int
board_checker::operator()(board& b)
  const
{
  if ( int id = b(1,1) )
    if ((b(0,0) == id && b(2,2) == id ) ||
        (b(0,2) == id && b(2,0) == id) ||
        (b(0,1) == id && b(2,1) == id) ||
        (b(1,0) == id && b(1,2) == id) )
      return id;
  if ( int id = b(0,0) )
    if ((b(0,1) == id && b(0,2) == id) ||
        (b(1,0) == id && b(2,0) == id) )
      return id;
  if ( int id = b(2,2) )
    if ((b(1,2) == id && b(0,2) == id) ||
        (b(2,1) == id && b(2,0) == id) )
      return id;

  bool draw=true;
  for (int x = 0; draw && x < 3; ++x )
    for (int y = 0; draw && y < 3; ++y )
      draw = b(x,y) != 0;
  if ( draw )
    return -1;
  // not finish
  return 0;
}