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In HPS Java terminology, a Driver driver processes events and can read or add collection data.  A Driver can driver may create histograms from physics data or implement a step of physics reconstruction algorithm.  Drivers are executed in order, and a Driver can driver may have child Drivers drivers in order to implement complex, hierarchical behavior.  A chain of Drivers is activated by the "mother of all drivers" which contains a list of the top level Drivers as children

The following is a toy example of a driver which prints information to the console at certain points in the job processing.

Driver Example

package org.hps.users.jeremym;

import org.lcsim.event.EventHeader;
import org.lcsim.geometry.Detector;
import org.lcsim.util.Driver;

public class ExampleDriver extends Driver {

    private int value;
  
    public void process(EventHeader event) {
        System.out.println("Processing event " + event.getEventNumber());   
    }

    protected void detectorChanged(Detector detector) {
        System.out.println("Detector name is " + detector.getDetectorName());
    }
    
    protected void endOfData() {
        System.out.println("Read all the data.");
    }

    protected void startOfData() {        
        System.out.println("Starting the job.");
    }
 
    public void setValue(int value) {
        this.value = value;
    }
}

Event Processing Hooks

Each driver may implement one or more job processing hooks which are activated by the framework in the following order.

Execution Order

process method

startOfData method

The startOfData method is activated at the beginning of data processing but before detector conditions are initialized.  Any setup that needs to be performed which is independent of the detector information can be implemented here.

detectorChanged  method

The detectorChanged method is called after the conditions system is initialized.

Steering Files

The lcsim xml format can describe describes a chain of drivers and their parameters at runtime using an XML data description.  The Guidelines for Creating Compatible Drivers documentation describes how to write a Driver driver so that it can be using in the within an XML steering files.

Here is sample XML file which calls the ExampleDriver defined above and sets a parameter value on it.

<lcsim xmlns:xs="http://www.w3.org/2001/XMLSchema-instance" xs:noNamespaceSchemaLocation="http://www.lcsim.org/schemas/lcsim/1.0/lcsim.xsd">       
    <execute>
        <driver name="ExampleDriver"/>
    </execute>    
    <drivers>
        <driver name="ExampleDriver" type="org.hps.users.jeremym.ExampleDriver">
            <value>1234</value>
        </driver>
    </drivers>
</lcsim>

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Reading Event Data

public void process(EventHeader event) {        
    List<CalorimeterHit> hits = event.get(CalorimeterHit.class, "EcalHits");
    for (CalorimeterHit hit : hits) {
        System.out.println("calorimeter hit has energy " + hit.getCorrectedEnergy() + " GeV.");
    }
}

public void process(EventHeader event) {        
    List<List<CalorimeterHit>> collections = event.get(CalorimeterHit.class);
    for (List<CalorimeterHit> hits : collections) {    
        for (CalorimeterHit hit : hits) {
            System.out.println("calorimeter hit has energy " + hit.getCorrectedEnergy() + " GeV.");
        }
    }
}

public void process(EventHeader event) {        
    List<List<CalorimeterHit>> collections = event.get(CalorimeterHit.class);
    for (List<CalorimeterHit> hits : collections) {    
        System.out.println("found Calorimeter collection: " + event.getMetaData(hits).getName());
    }
}

You must provide the list object as a key in order to retrieve this metadata.

Writing Event Data

New collections can be added to the event using the put method of EventHeader.

public void process(EventHeader event) {        
    // Create new collection somehow!
    List<Track> myFancyTracks = createFancyTracks(event);
 
    // Put new collection into the event.
    int flag = 1 << LCIOConstants.TRBIT_HITS;
    event.put("MyFancyTracks", fancyTrackCollection, Track.class, flag);
}

Flags is The flags are an integer bit mask with values defined in the LCIOConstants class.  This controls some of the IO behavior of the LCIO format.

AIDA Histogramming

Histogramming of event data can be done using the AIDA utility class which provides a simplified wrapper to the AIDA plotting interface.

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AIDA provides facilities for creating 1, 2 and 3D histograms, clouds, profiles, tuples, fitting and other functionality.  (many different types of analysis objects such as histogrmas.  Refer to the AIDA API documentation for full details.)