# Bullet Shielding

One thing that is very important is that a BulletShielder needs to know the **EXACT** firing angles of your targeting. Even if there is a little deviation, it just don't work. That's why SimpleBot and Roborio should has no problem with shielders.

Yeah, what intrigues me is where this deviation actually come from, since there is a lot going under the hood, and why Oculus doesn't have such "problem". I know it makes perfect sense. It's just too obscure.

First, a BulletShielder would simulate a traditional HOT that is aimed from one tick before, not real fire position, as that position is your position when aiming (onScannedRobot). If it still hits, it would fallback to simulate a state-of-the-art HOT that is fired from real position, which means an advanced firer that predicts its own movement one tick forward on aiming.

However, if you are firing at real position, it is impossible for a BulletShielder to shield without moving. Therefore, for those who fire at real position, a shielder must move a little to be able to shield. **And that's where the deviation comes from. **

Therefore for a learning gun which fires from real position and is able to learn that tiny move, BulletShielders don't work.

Hmmm, it should be obvious, right? Traditional HOT isn't actually going straight to the enemy's center, so the bullet's intersection has positive length. Anyway, I also thought that the condition for bullets intersecting considered somehow imprecise calculations and that this would be enough to catch traditional HOT bullets, like the use of some epsilon when checking for the intersection. But it seems it's more like the intersection of the segments having positive length or something.

Thanks for clearing it up!

No, the bullet intersection code of robocode is exact, and the deviation of traditional HOT must be calculated exactly as well.

What do you mean by "positive length"? I thought every length should be positive ;p

And I also thought an intersection of two segment (bullets are segments when calculating intersection) is a point ;p It has no length at all.

Sorry for the confusion caused. I just named things the wrong way. When I say segment intersection, I mean the safe area generated by the intersection of those segments, like in bullet shadowing. What I wonder is if, in a perfect universe where computers can compute with perfect precision, two bullets laying on opposite rays would collide or not. Note that the lines actually intersect, so the segments. The safe area, though, degenerates to a single point. So my doubt was like: Robocode actually checks for an intersection of those segments or for a non-degenerate safe area?

Note this degenerated from an actually valuable discussion into a stupid curiosity. :P

Robocode doesn't care about that safe area at all. In segment intersection algorithm, The first thing is to determine whether they are on the same line. If so, robocode just ignores the intersection. This behavior look strange at first, but it is friendly to those who new to robocode. This way they (when trying two stationary robot, etc. sample.Fire) won't confuse — why my robot collides its bullet with another all the time?

Yeah, I was just interested in the behavior. Thank you again, that was pretty enlightening and makes total sense, it would be stupid if those bullets collided. Gotta take some time to make sure I understand all the Robocode mechanics.

After a long night, I just have to say that Bullet Shielding is the hardest thing... ever. You deserve a huge prize, man.

Thanks ;) Anyway, are you trying to build your own BulletShielder? Worth mention that a lot of good shielders are open source (although I haven't looked into any of them yet).

Yes, I **was**. I started reading the tutorial, but I found it really counter intuitive, so I decided to go by the bullet shadowing approach (or very similar to it). I looked at EnergyDome after struggling for a few hours, just to notice that the only difference was the use of Virtual Guns and the multi-movement thing. So it was doing pretty much the same thing I was for the intersection. I got happy and thought I was going to find where my bug could be. At this point I couldn't even shield a single bullet, so the VG thing was not a game changer. It turns out that I couldn't fix it. I'm just supposing it's in the actual intersection algorithm, since I can't even shield against Tracker after moving a fraction of a pixel. It should work easily if was doing things right, correct?

Even if your algorithm is correct, it won't work with wrong target. You could try SimpleBot 0.022c, which uses traditional HOT (for stationary bot) with random fire power. If the algorithm is all right, it would at least shield a lot.

That version is not in http://robocode-archive.strangeautomata.com/robots/ :P

Got shooted some times, but shielded well. All the times I was hit, the error compared to my predicted angle was something around 10^(-16). I suppose it's not enough to cause a miss. Or is it?

No, it's not enough. Anything below 1e-8 can be considered as round-off errors, and the firing range you shadowed is much much bigger. You may try some debug graphics to see if everything works as expected, say, if they collide at calculated point.

That could be enough to cause a miss if you aren't moving to intersect the bullet. I think you should check what the predicted width of your shielded area is, and compare that to EnergyDome. It is a balance moving when you make the shot to get a bigger intersection area, because that means learning guns might learn your movement and not shoot HOT anymore. But not moving at all means your intersection width might be too small to be able to hit if enemies are predicting their movement.

Also remember that even if they aren't predicting their next location, if they make a shot while they are standing still (eg changing direction) then their aiming will be at your center and you will need to move when you fire to intercept the bullet.

Yes, this is crucial. But in my experience, when I set the threshold to 1e-14, it works pretty well — If the angle between your bullet and their bullet is below 1e-14, move. Else, don't move.

I also tried to take how good the intersection into account (etc. if the intersection is below 0.1, also move), but no improvement at all.

Therefore I think it's not because 1e-16 caused the miss — even 0 is not enough, when the angle is below 1e-14, the game just ignores the intersection.

- Oculus doesn't have such problem because it does some checking to be sure that the bullet would hit %100 if enemy movement predicted correctly.
- Something like this

`boolean fire = (gunHeat == 0 && a.getGunTurnRemainingRadians() < 18 / (currentBattleInfo.distance -18));`

I would say just use getGunTurnRemainingRadians() == 0, as any deviation would decrease your hit rate if your statistics is right.

In the next version I will make it even more precise(with anti-bullet shielding of course). This version uses the angular bot width.