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Nicky Meuleman
Nicky Meuleman

Posted on • Originally published at nickymeuleman.netlify.app

Advent of Code 2022 Day 13

#adventofcode #rust

Day 13: Distress Signal

https://adventofcode.com/2022/day/13

TL;DR: my solution in Rust

You communication device is receiving a signal, it's a distress signal!

It receives pairs of packets, and some of those pairs are out of order (because of course they are, it wouldn't be a puzzle if everything was hunky dory now would it?).

You need to be able to identify when a pair is ordered wrong so you can swap the packets.

Today's input are the pairs of packets you receive.

An example input looks like this:

[1,1,3,1,1]
[1,1,5,1,1]

[[1],[2,3,4]]
[[1],4]

[9]
[[8,7,6]]

[[4,4],4,4]
[[4,4],4,4,4]

[7,7,7,7]
[7,7,7]

[]
[3]

[[[]]]
[[]]

[1,[2,[3,[4,[5,6,7]]]],8,9]
[1,[2,[3,[4,[5,6,0]]]],8,9]
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Each pair is seperated by an empty line.
A packet is that array looking thing on a single line.
And they look familiar.

A packet consists of lists and integers.

  • A list starts with [ and ends with ].
  • A list can be empty
  • Each item in a list is seperated by a ,

The two packets in a pair are called left and right.

A pair where the left packet is smaller than the right packet is in the correct order.

To determine which of two packets is smaller, there are a few rules:

  • If both values are integers, the lower integer is smaller.
    • If both integers are identical, continue to the next part of the package.
  • If both values are lists, compare each value in the 2 lists step by step.
    • If the left list runs out of values to compare first, it is smaller.
    • If the right list runs out of values to compare first, it is smaller.
  • If one value is an integer and the other a list, turn the integer into a list of one integer, and run the comparison again.

Parsing

I parsed the input into a list of pairs.
Each item in a pair is a packet, so I used a Packet data structure.

enum Packet {
    Num(u8),
    List(Vec<Packet>),
}
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The main parse function splits the input in a double newline.
Parses the 2 packets seperatly, and return a list of those pairs.

fn parse() -> Vec<[Packet; 2]> {
    let input = std::fs::read_to_string("src/day13.txt").unwrap();

    input
        .split("\n\n")
        .map(|pair| {
            let mut lines = pair.lines();
            let left = lines.next().unwrap();
            let right = lines.next().unwrap();

            [parse_packet(left), parse_packet(right)]
        })
        .collect()
}
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Yeah, yeah, ... show us parse_packet!

Now. If you wanted to be cheeky, every packet is valid JSON.
So you could use your favourite way to parse JSON and call it a victory.

I didn't.

parse_packet takes a string, and returns a Packet.

fn parse_packet(s: &str) -> Packet {
    let chars: Vec<_> = s.chars().collect();
    // parse_list returns the first parsed packet and the rest of the input
    // the rest of the input will be empty when it is done
    let (packet, _) = parse_list(&chars);
    packet
}
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It uses the parse_list function, what's that?

It's one of 2 other helper functions.
A packet is either a Num(u8) or a Packet(Vec<Packet>).

  • The number variant gets parsed by parse_num.
  • The list variant gets parsed by parse_num.

They both use the same pattern.
It consumes a part of what is passed in, and returns the thing it parsed, and the remaining input after that thing.

  • for parse_num it returns (Packet::Num(u8), &[char])
  • for parse_list it returns (Packet::List(Vec<Packet>, &[char])

Where that &[char] bit is the remaining input.

At the very top, when we first call parse_list in parse_packet.
We call parse_list on an entire packet, the remaining part will be empty.
That's why I throw it away with that _.

This is a popular pattern.
An example of this pattern implemented in a much more robust and universal way is the nom crate

fn parse_num(list: &[char]) -> (Packet, &[char]) {
    // find the index where the first number ends
    let mut i = 0;
    while i < list.len() && list[i].is_ascii_digit() {
        i += 1;
    }

    // parse the number
    // uses math to concatenate numbers instead of turning them into a string first to parse that
    let mut num = 0;
    let mut offset = 1;
    for c in list[..i].iter().rev() {
        num += (*c as u8 - b'0') * offset;
        offset *= 10;
    }

    // return the number and the rest of the packet
    (Packet::Num(num), &list[i..])
}

fn parse_list(list: &[char]) -> (Packet, &[char]) {
    // start by removing the starting [ of the passed in list
    // at the end of this function, we remove the ending ] of the passed in list
    let mut list = &list[1..];
    let mut packets = Vec::new();

    loop {
        match list[0] {
            // list ended, break the loop
            ']' => break,
            // skip over ,
            ',' => list = &list[1..],
            // beginning of new list, time for recursion to parse the inner list
            '[' => {
                let (packet, rest) = parse_list(list);
                packets.push(packet);
                list = rest;
            }
            // beginning of a number
            _ => {
                let (n, rest) = parse_num(list);
                packets.push(n);
                list = rest;
            }
        }
    }

    // return the parsed list and the remaining characters minus the ] that terminates the list this function just parsed
    (Packet::List(packets), &list[1..])
}
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Part 1

The question asks what the sum of the indices of the pairs that arrived correctly is.

It's one of those days again where Advent of Code insist indices start at 1.

in pseudocode:

pairs
    .iter()
    .positions(|[left, right]| left < right)
    .map(|idx| idx + 1)
    .sum()
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It's that comparison where the next bit of interesting code happens.

I wanted to tell my code how it can compare two Packets.
Make it understand what a less than operation means.

The rules for such a comparison are described in the intro.

Helpers

To make that comparison possible, I implemented the Ord trait in Rust.

Basically, I wrote the code that Rust uses when you do a comparison.

Turns out that you can not only compare 2 numbers, but you can compare 2 lists!
And the logic it uses happens to be the same one described in the problem statement.
That's convenient!

I take the inner "thing" out of a Packet, and call them a and b.
Both are either a u8, or a Vec.

a.cmp(b), and if only one of them is a vec, wrap the other in a vec and compare that instead!

#[derive(PartialEq, Eq)]
enum Packet {
    Num(u8),
    List(Vec<Packet>),
}

impl Ord for Packet {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        match (self, other) {
            (Self::List(a), Self::List(b)) => a.cmp(b),
            (Self::List(a), Self::Num(b)) => a.cmp(&vec![Self::Num(*b)]),
            (Self::Num(a), Self::List(b)) => vec![Self::Num(*a)].cmp(&b),
            (Self::Num(a), Self::Num(b)) => a.cmp(b),
        }
    }
}

impl PartialOrd for Packet {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}
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With this trait implemented, that skeleton code from earlier works.

Final code 

use itertools::Itertools;

pub fn part_1() -> usize {
    let pairs = parse();

    pairs
        .iter()
        .positions(|[left, right]| left < right)
        .map(|idx| idx + 1)
        .sum()
}
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Part 2

We need to put all packets in the correct order now.
Ignoring the empty lines.

Two divider packets are inserted too.

[[2]]
[[6]]
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The question asks what the decoder key is.

The decoder key is the product of the indices of those 2 divider packets when all packets are in the correct order.

I reused the parsing logic for pairs.
Since we only care about individual packages, I turned the list of pair into a single list with flatten.

I parsed those 2 divider packets and inserted them into the list of packets.

Then, sort all packets and search for the divider packets.

The product of their indices (reminder: 1 based indices!) is the answer to part2!

Final code 

use itertools::Itertools;

pub fn part_2() -> usize {
    let pairs = parse();
    let mut packets: Vec<_> = pairs.iter().flatten().collect();

    let divider_1 = parse_packet("[[2]]");
    let divider_2 = parse_packet("[[6]]");

    packets.push(&divider_1);
    packets.push(&divider_2);

    packets.sort_unstable();

    packets
        .into_iter()
        .positions(|packet| packet == &divider_1 || packet == &divider_2)
        .map(|idx| idx + 1)
        .product()
}
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Final code 

use std::cmp::Ordering;

use itertools::Itertools;

#[derive(Debug, PartialEq, Clone, Eq)]
enum Packet {
    Num(u8),
    List(Vec<Packet>),
}

impl Ord for Packet {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        match (self, other) {
            (Self::List(a), Self::List(b)) => a.cmp(b),
            (Self::List(a), Self::Num(b)) => a.cmp(&vec![Self::Num(*b)]),
            (Self::Num(a), Self::List(b)) => vec![Self::Num(*a)].cmp(&b),
            (Self::Num(a), Self::Num(b)) => a.cmp(b),
        }
    }
}

impl PartialOrd for Packet {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

fn parse() -> Vec<[Packet; 2]> {
    let input = std::fs::read_to_string("src/day13.txt").unwrap();

    input
        .split("\n\n")
        .map(|pair| {
            let mut lines = pair.lines();
            let left = lines.next().unwrap();
            let right = lines.next().unwrap();

            [parse_packet(left), parse_packet(right)]
        })
        .collect()
}

fn parse_packet(s: &str) -> Packet {
    let chars: Vec<_> = s.chars().collect();
    // parse_list returns the first parsed packet and the rest of the input
    // the rest of the input will be empty when it is done
    let (packet, _) = parse_list(&chars);
    packet
}

fn parse_num(list: &[char]) -> (Packet, &[char]) {
    // find the index where the first number ends
    let mut i = 0;
    while i < list.len() && list[i].is_ascii_digit() {
        i += 1;
    }

    // parse the number
    // uses math to concatenate numbers instead of turning them into a string first to parse that
    let mut num = 0;
    let mut offset = 1;
    for c in list[..i].iter().rev() {
        num += (*c as u8 - b'0') * offset;
        offset *= 10;
    }

    // return the number and the rest of the packet
    (Packet::Num(num), &list[i..])
}

fn parse_list(list: &[char]) -> (Packet, &[char]) {
    // start by removing the starting [ of the passed in list
    // at the end of this function, we remove the ending ] of the passed in list
    let mut list = &list[1..];
    let mut packets = Vec::new();

    loop {
        match list[0] {
            // list ended, break the loop
            ']' => break,
            // skip over ,
            ',' => list = &list[1..],
            // beginning of new list, time for recursion to parse the inner list
            '[' => {
                let (packet, rest) = parse_list(list);
                packets.push(packet);
                list = rest;
            }
            // beginning of a number
            _ => {
                let (n, rest) = parse_num(list);
                packets.push(n);
                list = rest;
            }
        }
    }

    // return the parsed list and the remaining characters minus the ] that terminates the list this just parsed
    (Packet::List(packets), &list[1..])
}

pub fn part_1() -> usize {
    let pairs = parse();

    pairs
        .iter()
        .positions(|[left, right]| left < right)
        .map(|idx| idx + 1)
        .sum()
}

pub fn part_2() -> usize {
    let pairs = parse();
    let mut packets: Vec<_> = pairs.iter().flatten().collect();

    let divider_1 = parse_packet("[[2]]");
    let divider_2 = parse_packet("[[6]]");

    packets.push(&divider_1);
    packets.push(&divider_2);

    packets.sort_unstable();

    packets
        .into_iter()
        .positions(|packet| packet == &divider_1 || packet == &divider_2)
        .map(|idx| idx + 1)
        .product()
}
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