byte g_counter;

int box_number;
byte ball_number;

byte[] mslseq = new byte[20];
byte[] selected = new byte[20];

private void Form1_Load(object sender, EventArgs e)
{
           
  ball_number = 1;
  box_number = 0;
  g_counter = 1;

  DisplayText();
}

private void buttonEnter_Click(object sender, EventArgs e)
{  
   if (ball_number < 20)
   {
       mslseq[box_number] = ball_number;
       selected[ball_number] = 1;
   }

   //select next box
   box_number++;

   //find first available ball
    ball_number = GetNextBallNumber(UP_KEY, 1);
    g_counter = ball_number;

    if (box_number > 18)
    {
        box_number = 0;
        textBoxDisplay.Text = "Finished!!!";
    }

    DisplayText();
}


private void buttonUp_Click(object sender, EventArgs e)
{
    g_counter++;
    if (g_counter > 19)
       g_counter = 1;

    ball_number = GetNextBallNumber(UP_KEY, g_counter);
        g_counter = ball_number;

    DisplayText();

}

private void buttonDown_Click(object sender, EventArgs e)
{
    g_counter--;
    if (g_counter > 19)
        g_counter = 19;

    ball_number = GetNextBallNumber(DOWN_KEY, g_counter);
        g_counter = ball_number;

    DisplayText();
}

void DisplayText()
{
   textBoxBox.Text = (box_number + 1).ToString();
   textBoxBall.Text = ball_number.ToString();
}

byte GetNextMissleNumber(int key, byte b_num)
{
   int done = 0;
            
   while (done == 0)
   {
      if (selected[b_num] == 0)
      {
          done = 1;
          return b_num;
      }
      else
      {
          if (key == UP_KEY)
          {
               b_num++;
               if (b_num > 19)
               {
                  done = 1;
                  return 1;  //????
               }
          }
          else if (key == DOWN_KEY)
          {
              b_num--;
              if (b_num > 19) //overflow
              {
                 done = 1;
                 return 19; //????
              }
          }
       }
   }
   return 0;
}

byte GetNextBallNumber(int key, byte b_num)
        {
            int done = 0;
            int b_num_sel = b_num;

            while (done == 0)
            {
                if (selected[b_num] == 0)
                {
                    done = 1;
                    return b_num;
                }
                else
                {
                    if (key == UP_KEY)
                    {
                        b_num++;
                        if (b_num > 19)
                        {
                            b_num = 1;
                        }
                        if (b_num == b_num_sel)
                        {
                            done = 1;
                            return b_num;
                        }
                    }
                    else if (key == DOWN_KEY)
                    {
                        b_num--;
                        if (b_num < 1) 
                        {
                            b_num = 19;
                        }
                        if (b_num == b_num_sel)
                        {
                            done = 1;
                            return b_num;
                        }
                    }
                }
            }

            return 0;
        }