Advent of Code 2024 - Day 7

var timer = System.Diagnostics.Stopwatch.StartNew();
var input = await File.ReadAllLinesAsync("input.txt");

var equations = new List<(long Target, long[] Values)>();

foreach (var line in input)
{
    var split = line.Split(": ");
    var targetValue = long.Parse(split[0]);
    var values = split[1].Split(' ').Select(long.Parse).ToArray();

    equations.Add((targetValue, values));
}

Console.WriteLine($"Parsing done. ({timer.ElapsedMilliseconds}ms)");

var part1 = equations.Where(equation => IsValid(equation.Target, 0, 0, equation.Values))
    .Sum(equation => equation.Target);

Console.WriteLine($"Part 1: {part1} ({timer.ElapsedMilliseconds}ms)");

var part2 = equations.Where(equation => IsValid(equation.Target, 0, 0, equation.Values, true))
    .Sum(equation => equation.Target);

Console.WriteLine($"Part 2: {part2} ({timer.ElapsedMilliseconds}ms)");

return;

bool IsValid(long targetValue, long currentValue, int index, long[] values, bool isPart2 = false)
{
    if (index == values.Length)
    {
        return currentValue == targetValue;
    }

    if (currentValue > targetValue)
    {
        return false;
    }

    var result = IsValid(targetValue, currentValue + values[index], index + 1, values, isPart2) ||
                 IsValid(targetValue, currentValue * values[index], index + 1, values, isPart2);

    if (!isPart2 || result) return result;

    var concat = Concat(currentValue, values[index]);

    result = IsValid(targetValue, concat, index + 1, values, true);

    return result;
}

static long Concat(long a, long b)
{
    var pow = (int)Math.Log10(b) + 1;
    return a * (long)Math.Pow(10, pow) + b;
}