Secrets from the Robocode masters: Radar sweeps

Eivind Bjarte Tjore (eivind.tjore@c2i.net), Student, Norwegian University of Science and Technology

Eivind Bjarte Tjore hails from Bergen, Norway, and studies computer science at the Norwegian University of Science and Technology in Trondheim.

Summary: This tip was published in the "Cloak and turret: Learn secrets from the Robocode masters" article in the May 2002 issue of the IBM developerWorks journal.

Date: Jun 2002
Level: Introductory
Activity: 1376 views
Comments:

One of the keys to success in Robocode is always having the most updated information on every enemy bot. To accomplish this, you must use your radar in such a way that it does not sweep more of the battlefield than necessary. For instance, when your robot is standing in a corner, it makes no sense to do a full 360-degree sweep.

This tip discusses the code that I am using in et.Predator to control the radar. It assumes that your robot is an AdvancedRobot and that you have an Enemy class (a class that holds the information from ScannedRobotEvents) and an EnemyMap class (a collection of Enemies). The EnemyMap that I am using extends HashMap, but you also could use Hashtable or another suitable class.

The Enemy class must have an isUpdated() function, which checks whether the time since the information last was updated (getTime() from the Robot minus getTime() from the ScannedRobotEvent) is below some limit. I currently use 16 as the limit.

In the initialization part of the robot (in run(), before the loop), use the following:

addCustomEvent(new
  RadarTurnCompleteCondition(this));
setAdjustRadarForGunTurn(true);
setTurnRadarRight(360);

Next, add the event handler for the custom event:

public void onCustomEvent(CustomEvent e) {
  if (e.getCondition() instanceof
    RadarTurnCompleteCondition) sweep();
}

You also should use getScannedRobotEvents() each round to ensure that all ScannedRobotEvents get recorded.

Finally, add the code in Listing 1 to control the radar sweeps. Note that the code uses the int sign(double n) method that returns the sign of the number n (-1 or 1). It also uses normalRelativeAngle(double angle) to normalize the input angle to an angle between -180 and 180 degrees.

Listing 1. Radar sweep method

                
private int radarDirection=1;

private void sweep() {
  double maxBearingAbs=0, maxBearing=0;
  int scannedBots=0;
  Iterator iterator = theEnemyMap.values().
    iterator();

  while(iterator.hasNext()) {
    Enemy tmp = (Enemy)iterator.next();

    if (tmp!=null && tmp.isUpdated()) {
      double bearing=normalRelativeAngle
        (getHeading() + tmp.getBearing()
         - getRadarHeading());
      if (Math.abs(bearing)>maxBearingAbs) { 
        maxBearingAbs=Math.abs(bearing); 
        maxBearing=bearing; 
      }
      scannedBots++;
    }
  }

  double radarTurn=180*radarDirection;
  if (scannedBots==getOthers()) 		
    radarTurn=maxBearing+sign(maxBearing)*22.5; 

  setTurnRadarRight(radarTurn);
  radarDirection=sign(radarTurn);
}

The sweep() function determines the largest value for the absolute value of the bearing to the other robots. If all the robots are updated, it sets the radar to turn toward the enemy with the largest bearing from yours. If some robots have not been updated, it sets the radar to continue to sweep in the current direction.

Note that I add 22.5 degrees to the sweep, which is a safety margin because the robots have moved since the last scan. The motivation for choosing 22.5 as the constant is that the radar turns roughly 45 degrees per frame. By not using a constant, taking the distance to the robot with the largest bearing, and calculating the maximum value that the correct bearing can have, you can improve the algorithm.

Resources

Read all of the Secrets from the Robocode masters . This page will be updated as new tips become available.
Robocode's creator, Mathew Nelson, maintains the official Robocode site. This should be the first stop for anyone serious about Robocode.
RoboLeague by Christian Schnell is a league and season manager for Robocode. It ensures that all possible groupings indeed play their matches, manages the results, and produces HTML status reports.
"Rock 'em, sock 'em Robocode" (developerWorks, January 2002) disarms Robocode and starts you on your way to building your own customized lean, mean, fighting machine.
In "Rock 'em, sock 'em Robocode: Round 2" (developerWorks, May 2002), Sing Li looks at advanced robot construction and team play.
New to Java? Check out "Introduction to Java programming" (developerWorks, November 2004), a tutorial that steps you through the fundamentals of Java language programming.
developerWorks: Hundreds of articles about every aspect of Java programming.