# Robocode/Game Physics

## Coordinates and Direction Conventions

 Coordinates System: Robocode is using the Cartesian Coordinate System, which means that that the (0, 0) coordinate is located in the bottom left of the battle field. Robocode is using a clockwise direction convention where 0 / 360 deg is towards "North", 90 deg towards "East", 180 deg towards "South", and 270 deg towards "West".

Figure 1:

## Time and distance measurements in Robocode

 Time (t): Robocode time is measured in "ticks". Each robot gets one turn per tick. 1 tick = 1 turn. Robocode's units are basically measured in pixels, with two exceptions. First, all distances are measured with double precision, so you can actually move a fraction of a pixel. Second, Robocode automatically scales down battles to fit on the screen. In this case, the unit of distance is actually smaller than a pixel.

## Robot Movement Physics

 Acceleration (a): Robots accelerate at the rate of 1 pixel/turn/turn. Robots decelerate at the rate of 2 pixels/turn/turn. Robocode determines acceleration for you, based on the distance you are trying to move. v = at. Velocity can never exceed 8 pixels/turn. Note that technically, velocity is a vector, but in Robocode we simply assume the direction of the vector to be the robot's heading. d = vt. That is, distance = velocity * time

## Robot, Gun, and Radar rotation

 Max rate of rotation of robot: (10 - 0.75 * abs(velocity)) deg / turn. The faster you're moving, the slower you turn. 20 deg / turn. This is added to the current rate of rotation of the robot. 45 deg / turn. This is added to the current rate of rotation of the gun.

## Bullets

 Damage: 4 * firepower. If firepower > 1, it does an additional damage = 2 * (power - 1). 20 - 3 * firepower. 1 + firepower / 5. You cannot fire if gunHeat > 0. All guns are hot at the start of each round. 3 * firepower.

## Collisions

 With Another Robot: Each robot takes 0.6 damage. If a robot is moving away from the collision, it will not be stopped. AdvancedRobots take abs(velocity) * 0.5 - 1; (Never < 0).

## Robocode Processing Loop

The order that Robocode runs is as follows:

1. Battle view is (re)painted.
2. All robots execute their code until they take action (and then paused).
3. Time is updated (time = time + 1).
4. All bullets move and check for collisions. This includes firing bullets.
5. All robots move (gun, radar, heading, acceleration, velocity, distance, in that order).
6. All robots perform scans (and collect team messages).
7. All robots are resumed to take new action.
8. Each robot processes its event queue.

Most of this can be gleamed by following the method calls from BaseBattle.runRound() and Battle.runTurn() in the robocode.battle module.

Event dispatch happens from within commands that take a turn. So the call stack when an event is delivered usually looks like this:

Robocode internals → Robot's run method → Robocode internals → Event handler

However, event handlers can themselves make calls that take turns. If one of these happens to generate an event, we might see a call stack like

Robocode internals → Robot's run method → Robocode internals → First event handler → Robocode internals → Second event handler

But this kind of nesting could lead to a stack overflow. Or—more commonly—cases where the first handler finishes up its actions long after the response-provoking situation has passed. Thus, Robocode takes special steps for events generated within event handlers; these measures are implemented in EventManager.processEvents(). In particular, the call stack will get as far as

Robocode internals → Robot's run method → Robocode internals (including processEvents) → First event handler → Robocode internals (including processEvents)

but then the inner processEvents will detect the impending nesting and throw an EventInterruptedException, which unwinds the stack to the catch block in the outer processEvents:

Robocode internals → Robot's run method → Robocode internals (including processEvents)

effectively canceling whatever the running event handler was up to. Next, the event-delivering loop in the outer processEvents resumes delivering events, letting the second event handler execute unnested:

Robocode internals → Robot's run method → Robocode internals → Second event handler

It is possible, though not usually useful, to catch and respond to EventInterruptedExceptions in the first handler instead.

### Firing Pitfall

Because bullets are fired before the gun is moved, calling setFire() will cause the bullet to leave at the current gun heading. This may seem counter-intuitive if you are used to thinking in terms of pointing a gun, then shooting. It is also inconvenient because you can't call `setTurnGun(...)` and `setFire(...)` right after each other (not if you need perfect accuracy, anyway). Most of the time, the error will be so small you won't notice it, but if you're testing a pattern matcher against `sample.Walls`, you will occasionally spot the bug.

To get the bullet to leave after turning the gun, you will need to use code like this:

```long fireTime = 0;
void doGun() {
if (fireTime == getTime() && getGunTurnRemaining() == 0) {
setFire(2);
}

// ... aiming code ...

setTurnGunRight(...);
// Don't need to check whether gun turn will complete in single turn because
// we check that gun is finished turning before calling setFire(...).
// This is simpler since the precise angle your gun can move in one tick
// depends on where your robot is turning.
fireTime = getTime() + 1;
}```