AdventOfCode2025/AdventOfCode2026/Day2.cs

namespace AdventOfCode2026;

public class Day2 : Solution
{
    public override string SolvePart1()
    {
        return InputReader
            .ReadLines(2)
            .SelectMany(ToRanges)
            .Select(ToLimit)
            .SelectMany(GetIds)
            .Where(id => IsTwiceSequence(id.ToString()))
            .Sum()
            .ToString();
    }

    public override string SolvePart2()
    {
        return InputReader
            .ReadLines(2)
            .SelectMany(ToRanges)
            .Select(ToLimit)
            .SelectMany(GetIds)
            .Where(id => IsTwiceOrMoreSequence(id.ToString()))
            .Sum()
            .ToString();
    }

    private static IEnumerable<long> GetIds((string from, string to) limit) => GetIds(limit.from, limit.to);

    private static IEnumerable<long> GetIds(string from, string to)
    {
        var current = long.Parse(from);
        var end = long.Parse(to);
        while (current <= end)
        {
            yield return current;
            current++;
        }
    }

    private static bool IsTwiceOrMoreSequence(string input)
    {
        if (input.Length < 2)
        {
            return false;
        }

        for (var subLength = 1; subLength <= input.Length / 2; subLength++)
        {
            if (input.Length % subLength != 0)
            {
                continue;
            }

            var testBlock = input[..subLength];
            var theoreticalString = string.Concat(Enumerable.Repeat(testBlock, input.Length / subLength));
            if (theoreticalString == input)
            {
                return true;
            }
        }

        return false;
    }

    private static bool IsTwiceSequence(string input)
    {
        if (input.Length < 2)
        {
            return false;
        }

        if (input.Length % 2 != 0)
        {
            return false;
        }

        var firstHalfIndex = 0;
        var secondHalfIndex = input.Length / 2;
        while (secondHalfIndex < input.Length)
        {
            if (input[firstHalfIndex] != input[secondHalfIndex])
            {
                return false;
            }

            firstHalfIndex++;
            secondHalfIndex++;
        }

        return true;
    }

    private static (string from, string to) ToLimit(string range)
    {
        var parts = range.Split('-');
        return (parts[0], parts[1]);
    }

    private static IEnumerable<string> ToRanges(string line) => line.Split(',');
}