advent-of-code/2022/day13-part2/src/main.rs

use std::{cmp::Ordering, iter::zip, str::Chars};

#[derive(Debug)]
enum Packet {
    Number(i32),
    List(Vec<Packet>),
}

impl Packet {
    // Parse a single packet line into the Packet data structure.
    fn from_string(s: &mut Chars) -> Vec<Packet> {
        // println!("Entering with {}", s.clone().collect::<String>());
        // We're going to iterate through this character by character.
        // It is assumed that the opening '[' is already stripped.
        // Create an empty vector packet to fill in a loop.
        let mut packet_list: Vec<Packet> = Vec::new();
        // Now iterate through all the remaining characters.
        loop {
            // Collect a single element, i.e. all characters until the first occurence of '[', ']' or ','.
            // The delimiter itself will be collected into `c`.
            let mut elem = String::new();
            let mut c: char;
            loop {
                c = s.next().unwrap();
                if c == '[' || c == ']' || c == ',' {
                    break;
                } else {
                    elem.push(c);
                }
            }
            // Parse the element if it isn't empty and add it to the list.
            if !elem.is_empty() {
                let num = Packet::Number(elem.parse().unwrap());
                packet_list.push(num);
            }

            // Encountering a new list?
            // Call recursively and collect everything there.
            if c == '[' {
                let sublist = Packet::from_string(s);
                packet_list.push(Packet::List(sublist));
            }

            // Encountered the end of the list?
            // Then we're done here.
            if c == ']' {
                break;
            }
        }
        packet_list
    }
}

fn main() {
    // Use command line arguments to specify the input filename.
    let args: Vec<String> = std::env::args().collect();
    if args.len() < 2 {
        panic!("Usage: ./main <input-file>\nNo input file provided. Exiting.");
    }

    // Next, read the contents of the input file into a string for easier processing.
    let input = std::fs::read_to_string(&args[1]).expect("Error opening file");

    // --- TASK BEGIN ---

    // Collect all actual packets into a big packet vector.
    let mut packets: Vec<_> = input
        .lines()
        .filter(|l| !l.is_empty())
        .map(|l| Packet::from_string(&mut l[1..].chars()))
        .collect();
    
    // Create copies of the divider packages and add them to the big vector.
    packets.push(vec![Packet::List(vec![Packet::Number(2)])]);
    packets.push(vec![Packet::List(vec![Packet::Number(6)])]);

    // The magic line. Sort the vector using `packet_compare`.
    packets.sort_unstable_by(packet_compare);

    // Find the indices of the 2 and 6 divider.
    let idx2 = packets.iter().enumerate().find(|(_, x)| {
        packet_compare(
            x,
            &vec![Packet::List(vec![Packet::Number(2)])],
        ).is_eq()
    }).unwrap().0;

    let idx6 = packets.iter().enumerate().find(|(_, x)| {
        packet_compare(
            x,
            &vec![Packet::List(vec![Packet::Number(6)])],
        ).is_eq()
    }).unwrap().0;

    print_packet_list(&packets);
    println!("Decoder key: {}", (idx2 + 1) * (idx6 + 1));
}

fn print_packet_list(packet_list: &Vec<Vec<Packet>>) {
    for packet in packet_list {
        print_packet(packet);
        // Add newlines after every "top-level" packet.
        println!();
    }
}

fn print_packet(packet: &Vec<Packet>) {
    print!("[");
    for e in packet {
        match e {
            Packet::Number(x) => {
                print!("{x},");
            }
            Packet::List(l) => {
                print_packet(l);
            }
        }
    }
    print!("]");
}

fn packet_compare(left: &Vec<Packet>, right: &Vec<Packet>) -> Ordering {
    let mut result: Ordering;
    let mut zipper = zip(left, right);
    loop {
        match zipper.next() {
            Some(d) => {
                // Determine the Ordering result for the next two "elements", whatever they may be.
                result = match d {
                    (Packet::Number(lv), Packet::Number(rv)) => {
                        // Compare the integers directly.
                        lv.cmp(rv)
                    }
                    (Packet::Number(lv), Packet::List(rl)) => {
                        // Left is a number, right is a list.
                        // Convert the number to a list and then compare those.
                        let ll: Vec<Packet> = vec![Packet::Number(*lv)];
                        packet_compare(&ll, rl)
                    }
                    (Packet::List(ll), Packet::Number(rv)) => {
                        // Left is a list, right is a number.
                        // Convert the number to a list and then compare those.
                        let rl: Vec<Packet> = vec![Packet::Number(*rv)];
                        packet_compare(ll, &rl)
                    }
                    (Packet::List(ll), Packet::List(rl)) => {
                        // Recursively step into the lists.
                        packet_compare(ll, rl)
                    }
                };
            }
            None => {
                // Zipper empty?
                // We need to remember which of the lists ran out first.
                result = left.len().cmp(&right.len());
                // And definitely break, ofc, since the zipper is done.
                break;
            }
        }
        // Have we reached a conclusion already? If so, return.
        if result != Ordering::Equal {
            break;
        }
    }
    result
}