aoc/2025/02/p1.rs

#[unsafe(no_mangle)]
fn challenge_usize(b: &[u8]) -> usize {
    let mut total = 0;
    let s = unsafe { str::from_utf8_unchecked(b) };

    let ranges = s.trim_end().split(',');
    for range in ranges {
        let (start, end) = range.split_once('-').unwrap();
        if start.starts_with('0') || end.starts_with('0') {
            continue;
        }
        let (start, end): (usize, usize) = (start.parse().unwrap(), end.parse().unwrap());
        // println!("{start}-{end}");

        for id in start..=end {
            let mut id_buf = [0u8; MAX_USIZE_LEN];
            let id_s = fast_to_string(id, &mut id_buf);

            if is_repeating(id_s) {
                total += id;
                // println!("-->> invalid {id_s}");
            }
        }
    }

    total
}

const MAX_USIZE_LEN: usize = const { usize::MAX.ilog10() + 1 } as usize;
/// Output is reversed btw
fn fast_to_string(mut n: usize, into: &mut [u8; MAX_USIZE_LEN]) -> &mut [u8] {
    let mut len = 0;
    while n != 0 {
        into[len] = (n % 10) as u8 + b'0';
        n /= 10;
        len += 1;
    }

    &mut into[0..len]
}

fn is_repeating(s: &[u8]) -> bool {
    if !s.len().is_multiple_of(2) {
        return false;
    }

    let mid = s.len() / 2;
    let (left, right) = s.split_at(mid);
    left == right
}

// LMAO: I did p2 instead of p1 firstly bcs I misread it.

// fn is_repeating(s: &[u8]) -> bool {
//     for sublen in 0..(s.len() / 2) {
//         let sublen = sublen + 1;
//         if !s.len().is_multiple_of(sublen) {
//             continue;
//         }
//         let pref = &s[0..sublen];

//         let is_repeating = s[sublen..].chunks_exact(sublen).all(|c| c == pref);
//         if is_repeating {
//             return true;
//         }
//     }

//     false
// }