advent-2024-13.cs

List<Machine> machines = [];

foreach (var machine in input.Split("\n\n"))
{
    var lines = machine.Split("\n");

    var a = Split(lines[0]);
    var b = Split(lines[1]);
    var prize = Split(lines[2]);

    static string[] Split(string s) => s.Split([' ', ',', ':', '+', '='], StringSplitOptions.RemoveEmptyEntries);

    machines.Add(new Machine
    {
        A = (int.Parse(a[3]), int.Parse(a[5])),
        B = (int.Parse(b[3]), int.Parse(b[5])),
        Prize = (int.Parse(prize[2]) + 10000000000000L, int.Parse(prize[4]) + 10000000000000L)
    });
}

long sum = 0;
foreach (var machine in machines)
{
    if (ComputePresses(machine) is (long a, long b))
    {
        sum += Cost(a, b);
    }
}

Console.WriteLine(sum);

static long Cost(long a, long b) => a * 3 + b;

static (long a, long b)? ComputePresses(in Machine machine)
{
    return SolveEquations(machine.A.X, machine.B.X, machine.Prize.X, machine.A.Y, machine.B.Y, machine.Prize.Y) switch
    {
        (var a, var b) when double.IsInteger(a) && double.IsInteger(b) => ((long)a, (long)b),
        _ => null
    };
}

static (double a, double b)? SolveEquations(double x1, double x2, double x, double y1, double y2, double y)
{
    return (x1 * y2 - x2 * y1) switch
    {
        0 => null,
        var det => ((x * y2 - x2 * y) / det, (x1 * y - x * y1) / det)
    };
}

struct Machine
{
    public (int X, int Y) A;
    public (int X, int Y) B;
    public (long X, long Y) Prize;
}