capitalpb

I've had the damn song stuck in my head since I saw this post.

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()
    }
}