Advent of Code - 2025 - Day 3

var input = await File.ReadAllLinesAsync("input.txt");

var part1 = 0L;
var part2 = 0L;

foreach (var line in input)
{
    part1 += FindMaxNumber(line, 2);
    part2 += FindMaxNumber(line, 12);
}

Console.WriteLine($"Part 1: {part1}");
Console.WriteLine($"Part 2: {part2}");

static long FindMaxNumber(string line, int length)
{
    var max = 0L;
    var current = line.Select(c => int.Parse(c.ToString())).ToArray();

    for (var i = 1; i <= length; i++)
    {
        var digit = current[..^(length - i)].Max();
        max = max * 10 + digit;

        var index = Array.IndexOf(current, digit);
        current = current[(index + 1)..];
    }

    return max;
}