this post was submitted on 02 Dec 2025
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Advent Of Code

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An unofficial home for the advent of code community on programming.dev! Other challenges are also welcome!

Advent of Code is an annual Advent calendar of small programming puzzles for a variety of skill sets and skill levels that can be solved in any programming language you like.

Everybody Codes is another collection of programming puzzles with seasonal events.

EC 2025

AoC 2025

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๐Ÿง‘ - 2025 DAY 2 SOLUTIONS - ๐Ÿง‘ (programming.dev)
submitted 2 days ago by CameronDev@programming.dev to c/advent_of_code@programming.dev
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Day 2: Gift Shop

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[โ€“] CameronDev@programming.dev 4 points 2 days ago* (last edited 2 days ago) (2 children)

When you have regex, everything looks like a haystack.

Nearly solved pt2 by accident in pt1. Just showing the invalid checks, rest of the code is uninteresting.

fn check_invalid(p0: usize) -> bool {
        let str = format!("{}", p0);
        let len = str.len();
        if len % 2 == 1 {
            return false;
        }
        let len = len / 2;
        str[0..len] == str[len..]
    }

    fn check_invalid2(p0: usize) -> bool {
        let str = format!("{}", p0);
        let re = Regex::new(r"^([0-9]{1,})\1{1,}$").unwrap();
        if re.is_match(str.as_bytes()).unwrap() {
            return true;
        }
        false
    }

edit: The bot worked as well! With some slight human intervention. Tomorrow might be automatic if we are lucky.

[โ€“] CameronDev@programming.dev 3 points 2 days ago* (last edited 2 days ago)

Regex free solution. Much faster (233ms vs ~4s)

  fn check_invalid3(p0: usize) -> u32 {
        let mut i = 0;
        let mut found_count = 0;
        loop {
            let mut v = p0;
            i += 1;
            let mask = 10_usize.pow(i);
            if mask >= p0 {
                // Mask is larger than input, we have exhausted available matchers.
                return found_count;
            }
            let remainer = v % mask;
            if remainer < 10_usize.pow(i - 1) {
                // Zero prefix, won't be a pattern. (01, 002, etc)
                continue;
            }
            let mut count = 1;
            loop {
                let new_v = v / mask;
                if new_v % mask != remainer {
                    // doesnt repeat.
                    break;
                }
                if new_v / mask == 0 {
                    // has repeated, so we have found at least one pattern. Lets keep going to see if there is a simpler pattern.
                    found_count = count;
                    break;
                }
                count += 1;
                v = new_v;
            }
        }
    }
[โ€“] CameronDev@programming.dev 2 points 2 days ago (1 children)

heh, recompiling the regex was wasting 80% of my time :D

[โ€“] Deebster@programming.dev 2 points 1 day ago (1 children)

Every so often I look for a library that will compile the regex at compile time - iirc, there's some stuff needing to made const fn before that can happen.

Last time I used regex, lazy_static was the way to go; I assume that regex can go in OnceCell nowadays.

[โ€“] CameronDev@programming.dev 2 points 1 day ago (1 children)

I just passed it around, felt easier and rust-like. Compile time would be nice, but I have a vague feeling this would be too restrictive for some regexes/engines?

[โ€“] Deebster@programming.dev 3 points 1 day ago

Maybe? There's some deep wizardry shown in some people's macros so a regex feels fairly basic in comparison.