# Robocode/Game Physics

## Coordinates and directions

Coordinates system
Robocode uses the Cartesian coordinate system, which means that that the (0, 0) coordinate is located at the bottom-left corner of the battlefield.
Rotational direction system
Robocode uses a clockwise direction convention where 0° & 360° is north, 90° is east, 180° is south, and 270° is west.
In radians it's 0 & 2𝜋 ~= 6.28 is north, 𝜋/2 ~= 1.57 is east, 𝜋 ~= 3.14 is south, and 3𝜋/2 ~= 4.71 is west.

## Time and distance

Time measurement
Robocode time is measured in "ticks". Each robot gets one turn per tick. 1 tick = 1 turn.
Distance measurement
Robocode's distance units are measured with double precision, so you can move a fraction of a unit. Generally, 1 Robocode distance unit = 1 pixel, except when Robocode automatically scales down battles to fit on the screen.

## Movement physics

Acceleration (a)
Robots accelerate at the rate of 1 pixel/turn every turn. Robots decelerate at the rate of 2 pixels/turn every turn. Robocode determines acceleration for you, based on the distance you are trying to move.
Velocity (v)
The velocity equation is: 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.
Distance (d)
The distance formula is: d = vt. That is, distance = velocity * time

## Rotation

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

## Bullets

Bullet damage
4 * firepower. If firepower > 1, it does an additional damage = 2 * (power - 1).
Bullet velocity
20 - 3 * firepower.
Gun heat generated on firing
1 + firepower / 5. You cannot fire if gunHeat > 0. All guns are hot at the start of each round.
Energy returned on hit
3 * firepower.

## Collisions

Collision with another robot
Each robot takes 0.6 damage. If a robot is moving away from the collision, it will not be stopped.
Collision with a wall
AdvancedRobots take damage = 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 are then paused).
3. Time is updated (time++).
4. All bullets move (including the bullet fired in the last tick) and are checked for collisions.
5. All robots move (gun, radar, heading, acceleration, velocity, distance, in that order. gun heat is also decreased in this step).
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 gleaned by following the method calls from `BaseBattle.runRound()` and `Battle.runTurn()` in the `robocode.battle` module.

### 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;
}```

## Event handling

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.