Advent of Code 2025 — Day 02

Language: Rust

Problem https://adventofcode.com/2025/day/2

Day 2 is a scan of numeric ranges and sum of IDs that have a repeated digit pattern.

The input is a single comma seperated line of inclusive ranges like 11-22. I split on commas, then pare each start-end:

let (a, b) = range.split_once('-').unwrap();
let start: u64 = a.parse().unwrap();
let end: u64 = b.parse().unwrap();

Then each part does the same thing: iterate start..=end, filter by a predicate, sum, and then sum accross ranges.

Part 1: Invalid IDs are of the form XYXY (some digit sequence repeated twice). That means the digit count is even, and the halves match.

if s.len() % 2 != 0 { return false; }
let (l, r) = s.split_at(s.len() / 2);
l == r

Part 2: Now the number is invalid if it’s one block repeated k times, k >= 2. I try every possible block length up to len/2, require divisibility, then verify all chunks equal the first:

for block_len in 1..=(len / 2) {
    if len % block_len != 0 { continue; }
    let block = &s[..block_len];
    for i in (block_len..len).step_by(block_len) {
        if &s[i..i + block_len] != block { ok = false; break; }
    }
    if ok { return true; }
}

Implementation shape Both parts follow the same pattern, only the predicate changes:

(start..=end)
    .filter(|&n| is_repeated_at_least_twice(n))
    .sum::<u64>()

Solution: https://github.com/Elyrial/AdventOfCode/blob/main/src/solutions/year2025/day02.rs

Approach (C)

Explain it.