def plot(x,y)
  puts "(#{x},#{y})"
end

def les(x1,y1,x2,y2, edge=:left)
  # sort by y
  if y1 > y1
    x1,y1,x2,y2 = x2,y2,x1,y1
  end

  # precalculations
  dx = x2-x1
  dy = y2-y1
  m = dy.to_f/dx

  x = x1
  s = dx / dx.abs
  if edge == :left
    if s > 0
      inc = dy
      o = dy
    else
      inc = 0
      o = dy-1
    end
  else
    if s > 0
      inc = 0
      o = dy-1
    else
      inc = -dy
      o = dy
    end
  end

  y1.upto(y2) do |y|
    plot(x,y)
    inc += dx
    while inc.abs > o
      x += s
      inc -= s*dy
    end
  end
  puts
end

les(0,0,3,12)
les(0,0,-3,12)
les(0,0,11,3)
les(0,0,-11,3)

les(0,0,3,12,:right)
les(0,0,-3,12,:right)
les(0,0,11,3,:right)
les(0,0,-11,3,:right)