namespace ClosestResolution
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void TextChanged(object sender, EventArgs e)
        {
            //try get int from textbox
            if (int.TryParse(txtX.Text, out int width) && int.TryParse(txtY.Text, out int height))
            {
                //get closest resolution
                var closest = ClosestResolution(width, height);
                //set label text
                lblX.Text = closest.Width.ToString();
                lblY.Text = closest.Height.ToString();
            }
            else
            {
                lblX.Text = "Error";
                lblY.Text = "Error";
            }
        }

        private Size ClosestResolution(int width, int height)
        {
            //always go down in size in only one dimension
            if (width > height) //landscape
            {
                if ((width / 3) * 4 > height) //height needs to be kept
                {
                    return new Size((height * 4) / 3, height);
                }
                else
                {
                    return new Size(width, (width * 3) / 4);
                }
            }
            else //portrait
            {
                if ((height / 3) * 4 < width) //width needs to be kept
                {
                    return new Size(width, (width * 4) / 3);
                }
                else
                {
                    return new Size((height * 3) / 4, height);
                }
            }
        }
    }
}