advent of code 2020 day 10 part 2

aoc2020_1002.cs

// Problem: Find the number of paths through a tree where each node can have 1,2 or 3 children?

string SolvePart2(IEnumerable<string> inputs)
{
    var adapters = inputs.ToInts().Append(0).OrderBy(x => x).ToList();

    long total = 1;
    int branches = 0;
    var joltage = 0;

    for (var index = 1; index < adapters.Count; index++)
    {
        var choices = adapters.Skip(index).Count(x => x - joltage <= 3);
        joltage += adapters[index] - joltage;
        branches++;

        if (choices == 1)
        {
            total *= branches switch
            {
                1 => 1,
                2 => 2,
                3 => 4,
                4 => 7,
                _ => throw new InvalidOperationException($"{branches} is an invalid length")
            };

            branches = 0;
        }
    }

    return $"{total} possible adapter stacking options";
}