d15.rs

use crate::Day;

pub struct Day15 {}

impl Day for Day15 {
    fn year(&self) -> u16 {
        2023
    }

    fn day(&self) -> u8 {
        15
    }

    fn part_one(&self) -> String {
        self.read_default_input()
            .lines()
            .next()
            .unwrap()
            .split(',')
            .map(hash)
            .sum::<u32>()
            .to_string()
    }

    fn part_two(&self) -> String {
        let mut boxes: Vec<Vec<(String, u8)>> = vec![Vec::new(); 256];

        self.read_default_input()
            .lines()
            .next()
            .unwrap()
            .split(',')
            .for_each(|item| {
                if item.contains('-') {
                    let tokens = item.split('-').collect::<Vec<_>>();
                    let label = tokens[0];
                    let box_index = hash(label) as usize;

                    boxes[box_index].retain(|lens| lens.0 != label);
                } else if item.contains('=') {
                    let tokens = item.split('=').collect::<Vec<_>>();
                    let label = tokens[0];
                    let box_index = hash(label) as usize;
                    let focal_length = tokens[1].parse::<u8>().unwrap();

                    match boxes[box_index].iter_mut().find(|lens| lens.0 == label) {
                        Some(lens) => lens.1 = focal_length,
                        None => boxes[box_index].push((label.to_string(), focal_length)),
                    }
                } else {
                    unreachable!()
                };
            });

        let mut lenses_focus_power = 0;
        for (box_index, r#box) in boxes.iter().enumerate() {
            for (slot_index, lens) in r#box.iter().enumerate() {
                lenses_focus_power += (box_index + 1) * (slot_index + 1) * lens.1 as usize;
            }
        }

        lenses_focus_power.to_string()
    }
}

fn hash(input: &str) -> u32 {
    let chars = input.chars().map(|c| c as u32).collect::<Vec<u32>>();

    let mut current_value = 0;
    for c in chars {
        current_value += c;
        current_value *= 17;
        current_value %= 256;
    }

    current_value
}