Day 19 - Linen Layout

Megathread guidelines

  • Keep top level comments as only solutions, if you want to say something other than a solution put it in a new post. (replies to comments can be whatever)
  • You can send code in code blocks by using three backticks, the code, and then three backticks or use something such as https://topaz.github.io/paste/ if you prefer sending it through a URL

FAQ

  • Gobbel2000@programming.dev
    2·
    10 days ago

    Rust

    First part is solved by making a regex of the available towels, like ^(r|wr|bg|bwu|rb|gb|br)*$ for the example. If a design matches it, then it can be made. This didn’t work for the second part, which is done using recursion and memoization instead. Again, it was quite surprising to see such a high solution number. 32 bits were not enough (thanks, debug mode overflow detection).

    Solution 
    use regex::Regex;
use rustc_hash::FxHashMap;

fn parse(input: &str) -> (Vec<&str>, Vec<&str>) {
    let (towels, designs) = input.split_once("\n\n").unwrap();
    (towels.split(", ").collect(), designs.lines().collect())
}

fn part1(input: String) {
    let (towels, designs) = parse(&input);
    let pat = format!("^({})*$", towels.join("|"));
    let re = Regex::new(&pat).unwrap();
    let count = designs.iter().filter(|d| re.is_match(d)).count();
    println!("{count}");
}

fn n_arrangements<'a>(
    design: &'a str,
    towels: &[&str],
    cache: &mut FxHashMap<&'a str, u64>,
) -> u64 {
    if design.is_empty() {
        return 1;
    }
    if let Some(n) = cache.get(design) {
        return *n;
    }
    let n = towels
        .iter()
        .filter(|t| design.starts_with(*t))
        .map(|t| n_arrangements(&design[t.len()..], towels, cache))
        .sum();
    cache.insert(design, n);
    n
}

fn part2(input: String) {
    let (towels, designs) = parse(&input);
    let sum: u64 = designs
        .iter()
        .map(|d| n_arrangements(d, &towels, &mut FxHashMap::default()))
        .sum();
    println!("{sum}");
}

util::aoc_main!();

    Also on github

      • Gobbel2000@programming.dev
        3·
        10 days ago

        About 3ms. A manual implementation might be a bit faster, but not by much. The regex crate is quite optimized for pretty much these problems.

        • CameronDev@programming.devOPM
          2·
          10 days ago

          Wow, that is very fast, nice. I was happy with 120ms, seems I’m leaving a lot of performance on the table.

          Edit: Regex cut my total time in half, but I am measuring the whole execution, still a massive improvement.

          • Gobbel2000@programming.dev
            3·
            10 days ago

            The 3ms are for part 1 only, part 2 takes around 27ms. But I see that our approaches there are very similar. One difference that might make an impact is that you copy the substrings for inserting into the hashmap into Strings.

            • CameronDev@programming.devOPM
              2·
              10 days ago

              Removing the string copy with the length->count array from @sjmulder saved me 20ms, so not super significant. I’ll have to play the the profiler and see what I am doing wrong.

              I think your approach looks a lot more Rust-like, which I like. Part 1 in 4 lines is very nice.