day_15.rs

use common::{Answer, Solution};
use itertools::Itertools;

pub struct Day15;

impl Solution for Day15 {
    fn name(&self) -> &'static str {
        "Lens Library"
    }

    fn part_a(&self, input: &str) -> Answer {
        input
            .trim()
            .split(',')
            .map(|x| hash(x) as u32)
            .sum::<u32>()
            .into()
    }

    fn part_b(&self, input: &str) -> Answer {
        let input = parse(input);
        let mut boxes = vec![Vec::new(); 256];

        for (label, focal_len) in input {
            let key = hash(label) as usize;
            if let Some(focal_len) = focal_len {
                if let Some((_, e)) = boxes[key]
                    .iter_mut()
                    .find(|x: &&mut (&str, u32)| x.0 == label)
                {
                    *e = focal_len;
                } else {
                    boxes[key].push((label, focal_len));
                }
            } else {
                boxes[key].retain(|x| x.0 != label);
            }
        }

        let mut acc = 0;
        for (i, e) in boxes.iter().enumerate() {
            for (j, f) in e.iter().enumerate() {
                acc += (i + 1) * (j + 1) * f.1 as usize;
            }
        }

        acc.into()
    }
}

fn parse(input: &str) -> Vec<(&str, Option<u32>)> {
    let mut out = Vec::new();

    for i in input.trim().split(',') {
        let (label, focal_len) = i.split(['=', '-'].as_ref()).collect_tuple().unwrap();
        out.push((label, focal_len.parse::<u32>().ok()));
    }

    out
}

fn hash(input: &str) -> u8 {
    let mut out = 0u8;
    for c in input.chars() {
        out = out.wrapping_add(c as u8).wrapping_mul(17);
    }

    out
}

#[cfg(test)]
mod test {
    use common::Solution;
    use indoc::indoc;

    use super::Day15;

    const CASE: &str = indoc! {"
        rn=1,cm-,qp=3,cm=2,qp-,pc=4,ot=9,ab=5,pc-,pc=6,ot=7
    "};

    #[test]
    fn part_a() {
        assert_eq!(Day15.part_a(CASE), 1320.into());
    }

    #[test]
    fn part_b() {
        assert_eq!(Day15.part_b(CASE), 145.into());
    }
}