advent_of_code/year2023/
day04.rs

use crate::input::{on_error, Input};

pub fn solve(input: &Input) -> Result<u64, String> {
    const MAX_WINNING_NUMBERS: usize = 16;
    let multiplier_stack = &mut [1; MAX_WINNING_NUMBERS];
    let mut multiplier_idx = 0;

    input
        .text
        .lines()
        .map(|card_str| {
            let card_str = card_str.split_once(": ").ok_or_else(on_error)?.1;
            let (win_numbers, my_numbers) = card_str.split_once(" | ").ok_or_else(on_error)?;

            let mut winning_bitmask = 0_u128;
            for number in win_numbers.split_ascii_whitespace() {
                let number = parse_number(number)?;
                winning_bitmask |= 1 << number;
            }

            let mut points = 0;
            for number in my_numbers.split_ascii_whitespace() {
                let number = parse_number(number)?;
                if winning_bitmask & (1 << number) != 0 {
                    points = if input.is_part_one() && points != 0 {
                        points * 2
                    } else {
                        points + 1
                    };
                }
            }

            Ok(if input.is_part_one() {
                points as u64
            } else {
                let num_copies = multiplier_stack[multiplier_idx];
                multiplier_stack[multiplier_idx] = 1;
                for i in 1..=points {
                    multiplier_stack[(multiplier_idx + i) % MAX_WINNING_NUMBERS] += num_copies;
                }
                multiplier_idx = (multiplier_idx + 1) % MAX_WINNING_NUMBERS;
                num_copies
            })
        })
        .sum()
}

fn parse_number(num_str: &str) -> Result<u8, String> {
    let n = num_str.parse::<u8>().map_err(|_| on_error())?;
    if n >= 128 {
        return Err(on_error());
    }
    Ok(n)
}

#[test]
pub fn tests() {
    use crate::input::{test_part_one_no_allocations, test_part_two_no_allocations};

    let test_input = "Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11";
    test_part_one_no_allocations!(test_input => 13);
    test_part_two_no_allocations!(test_input => 30);

    let real_input = include_str!("day04_input.txt");
    test_part_one_no_allocations!(real_input => 17803);
    test_part_two_no_allocations!(real_input => 5_554_894);
    let real_input = include_str!("day04_input_other.txt");
    test_part_two_no_allocations!(real_input => 6_420_979);
}
advent_of_code/year2023/day04.rs

advent_of_code/year2023/
day04.rs
