Discussion on: AoC Day 3: No Matter How You Slice It

[Ryan Palo]

Part 1

I really struggled with part 1. Not because I couldn't figure out the problem, or get my code to compile. I had my tests running pretty quickly! But I kept getting the wrong answer! I was on the very precipice of giving up and checking the subreddit when I realized that str.matches(|c| c.is_digit(10)) only finds single digit at a time -- it doesn't build consecutive digits into a single "find." So with input string #1 @ 55,22: 10x10, it was reading this as id: 1, left: 5, top: 5, width: 2, height: 2 and throwing away the rest. 💩

After scratching my head and then bringing in my first external dependency ever in Rust (pretty painless, all things considered) things worked out just fine.

Since the dependency I brought in was just a regex crate, which would be built-in in some languages, I figured that was OK. I wasn't bringing in the find-overlapping-squares crate.

Anyhow, here's part 1:

use regex::Regex;
use std::collections::HashMap;

/// An X, Y grid of Santa's fabric that elves can lay claim to
struct Fabric {
    squares: HashMap<(usize, usize), usize>,
}

/// The data for a rectangular claim an elf makes on a section of fabric
struct Claim {
    id: usize,
    left: usize,
    top: usize,
    width: usize,
    height: usize,
}

impl Fabric {
    fn new() -> Self {
        Self { squares: HashMap::new() }
    }

    /// Increments the amount of claims covering each of the cells inside
    /// the rectangle.
    fn claim(&mut self, claim: &Claim) {
        for x in claim.left..(claim.left + claim.width) {
            for y in claim.top..(claim.top + claim.height) {
                if x > 999 || y > 999 {
                    continue;
                }
                *self.squares.entry((x, y)).or_insert(0) += 1;
            }
        }
    }

    /// Counts how many cells have more than one claim on them
    fn count_conflicts(&self) -> usize {
        self.squares.values().filter(|count| **count >= 2).count()
    }

    /// Counts the total squares claimed
    /// 
    /// A helper function I wrote to help with debugging... #didnthelp
    fn total_squares(&self) -> usize {
        self.squares.iter().count()
    }
}



/// Processes a claim string into an actual Claim
/// 
/// claim string pattern is #<id> @ <left>,<top>: <width>x<height>
/// Since all the numbers are disjoint, we can just match all the 
/// separated numbers in order.
fn process_claim(claim_text: &str) -> Claim {
    // This makes it so we only compile the regex once
    lazy_static! {
        static ref claim_re: Regex = Regex::new(r"#(?P<id>\d+) @ (?P<left>\d+),(?P<top>\d+): (?P<width>\d+)x(?P<height>\d+)").unwrap();
    }

    let claim_parts = claim_re.captures(claim_text).unwrap();
    Claim {
        id: claim_parts["id"].parse().unwrap(),
        left: claim_parts["left"].parse().unwrap(),
        top: claim_parts["top"].parse().unwrap(),
        width: claim_parts["width"].parse().unwrap(),
        height: claim_parts["height"].parse().unwrap(),
    }
}

/// Counts the number of squares with more than one claim on them
pub fn count_conflicting_squares(text: &str) -> usize {
    let mut fabric = Fabric::new();

    for line in text.lines() {
        let claim = process_claim(line);
        fabric.claim(&claim);
    }

    fabric.count_conflicts()
}

Part 2

Actually, for part 1, I didn't even keep track of the ID of the claims -- I just threw that data away! And then I read part two and sat there in sadness for a few minutes.

But!

Then I realized that I wouldn't have to come up with an entirely new approach. I could process the claims like normal, and then re-run through the claims and recheck all of their squares, to see if any have all cells with only one claim on them. Yeah, it doubles the run-time, but O(n) is O(n), even if you double it (sort of). Anyways, I'm pretty happy with today's challenge. Especially my top level functions count_conflicting_squares and find_unconflicting_id: I was able to make them abstract enough that they're pretty easy to read and figure out.

impl Fabric {
    /// Checks whether or not a given claim has any overlapping cells
    fn check_overlap(&self, claim: &Claim) -> bool {
        for x in claim.left..(claim.left + claim.width) {
            for y in claim.top..(claim.top + claim.height) {
                if self.squares.get(&(x, y)) != Some(&1) {
                    return true;
                }
            }
        }
        false
    }
}

/// Finds out if a claim in a group of claims doesn't overlap.  Returns
/// the first one that doesn't.
pub fn find_unconflicting_id(text: &str) -> usize {
    let mut fabric = Fabric::new();
    let mut claims: Vec<Claim> = Vec::new();

    // Load all the claims in
    for line in text.lines() {
        let claim = process_claim(line);
        fabric.claim(&claim);
        claims.push(claim);
    }

    // Check them all for overlaps
    for claim in claims {
        if !fabric.check_overlap(&claim) {
            return claim.id;
        }
    }
    return 0;
}

[jess unrein]

I made the exact same mistake with my initial attempt, where I was only grabbing the first digit with my regex. I'm glad it wasn't just me 😅I was so frustrated because the test input worked, since each number in the test input was only one digit! Sometimes I wish that AoC gave just a little more test data to work with before grabbing your final input.

[Ryan Palo]

Yes! The first test input kept passing, and then I wrote like 4 or 5 more tests to check different things, and they all passed! But I never checked double-digit numbers. :|