using System;
using System.Windows.Forms;

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

        float prev_value;
        Keys curr_operator = Keys.None;
        bool wait_for_new = false;

        void Calc()
        {
            float curr_value = float.Parse(textBox1.Text);
            float new_value = 0;
            switch(curr_operator)
            {
                case Keys.Add:
                    new_value = prev_value + curr_value;
                    textBox1.Text = new_value.ToString();
                    prev_value = new_value;
                    wait_for_new = true;
                    break;
                case Keys.Subtract:
                    new_value = prev_value - curr_value;
                    textBox1.Text = new_value.ToString();
                    prev_value = new_value;
                    wait_for_new = true;
                    break;
                case Keys.Multiply:
                    new_value = prev_value * curr_value;
                    textBox1.Text = new_value.ToString();
                    prev_value = new_value;
                    wait_for_new = true;
                    break;
                case Keys.Divide:
                    new_value = prev_value / curr_value;
                    textBox1.Text = new_value.ToString();
                    prev_value = new_value;
                    wait_for_new = true;
                    break;
                default: break;
            }
            curr_operator = Keys.None;
        }

        private void textBox1_KeyDown(object sender, KeyEventArgs e)
        {
            switch(e.KeyCode)
            {
                case Keys.Add:
                    Calc();
                    curr_operator = Keys.Add;
                    wait_for_new = true;
                    e.SuppressKeyPress = true;
                    break;
                case Keys.Subtract:
                    Calc();
                    curr_operator = Keys.Subtract;
                    wait_for_new = true;
                    e.SuppressKeyPress = true;
                    break;
                case Keys.Multiply:
                    Calc();
                    curr_operator = Keys.Multiply;
                    wait_for_new = true;
                    e.SuppressKeyPress = true;
                    break;
                case Keys.Divide:
                    Calc();
                    curr_operator = Keys.Divide;
                    wait_for_new = true;
                    e.SuppressKeyPress = true;
                    break;
                case Keys.Enter:
                    Calc();
                    wait_for_new = true;
                    e.SuppressKeyPress = true;
                    break;
                case Keys.Escape:
                    e.SuppressKeyPress = true;
                    wait_for_new = true;
                    textBox1.Text = string.Empty;
                    break;
                default:
                    if(wait_for_new)
                    {
                        prev_value = (textBox1.Text.Length > 0) ? float.Parse(textBox1.Text) : 0;
                        textBox1.Text = string.Empty;
                        wait_for_new = false;
                    }
                    break;
            };
        }

    }
}