day15.rs

use std::fs;

pub(crate) fn day15() {
    println!("{}", part_a(fs::read_to_string("input/2023/day15/input.txt").unwrap()));
    println!("{}", part_b(fs::read_to_string("input/2023/day15/input.txt").unwrap()));
}

fn part_a(input: String) -> u32 {
    input.split(",").map(|seq| {
        let mut ans = 0;
        for c in seq.chars() {
            let ci = c as u32;
            ans += ci;
            ans *= 17;
            ans = ans % 256;
        }
        ans
    }).sum()
}

fn part_b(input: String) -> usize {
    let mut boxes: Vec<Vec<(&str, usize)>> = vec! { vec!(); 256 };
    input.split(",").for_each(|seq| {
        let mut label = String::new();
        let mut ans = 0;
        for c in seq.chars() {
            if c == '-' || c == '=' { break; }
            label.push(c);

            let ci = c as usize;
            ans += ci;
            ans *= 17;
            ans = ans % 256;
        }

        if seq.ends_with("-") {
            boxes[ans] = boxes[ans].iter()
                .filter(|(l2, _)| l2 != &label.as_str())
                .map(|v| *v)
                .collect();
        } else {
            let (label, focal_length) = seq.split_once("=").unwrap();
            if let Some(pos) = boxes[ans].iter().position(|(l2, _)| l2 == &label) {
                boxes[ans][pos] = (label, focal_length.parse::<usize>().unwrap());
            } else {
                boxes[ans].push((label, focal_length.parse::<usize>().unwrap()));
            }
        }
    });

    let mut ans = 0;
    for box_i in 0..256 {
        for slot_i in 0..boxes[box_i].len() {
            let (_label, focal_length) = boxes[box_i][slot_i];
            ans += (box_i + 1) * (slot_i + 1) * focal_length;
        }
    }
    ans
}

#[cfg(test)]
mod day15_tests {
    use std::fs;

    use crate::y2023::day15::{part_a, part_b};

    #[test]
    fn test_works() {
        assert_eq!(1320, part_a(fs::read_to_string("input/2023/day15/test.txt").unwrap()));
        assert_eq!(145, part_b(fs::read_to_string("input/2023/day15/test.txt").unwrap()));
    }

    #[test]
    fn input_works() {
        assert_eq!(495972, part_a(fs::read_to_string("input/2023/day15/input.txt").unwrap()));
        assert_eq!(245223, part_b(fs::read_to_string("input/2023/day15/input.txt").unwrap()));
    }
}