contests/advent_of_code/2022/13/src/main.rs

use nom::{IResult, 
    multi::{many1, separated_list0},
    sequence::{separated_pair, terminated}, 
    character::complete::{self, multispace1, multispace0}, 
    bytes::complete::tag,
    branch::alt, Parser};

#[derive(Debug, Clone)]
enum Item {
    NestedList(List),
    Num(u8),
} use Item::*;

type List = Vec<Item>;

fn parse_item(input: &str) -> IResult<&str, Item> {
    let (input, item) = alt((
        complete::u8.map(|n| Num(n)), 
        parse_list.map(|l| NestedList(l))
    ))(input)?;

    Ok((input, item))
}

fn parse_list(input: &str) -> IResult<&str, List> {
    let (input, _) = tag("[")(input)?;
    let (input, list) = separated_list0(tag(","), parse_item)(input)?;

    let (input, _) = tag("]")(input)?;

    Ok((input, list))
}

fn parse_input(input: &str) -> IResult<&str, Vec<(List, List)>> {
    let (input, result) = many1(
        terminated(
            separated_pair(
                parse_list, 
                multispace1, 
                parse_list
            ), 
            multispace0
        )
    )(input)?;

    Ok((input, result))
}

fn print_list(list: &List) {
    print!("[");
    for item in list {
        match item {
            NestedList(l) => print_list(l),
            Num(n) => print!("{n}"),
        }
        print!(",")
    }
    print!("]")
}

fn print_list_pair(lists: &(List, List)) {
    print_list(&lists.0);
    println!();
    print_list(&lists.1);
    println!();
    println!();
}

fn check_lists(lists: &(List, List)) -> Option<bool> {
    let mut l1 = lists.0.iter();
    let mut l2 = lists.1.iter();

    loop {
        match (l1.next(), l2.next()) {
            (None, Some(_)) => return Some(true),
            (Some(_), None) => return Some(false),
            (None, None) => return None,
            (Some(Num(n)), Some(Num(m))) => if n < m {
                return Some(true);
            } else if n > m {
                return Some(false);
            } else {
                continue;
            },
            (Some(NestedList(left)), Some(NestedList(right))) => match check_lists(&(left.to_vec(), right.to_vec())) {
                None => continue,
                result => return result,
            },
            (Some(NestedList(left)), Some(Num(m))) => match check_lists(&(left.to_vec(), vec![Num(*m)])) {
                None => continue,
                result => return result,
            },
            (Some(Num(n)), Some(NestedList(right))) => match check_lists(&(vec![Num(*n)], right.to_vec())) {
                None => continue,
                result => return result,
            },
        }
    }
}



fn main() {
    let input_text = include_str!("../input.txt");
    let (_rest, list_pairs) = parse_input(input_text).unwrap();

    // Checking the parsing (which worked first try btw like wtf)
    // for list_pair in &lists {
    //     print_list_pair(list_pair)
    // }

    let mut index = 0;
    let mut result = 0;
    for list_pair in &list_pairs {
        index += 1;

        match check_lists(list_pair) {
            Some(true) => result += index,
            Some(false) => continue,
            None => unreachable!(),
        }
    }

    println!("Sum of indices is: {result}");

    let first_divider_packet = vec![NestedList(vec![Num(2)])];
    let second_divider_packet = vec![NestedList(vec![Num(6)])];

    let mut list = Vec::new();
    for list_pair in list_pairs {
        list.push(list_pair.0);
        list.push(list_pair.1);
    }

    list.push(first_divider_packet.clone());
    list.push(second_divider_packet.clone());

    list.sort_by(|l, r| match check_lists(&(l.to_vec(), r.to_vec())) {
        None => std::cmp::Ordering::Equal,
        Some(true) => std::cmp::Ordering::Less,
        Some(false) => std::cmp::Ordering::Greater,
    });

    // Checking the sorting (which works :D)
    // for l in &list {
    //     print_list(l);
    //     println!();
    // } 

    let mut first_index = 0;
    let mut second_index = 0;
    index = 1;

    for l in & list {
        if check_lists(&(l.to_vec(), first_divider_packet.clone())) == None {
            first_index = index;
        } else if check_lists(&(l.to_vec(), second_divider_packet.clone())) == None {
            second_index = index;
        }

        if first_index != 0 && second_index != 0 {
            break;
        }

        index += 1;
    }

    println!("Divider packets' indices multiplied: {}", first_index*second_index);
}