
            .sum()
    }

    fn tilt_north(&mut self) {
        for row in 1..self.tiles.len() {
            for col in 0..self.tiles[0].len() {
                if self.tiles[row][col] != 'O' {
                    continue;
                }

                let mut new_row = row;
                for check_row in (0..row).rev() {
                    if self.tiles[check_row][col] == '.' {
                        new_row = check_row;
                    } else {
                        break;
                    }
                }

                self.tiles[row][col] = '.';
                self.tiles[new_row][col] = 'O';
            }
        }
    }

    fn tilt_west(&mut self) {
        for col in 1..self.tiles[0].len() {
            for row in 0..self.tiles.len() {
                if self.tiles[row][col] != 'O' {
                    continue;
                }

                let mut new_col = col;
                for check_col in (0..col).rev() {
                    if self.tiles[row][check_col] == '.' {
                        new_col = check_col;
                    } else {
                        break;
                    }
                }

                self.tiles[row][col] = '.';
                self.tiles[row][new_col] = 'O';
            }
        }
    }

    fn tilt_south(&mut self) {
        for row in (0..(self.tiles.len() - 1)).rev() {
            for col in 0..self.tiles[0].len() {
                if self.tiles[row][col] != 'O' {
                    continue;
                }

                let mut new_row = row;
                for check_row in (row + 1)..self.tiles.len() {
                    if self.tiles[check_row][col] == '.' {
                        new_row = check_row;
                    } else {
                        break;
                    }
                }

                self.tiles[row][col] = '.';
                self.tiles[new_row][col] = 'O';
            }
        }
    }

    fn tilt_east(&mut self) {
        for col in (0..(self.tiles[0].len() - 1)).rev() {
            for row in 0..self.tiles.len() {
                if self.tiles[row][col] != 'O' {
                    continue;
                }

                let mut new_col = col;
                for check_col in (col + 1)..self.tiles[0].len() {
                    if self.tiles[row][check_col] == '.' {
                        new_col = check_col;
                    } else {
                        break;
                    }
                }

                self.tiles[row][col] = '.';
                self.tiles[row][new_col] = 'O';
            }
        }
    }
}

pub struct Day14;

impl Solver for Day14 {
    fn star_one(&self, input: &str) -> String {
        let mut platform_map = PlatformMap::from(input);
        platform_map.tilt_north();
        platform_map.load().to_string()
    }

    fn star_two(&self, input: &str) -> String {
        let mut platform_map = PlatformMap::from(input);
        let mut map_history: Vec>> = vec![];

        for index in 0..1_000_000_000 {
            platform_map.tilt_north();
            platform_map.tilt_west();
            platform_map.tilt_south();
            platform_map.tilt_east();

            if let Some(repeat_start) = map_history
                .iter()
                .position(|tiles| tiles == &platform_map.tiles)
            {
                let repeat_length = index - repeat_start;
                let delta = (1_000_000_000 - repeat_start) % repeat_length;
                let solution_index = repeat_start + delta - 1;

                return PlatformMap {
                    tiles: map_history[solution_index].clone(),
                }
                .load()
                .to_string();
            }

            map_history.push(platform_map.tiles.clone());
        }

        platform_map.load().to_string()
    }
}

Had to take a couple days off, but this was a nice one to come back to. Will have to find some time today to go back and do one or two of the 3 that I missed. I don’t have much to say about this one - I had an idea almost immediately and it worked out without much struggle. There’s probably some cleaner ways to write parts of this, but I’m not too disappointed with how it turned out.

https://github.com/capitalpb/advent_of_code_2023/blob/main/src/solvers/day15.rs

use crate::Solver;
use std::collections::HashMap;

#[derive(Debug)]
struct Lens {
    label: String,
    focal_length: u32,
}

fn hash_algorithm(input: &str) -> u32 {
    input
        .chars()
        .fold(0, |acc, ch| (acc + ch as u32) * 17 % 256)
}

pub struct Day15;

impl Solver for Day15 {
    fn star_one(&self, input: &str) -> String {
        input
            .trim_end()
            .split(',')
            .map(hash_algorithm)
            .sum::()
            .to_string()
    }

    fn star_two(&self, input: &str) -> String {
        let mut boxes: HashMap> = HashMap::new();

        for instruction in input.trim_end().split(',') {
            let (label, focal_length) = instruction
                .split_once(|ch| char::is_ascii_punctuation(&ch))
                .unwrap();

            let box_number = hash_algorithm(label);
            let lenses = boxes.entry(box_number).or_insert(vec![]);

            if focal_length == "" {
                lenses.retain(|lens| lens.label != label);
                continue;
            }

            let new_lens = Lens {
                label: label.to_string(),
                focal_length: focal_length.parse().unwrap(),
            };

            if let Some(lens_index) = lenses.iter().position(|lens| lens.label == new_lens.label) {
                lenses[lens_index].focal_length = new_lens.focal_length;
            } else {
                lenses.push(new_lens);
            }
        }

        boxes
            .iter()
            .map(|(box_number, lenses)| {
                lenses
                    .iter()
                    .enumerate()
                    .map(|(lens_index, lens)| {
                        (box_number + 1) * (lens_index as u32 + 1) * lens.focal_length
                    })
                    .sum::()
            })
            .sum::()
            .to_string()
    }
}

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()
    }
}