aoc/year2023/
day15.rs

1//! # Lens Library
2//!
3//! Calculates part one and two at the same time as a speed optimization.
4use crate::util::parse::*;
5use std::array::from_fn;
6
7type Input = (usize, usize);
8
9struct Item<'a> {
10    label: &'a [u8],
11    lens: usize,
12}
13
14pub fn parse(input: &str) -> Input {
15    let mut part_one = 0;
16    let mut part_two = 0;
17    let mut boxes: [Vec<Item<'_>>; 256] = from_fn(|_| Vec::new());
18
19    for step in input.trim().as_bytes().split(|&b| b == b',') {
20        let size = step.len();
21        part_one += hash(step);
22
23        if step[size - 1] == b'-' {
24            let label = &step[..size - 1];
25            // If the label exists then remove it.
26            boxes[hash(label)].retain(|item| item.label != label);
27        } else {
28            let label = &step[..size - 2];
29            let hash = hash(label);
30            let slot = &mut boxes[hash];
31            let lens = step[size - 1].to_decimal() as usize;
32
33            // Replace or append new lens.
34            if let Some(i) = slot.iter().position(|item| item.label == label) {
35                slot[i].lens = lens;
36            } else {
37                slot.push(Item { label, lens });
38            }
39        }
40    }
41
42    for (i, next) in boxes.iter().enumerate() {
43        for (j, item) in next.iter().enumerate() {
44            part_two += (i + 1) * (j + 1) * item.lens;
45        }
46    }
47
48    (part_one, part_two)
49}
50
51pub fn part1(input: &Input) -> usize {
52    input.0
53}
54
55pub fn part2(input: &Input) -> usize {
56    input.1
57}
58
59/// Custom hash function.
60#[inline]
61fn hash(slice: &[u8]) -> usize {
62    slice.iter().fold(0, |acc, &b| ((acc + b as usize) * 17) & 0xff)
63}
aoc/year2023/day15.rs

aoc/year2023/
day15.rs
