Advent of Code 2023 - Day 15

var input = File.ReadAllText("input.txt").Split(',');

var hashes = input.Select(Hash).ToList();

Console.WriteLine($"Part 1: {hashes.Sum()}");

var boxes = new List<List<(string Name, int FocalLength)>>();
Enumerable.Range(0, 256).ToList().ForEach(_ => boxes.Add(new()));
var separator = new[] { '=', '-' };

foreach (var step in input)
{
    var instructions = step.Split(separator, StringSplitOptions.RemoveEmptyEntries);
    var lensName = instructions[0];
    var box = boxes[Hash(lensName)];

    if (instructions.Length == 2)
    {
        var focalLength = int.Parse(instructions[1]);
        var lens = (lensName, focalLength);
        var lensIndex = box.FindIndex(l => l.Name == lensName);

        if (lensIndex > -1)
        {
            box[lensIndex] = lens;
        }
        else
        {
            box.Add(lens);
        }
    }
    else
    {
        var lens = box.FirstOrDefault(l => l.Name == lensName);

        if (lens != default)
        {
            box.Remove(lens);
        }
    }
}

var part2 = 0;
for (var boxIndex = 0; boxIndex < boxes.Count; boxIndex++)
{
    part2 += boxes[boxIndex]
        .Select((lens, lensIndex) => lens.FocalLength * (lensIndex + 1) * (boxIndex + 1))
        .Sum();
}

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

static int Hash(string s)
{
    var i = 0;
    foreach (var c in s)
    {
        i += c;
        i *= 17;
        i %= 256;
    }

    return i;
}