diff -u -r1.7 -r1.8
--- SensitiveDetectorFactory.hh	30 Mar 2006 19:43:58 -0000	1.7
+++ SensitiveDetectorFactory.hh	7 Feb 2007 05:18:02 -0000	1.8
@@ -1,4 +1,4 @@

-// $Header: /cvs/lcd/lcdd/include/SensitiveDetectorFactory.hh,v 1.7 2006/03/30 19:43:58 jeremy Exp $

+// $Header: /cvs/lcd/lcdd/include/SensitiveDetectorFactory.hh,v 1.8 2007/02/07 05:18:02 jeremy Exp $

 
 // LCDD
 #include "LCDDProcessor.hh"

@@ -52,6 +52,11 @@

 
   static G4ScorerSD* createScorerSD(const SAXObject* object);
 

+  /**
+   * Lookup the IdSpec of a detector SAX object based on the idspecref.
+   * @throw G4Exception if the idspecref has a bad reference value.  
+   * @return The IdSpec of this detector, or null if one does not exist.
+   */

   static IdSpec* findIdSpec(const SensitiveDetectorType* sdt);
 
   static void setBaseSensitiveDetectorAttributes(G4SensitiveDetector* sd, const SensitiveDetectorType* sdt);

diff -u -r1.12 -r1.13
--- SensitiveDetectorFactory.cc	27 Sep 2005 00:12:25 -0000	1.12
+++ SensitiveDetectorFactory.cc	7 Feb 2007 05:18:02 -0000	1.13
@@ -1,4 +1,4 @@

-// $Header: /cvs/lcd/lcdd/src/SensitiveDetectorFactory.cc,v 1.12 2005/09/27 00:12:25 jeremy Exp $

+// $Header: /cvs/lcd/lcdd/src/SensitiveDetectorFactory.cc,v 1.13 2007/02/07 05:18:02 jeremy Exp $

 #include "SensitiveDetectorFactory.hh"
 
 /* LCDD */

@@ -98,16 +98,34 @@

   const calorimeter* cal = dynamic_cast<const calorimeter*>( object );
   const std::string& hitCompareStr = cal->get_compare();
 

+  // Comparison algorithm to be used for combining hits, to be determined
+  // from attributes on sensitive detector.

   HitComparator* hitCompare = 0;

+
+  // Find the IdSpec.  
+  IdSpec* idspec = findIdSpec( sdt ); 
+
+  // The attribute value for hit_compare is not valid.
+  if ( hitCompareStr != "id" && hitCompareStr != "position" ) {
+    G4Exception("Invalid selection for hit comparison <" + hitCompareStr + ">, for calorimeter <" + sdt->get_name() + ">.");
+  }
+
+  // Compare on IDs.

   if ( hitCompareStr == "id" ) {

-    hitCompare = new IdComparator();

+    if ( idspec ) {
+      //std::cout << "id compare" << std::endl;
+      hitCompare = new IdComparator();
+    }
+    else {
+      std::cerr << "WARNING: IdSpec for <" << sdt->get_name() << "> does not exist!  Position comparison will be used for hit aggregation instead of id." << std::endl;
+    }

   }

-  else if ( hitCompareStr == "position" ) {

+
+  // Compare on position.
+  if ( !hitCompare ) {
+    //std::cout << "position compare" << std::endl;

     hitCompare = new PositionComparator();
   }

-  else {
-    G4Exception("Invalid hit compare type " + hitCompareStr);
-  }

 
   // create SD with seg and comparator
   if ( seg ) {

@@ -183,11 +201,17 @@

     // find idspec
     if ( child_tag == "idspecref" ) {
 

+      //std::cout << "found idspecref" << std::endl;
+

       IdSpecType::idspecref* id_ref =
 	dynamic_cast<IdSpecType::idspecref*> ( seq->content(i).object );
 
       idspec = IdManager::instance()->getIdSpec( id_ref->get_ref() );
 

+      if ( !idspec ) {
+	G4Exception("The IdSpec <" + id_ref->get_ref() + "> referenced by detector <" + sdt->get_name() + "> does not exist!");
+      }
+

       break;
     }
   }