function init(s)
  move_delta=-1
  invader_y=0
  invaders={
    {x=10, y=0, live=true},
    {x=20, y=0, live=true},
    {x=40, y=0, live=true},
    {x=50, y=0, live=true},
    {x=15, y=8, live=true},
    {x=25, y=8, live=true},
    {x=45, y=8, live=true},
    {x=55, y=8, live=true},
  }
  invader_speed=s
end
start_invader_speed=10
init(start_invader_speed)

function vblank()
  counter=counter+1
  cls()
  --if counter==60 then counter=0 end
  process_keys()
  draw(ship.sprite, ship.x, ship.y)
  if ship.shooting then
    process_bullet()
    draw(bullet.sprite, bullet.x, bullet.y)
  end
  if counter%(invader_speed+1)==0 then
    process_invaders()
  end
  draw_invaders()
end

function process_keys()
  if key_pressed(4) and ship.x>0 then
    ship.x=ship.x-1
  elseif key_pressed(6) and ship.x <=63-7 then
    ship.x = ship.x + 1
  elseif key_pressed(5) and not ship.shooting then
    shoot()
  end
end

function shoot()
  ship.shooting = true
  bullet.x=ship.x+3
  bullet.y=ship.y-1
end

function process_bullet()
  if bullet.y>0 then
    bullet.y = bullet.y-1
  else
    ship.shooting=false
  end
  for i,inv in pairs(invaders) do
    if inv.live and bullet.y<=inv.y+#invader_sprite and bullet.x>=inv.x and bullet.x<inv.x+8 then
      ship.shooting=false
      inv.live=false
      sound_timer=4
    end
  end
end

function abs(x)
  if x<0 then return -x else return x end
end

function process_one_invader(inv)
end

function process_invaders()
  local drop=false
  local alives=0
  for i, inv in pairs(invaders) do
    if inv.live and (inv.x<=0 or inv.x+7>=63) then
      move_delta=-move_delta
      drop=true
    end
  end
  for i, inv in pairs(invaders) do
    if inv.live then
      alives=alives+1
      inv.x=inv.x+move_delta
      if drop then inv.y=inv.y+1 end
      if inv.y+#invader_sprite>=32-#ship.sprite then
	sound_timer=30
	init(start_invader_speed)
      end
    end
  end
  if alives==0 then
    sound_timer=15
    init(invader_speed-1)
  end
end

function draw_invaders()
  for i, inv in pairs(invaders) do
    if inv.live then
      draw(invader_sprite, inv.x, inv.y)
    end
  end
end

ship = {
  x=20,
  y=28,
  sprite={0x10, 0x38, 0x7C, 0xFE},
  shooting=false
}

bullet = {
  x=0,
  y=0,
  sprite={0x80}
}

counter=0

leeway=5
invader_sprite={0x3C, 0x7E, 0xFF, 0xFF, 0x99, 0x99}