Sweeping !20374 from 21.3 to 21.0.SCT RDO updates for overlay MC (ATLASSIM-3903)

SCT RDO updates to reduce size for overlay MC (ATLASSIM-3903)

See merge request !20374 (merged)

Although physics results will not be changed, containers in RDO will be changed. (ESD and AOD will not be changed.)

!20213 (merged) (merged) is the corresponding MR for the master branch.

!20374 (merged) (merged) is the corresponding MR for the 21.3 branch.

Now this MR is to have the forward compatibility in 21.0 to RDO files produced with master or 21.3, and not to reduce the RDO file size because the old SCT_RawDataContainer_p3 is used for persistification.

InnerDetector/InDetDigitization/SCT_Digitization/src/SCT_FrontEnd.cxx

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration
 */
 
 #include "SCT_FrontEnd.h"
@@ -21,10 +21,10 @@
 
 // C++ Standard Library
 #include <algorithm>
+#include <iostream>
 
 // #define SCT_DIG_DEBUG
 
-#include <iostream>
 using namespace std;
 // ---------------
 
@@ -1383,25 +1383,36 @@ StatusCode SCT_FrontEnd::doClustering(SiChargedDiodeCollection &collection)
         } while (strip < m_strip_max);
     }
     else {
-        do { // Expanded read out mode, one RDO/strip per cluster
-            if (m_StripHitsOnWafer[strip] > 0) {
-                clusterSize = 1;
-                hitStrip = m_sct_id->strip_id(collection.identify(), strip);
-                SiChargedDiode &HitDiode = *(collection.find(hitStrip));         
-                                                                                 
-                SiHelper::SetStripNum(HitDiode, clusterSize);                     
-                                                                                  
-
+      // Expanded read out mode, basically one RDO/strip per cluster
+      // But if consecutively fired strips have the same time bin, those are converted into one cluster.
+      do {
+        clusterSize = 1;
+        if (m_StripHitsOnWafer[strip] > 0) {
+          hitStrip = m_sct_id->strip_id(collection.identify(), strip);
+          SiChargedDiode& hitDiode = *(collection.find(hitStrip));
+          int timeBin = SiHelper::GetTimeBin(hitDiode);
+          SiChargedDiode* previousHitDiode = &hitDiode;
+          // Check if consecutively fired strips have the same time bin
+          for (int newStrip=strip+1; newStrip<m_strip_max; newStrip++) {
+            if (not (m_StripHitsOnWafer[newStrip]>0)) break;
+            Identifier newHitStrip = m_sct_id->strip_id(collection.identify(), newStrip);
+            SiChargedDiode& newHitDiode = *(collection.find(newHitStrip));
+            if (timeBin!=SiHelper::GetTimeBin(newHitDiode)) break;
+            SiHelper::ClusterUsed(newHitDiode, true);
+            previousHitDiode->setNextInCluster(&newHitDiode);
+            previousHitDiode = &newHitDiode;
+            clusterSize++;
+          }
+          SiHelper::SetStripNum(hitDiode, clusterSize);
 #ifdef SCT_DIG_DEBUG
-                ATH_MSG_DEBUG("RDO Strip = " << strip << ", tbin = " <<
-                    SiHelper::GetTimeBin(HitDiode) <<
-                    ", HitInfo(1=real, 2=crosstalk, 3=noise): " <<
-                    m_StripHitsOnWafer[strip]);
+          ATH_MSG_DEBUG("RDO Strip = " << strip << ", tbin = " <<
+                        SiHelper::GetTimeBin(hitDiode) <<
+                        ", HitInfo(1=real, 2=crosstalk, 3=noise): " <<
+                        m_StripHitsOnWafer[strip]);
 #endif
-            }
-            ++strip;                // !< This is the starting point of the next
-                                    // cluster within this collection
-        } while (strip < m_strip_max);
+        }
+        strip += clusterSize; // If more than one strip fires, those following strips are skipped.
+      } while (strip < m_strip_max);
     }
 
     // clusters below threshold, only from pre-digits that existed before no