You get inside and take the elevator to its only other stop: the gift shop. "Thank you for visiting the North Pole!" gleefully exclaims a nearby sign. You aren't sure who is even allowed to visit the North Pole, but you know you can access the lobby through here, and from there you can access the rest of the North Pole base.

As you make your way through the surprisingly extensive selection, one of the clerks recognizes you and asks for your help.

Visit the website for the full story and full puzzle description.

I just went with checking all numbers in the ranges, no need to be super smart, this already runs in a few hundred milliseconds.

namespace AdventOfCode.Y2025.Day02;

using System;
using System.Linq;
using System.Collections.Generic;

[ProblemName("Gift Shop")]
class Solution : Solver {

    public object PartOne(string input) => Solve(input, st => 2);

    public object PartTwo(string input) => Solve(input, st => st.Length);

    long Solve(string input, Func<string, int> maxRepetition) => (
        from id in GetIds(input)
        let st = id.ToString()
        where Enumerable.Range(2, maxRepetition(st)-1).Any(k => Periodic(st, k))
        select id
    ).Sum();

    public IEnumerable<long> GetIds(string input) {
        foreach (var range in input.Split(",")) {
            var parts = range.Split("-").Select(long.Parse).ToArray();
            for (var i = parts[0]; i <= parts[1]; i++) {
                yield return i;
            }
        }
    }

    // Returns true if the string consists of the same substring repeated repetitionCount times.
    // Example: repetitionCount == 3 returns true for "abcabcabc".
    bool Periodic(string st, int repetitionCount) {
        if (st.Length % repetitionCount != 0) {
            return false;
        }

        int period = st.Length / repetitionCount;
        for (int i = period; i < st.Length; i += period) {
            if (st[..period] != st[i..(i + period)]) {
                return false;
            }
        }
        return true;
    }
}

Please ☆ my repo if you like it!