Radar

Radar is one of the most vital components of your robot. Without it, targeting is effectively impossible and movement is purely random. Just as with movement and targeting, there are many simple and complex algorithms for radar control. In most robots the radar takes up the smallest portion of code.

Radar in Robocode

Technical Information

A radar in Robocode can turn a maximum of 45° or π/4^rad in a single tick. The radar scans robots up to 1200 units away. The angle that the radar rotates between two ticks creates what is called a radar arc, and every robot detected within the arc is sent to the onScannedRobot() method in order of distance from the scanning bot. The closest bot is detected first, while the furthest bot is detected last. By default, the onScannedRobot() method has the lowest event priority of all the event handlers in Robocode, so it is the last one to be triggered each tick.

1-vs-1 Radars

1-vs-1 radars are the smallest of the bunch and many can get a scan in every turn, producing a perfect lock.

Spinning radar

A simple spin of the radar, this is very ineffective in one on one, but still used in NanoBots.

Here is an example of this type of radar:

public void run() {
    // ...

    do {
        turnRadarRightRadians(Double.POSITIVE_INFINITY);
    } while (true);
}

The infinity lock

The infinity lock is the simplest radar lock and it is used frequently in NanoBots. It has the disadvantage of "slipping" and losing its lock frequently. But is a much better alternative to the Spinning radar in one on one combat.

Here is an example of this type of radar:

public void run() {
    // ...
    turnRadarRightRadians(Double.POSITIVE_INFINITY);
}

public void onScannedRobot(ScannedRobotEvent e) {
    // ...
    setTurnRadarLeftRadians(getRadarTurnRemainingRadians());
}

Perfect radar locks

There are several "perfect" radar locks that will not slip once they have a lock.

Narrow lock

Point the radar at the enemy's last known location. This results in a thin beam which follows the enemy around the battlefield. Many recent one on one bots use this type of radar. This radar lock is similar to, and considered an enhancement on the wide radar lock described below.

If implemented correctly, it is not possible for the enemy escape this lock. A robot's width is 36px (that's 18px from the middle) and it can only move at up to 8px/turn, so if your beam is pointing at its centre, it won't be able to move entirely out of it in one turn. However, a robot will only automatically scan for enemies if its radar is turning, which might not happen if you are using a Narrow Lock and your enemy decides to stay still for 2 turns. For this reason, if you decide to use an unmultiplied Narrow Lock, you must call scan() yourself to avoid losing lock. Furthermode, if you skip turns, your enemy might be able to move out from your radar beam before you recover.

To overcome these issues, a Narrow Lock is often multiplied by a factor (as described below) to keep the radar moving and ensure the enemy does not escape the scan arc.

Here is an example of this kind of radar:

import robocode.util.Utils;

public void run() {
    // ...

    turnRadarRightRadians(Double.POSITIVE_INFINITY);
    do {
        // Check for new targets.
        // Only necessary for Narrow Lock because sometimes our radar is already
        // pointed at the enemy and our onScannedRobot code doesn't end up telling
        // it to turn, so the system doesn't automatically call scan() for us
        // [see the javadocs for scan()].
        scan();
    } while (true);
}

public void onScannedRobot(ScannedRobotEvent e) {
    double radarTurn =
        // Absolute bearing to target
        getHeadingRadians() + e.getBearingRadians()
        // Subtract current radar heading to get turn required
        - getRadarHeadingRadians();

    setTurnRadarRightRadians(Utils.normalRelativeAngle(radarTurn));

    // ...
}

The following factors affect the behaviour in the following ways (eg. factor * Utils.normalRelativeAngle(radarTurn))

• 1.0 - Thin radar lock. Must call scan() to avoid losing lock. God help you if you ever skip a turn.
• 1.9 - Radar arc starts wide and slowly narrows as much as possible while staying on target.
• 2.0 - Radar arc sweeps through a fixed angle. Exact angle chosen depends on positions of enemy and radar when enemy is first picked up. Angle will be increased if necessary to maintain a lock. Most used corrective factor.

Wide lock

The Wide Radar Lock tries to scan a fixed distance to either side of the enemy. It was to the best of knowledge created before the Narrow Radar Lock. Its constant motion means the radar will not slip as long as you don't miss any turns.

Here is an example of this type of radar:

import robocode.util.Utils;

public void run() {
    // ...

    do {
        // ...
        if (getRadarTurnRemaining() == 0.0)
            setTurnRadarRightRadians(Double.POSITIVE_INFINITY);
        execute();
    } while (true);
}

public void onScannedRobot(ScannedRobotEvent e) {
    double radarTurn = Utils.normalRelativeAngle(
        // Absolute bearing to target
        getHeadingRadians() + e.getBearingRadians()
        // Subtract current radar heading to get turn required
        - getRadarHeadingRadians() );

    // Distance we want to scan from middle of enemy to either side
    double extraTurn = Math.min(Math.atan(36.0 / e.getDistance()), Math.PI/4.0);

    setTurnRadarRightRadians(radarTurn + (radarTurn < 0) ? -extraTurn : extraTurn);

    // ...
}

You can replace 36.0 with any distance you want covered. Too large a value and your robot will be caught in an infinity lock (described above).

Melee radars

Melee radars are more complex and take up considerable more room inside a robot. Since the field of opponents does not usually fall within a 45° area, compromises must be made between frequent data of one bot (e.g., the firing target) and consistently updated data of all bots.

Spinning radar

Just as with one on one, there is the generic spinning radar. This is the most used melee radar as it is by far the easiest to implement.

public void run() {
    // ...

    while(true) {
        turnRadarRightRadians(Double.POSITIVE_INFINITY);
    }
}

Corner Arc

A variation on the spinning radar, if the robot is in a corner it scans back and forth across the 90 degree arc away from the corner, as not to waste time scanning where there cannot be any robots. Typically, this is used only in smaller melee robots, which do not have room for a more complicated radar system. Melee bots with room tend to use one of the implementations below.

Oldest Scanned

This type of melee radar spins towards the robot it hasn't seen in the longest amount of time. Here is an example of this type of radar.

import java.util.*;
import robocode.*;
import robocode.util.*;

// ... Within robot class

static LinkedHashMap<String, Double> enemyHashMap;
static double scanDir;
static Object sought;

public void run() {
    scanDir = 1;
    enemyHashMap = new LinkedHashMap<String, Double>(5, 2, true);

    // ...

    while(true) {
        setTurnRadarRightRadians(scanDir * Double.POSITIVE_INFINITY);
        scan();
    }
}

public void onRobotDeath(RobotDeathEvent e) {
    enemyHashMap.remove(e.getName());
    sought = null;
}

public void onScannedRobot(ScannedRobotEvent e) {
    String name = e.getName();
    LinkedHashMap<String, Double> ehm = enemyHashMap;

    ehm.put(name, getHeadingRadians() + e.getBearingRadians());

    if ((name == sought || sought == null) && ehm.size() == getOthers()) {
	scanDir = Utils.normalRelativeAngle(ehm.values().iterator().next()
            - getRadarHeadingRadians());
        sought = ehm.keySet().iterator().next();
    }

    // ...
}

With above code, the radar will spin past every bot, and then reverse it's direction until its passed all bots, then repeat. If the spin takes more than 4 ticks, the radar will continue spinning like the spinning radar above. This kind of radar is used in many top bots include Shadow and Phoenix. This kind of radar should be used unless you need a melee radar lock or your targeting system requires you to have enemy scans every 8 ticks.

Gun Heat Lock

A technique developed by Paul Evans/Kawagi for SandboxDT/FloodHT. This radar locks onto a target when the robot has low gunheat. Normally this is best for melee robots which use GuessFactor Targeting.

public void run() {
    // ...
    setTurnRadarRightRadians(Double.POSITIVE_INFINITY);
}

public void onScannedRobot(ScannedRobotEvent e) {
    // ... your own target selection
    double absoluteBearing = getHeadingRadians() + e.getBearingRadians();
    if (isCurrentTarget && getGunHeat() < 0.5) { // Lock for 5 ticks
        setTurnRadarRightRadians(3.5 * Utils.normalRelativeAngle(absoluteBearing - getRadarHeadingRadians()));
    else
        setTurnRadarRightRadians(Double.POSITIVE_INFINITY); 
}

The code above uses a Narrow Lock but the simpler Infinity Lock works just as well. For a narrow lock, it is recommended you multiply by a large factor to continue scanning other robots while locked.

Notes

For most of these radar locks, you will need to add one of the following to your run() method:

setAdjustRadarForRobotTurn(true);
setAdjustGunForRobotTurn(true);
setAdjustRadarForGunTurn(true);