advent-2024-07.cs

long sum = 0;

var reader = new StringReader(input);
while (reader.ReadLine() is string line)
{
    var numbers = line.Split([' ', ':'], StringSplitOptions.RemoveEmptyEntries).Select(long.Parse).ToArray();
    if (numbers is [var test, ..var n] && SolveEquation(test, n))
    {
        sum += test;
    }
}

Console.WriteLine(sum);

static bool SolveEquation(long test, ReadOnlySpan<long> numbers)
{
    return numbers switch
    {
        [var n] => n == test,
        [var a, var b, .. var rest] => SolveEquation(test, [a + b, .. rest])
            || SolveEquation(test, [a * b, .. rest])
            || SolveEquation(test, [long.Parse(a.ToString() + b.ToString()), .. rest]),
        _ => false
    };
}