def to_direction(start_x, start_y, target_x, target_y)
  #radians = Math.atan2(target_x - start_x, target_y - start_y)
  sx = start_x - target_x
  sy = start_y - target_y
  if sx != 0 && sy != 0
    return sx > 0 ? 4 : 6, sy > 0 ? 8 : 2
  elsif sx != 0
    return sx > 0 ? 4 : 6
  elsif sy != 0
    return sy > 0 ? 8 : 2
  end
end

# cima (8)
p to_direction(2, 1, 2, 0)
# baixo (2)
p to_direction(2, 1, 2, 3)
# esquerda (4)
p to_direction(2, 1, 0, 1)
# direita (6)
p to_direction(2, 1, 3, 1)