05. Remove Bullets

Image (Original Source)

The last step ended with a warning about a defect in our game. Defects happen all the time when we code so it is good idea to learn how to find, reproduce, and fix them.

Let's imagine that a player of our game reported a bug complaining that the game was exhibiting unbounded resource consumption. The best bug reports come with steps to reproduce, but we are not always so lucky.

Without steps to reproduce, the first step is to develop a theory for what could be leading to the behavior described in the report. In this case, our game is really simple. We display a single ship, can move it around, and can repeatedly press a button to fire bullets.

Our game displays itself at 30 frames per second. It is theoretically possible that something in the display logic has a memory leak, but such a defect would most likely be showing up in other games. As a general rule of thumb, don't blame your tools! Most of the time the problem is in the code you have just written and not in a tool that has gone through a rigorous testing process.

Looking back at the code we've written, the only place where we accumulate additional state instead of just overwriting it is the code to add new bullets so that seems like the best place to focus.

Now that we have a theory (that the bullet handling code is causing the resource consumption), it would be nice to be able to prove it.

Let's add some code to illustrate the problem. We will display the number of bullets we are tracking. If the bullets are not being cleaned up, the count should only ever go up.

 
function _draw()
cls()
print(#bullets,6)
 

Hit ctrl-r to run the game and fire some bullets.

In the screenshot above, we have fired a total of 10 bullets, but only 3 are visible on the screen. This clearly indicates the problem. We should only be tracking bullets that are visible. Anything that has gone off the screen is a waste of memory and processing time.

That gives us an outline of what the fix should be. Let's implement it.

 
function _update()
t=t+1
for b in all(bullets) do
b.x+=b.dx
b.y+=b.dy
if b.x<0 or b.x>128 or
b.y<0 or b.y>128 then
del(bullets,b)
end
end
 

If a bullet's x or y coordinates ever go outside of the screen bounds, we can safely get rid of it. The screen bounds are 0-128.

Before we test it, let's make a change to our bullet sprite to the bullets stand out a bit more.

Run the game with ctrl-r and let's see if we can get the number of bullets being tracked to exceed the number of bullets on the screen.

The numbers now match no matter how many bullets we fire. Success! Bug fixed and crisis averted!

End Result

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