ITS digi: Ability to process events before the first RO

This commit makes it possible to inject events/hits into the
ITS digitizer which happen in time before the "first interaction sampled"
(start of the first readout-frame given by HBFUtils).

The commit avoids/fixes a segmenation fault which, so far, occurs
in these situations (because of overflow of unsigned int32 newROFrame)

The commit allows to reduce "startup" effects in a timeframe by
capturing hits that come from events before data taking starts. This makes
MC more realistic.

With these "startup effects" removed, one could now play with short
timeframe lengths for skimming studies.

Detectors/ITSMFT/common/simulation/include/ITSMFTSimulation/Digitizer.h

@@ -118,6 +118,7 @@ class Digitizer : public TObject
   uint32_t mROFrameMin = 0; ///< lowest RO frame of current digits
   uint32_t mROFrameMax = 0; ///< highest RO frame of current digits
   uint32_t mNewROFrame = 0; ///< ROFrame corresponding to provided time
+  bool mIsBeforeFirstRO = false;
 
   uint32_t mEventROFrameMin = 0xffffffff; ///< lowest RO frame for processed events (w/o automatic noise ROFs)
   uint32_t mEventROFrameMax = 0;          ///< highest RO frame forfor processed events (w/o automatic noise ROFs)

Detectors/ITSMFT/common/simulation/src/Digitizer.cxx

@@ -102,6 +102,10 @@ void Digitizer::init()
   }
   mParams.print();
   mIRFirstSampledTF = o2::raw::HBFUtils::Instance().getFirstSampledTFIR();
+
+  //
+  LOG(info) << "First IR sampled in digitization is: " << mIRFirstSampledTF;
+  LOG(info) << "First IR ns " << mIRFirstSampledTF.bc2ns();
 }
 
 auto Digitizer::getChipResponse(int chipID)
@@ -167,7 +171,19 @@ void Digitizer::setEventTime(const o2::InteractionTimeRecord& irt)
     if (mCollisionTimeWrtROF < 0 && nbc > 0) {
       nbc--;
     }
-    mNewROFrame = nbc / mParams.getROFrameLengthInBC();
+
+    // we might get interactions to digitize from before
+    // the first sampled IR
+    if (nbc < 0) {
+      mNewROFrame = 0;
+      // this event is before the first RO
+      mIsBeforeFirstRO = true;
+    } else {
+      mNewROFrame = nbc / mParams.getROFrameLengthInBC();
+      mIsBeforeFirstRO = false;
+    }
+    LOG(info) << " NewROFrame " << mNewROFrame << " nbc " << nbc;
+
     // in continuous mode depends on starts of periodic readout frame
     mCollisionTimeWrtROF += (nbc % mParams.getROFrameLengthInBC()) * o2::constants::lhc::LHCBunchSpacingNS;
   } else {
@@ -275,6 +291,11 @@ void Digitizer::processHit(const o2::itsmft::Hit& hit, uint32_t& maxFr, int evID
   if (isContinuous()) {
     timeInROF += mCollisionTimeWrtROF;
   }
+  if (mIsBeforeFirstRO && timeInROF < 0) {
+    // disregard this hit because it comes from an event before readout starts and it does not effect this RO
+    return;
+  }
+
   // calculate RO Frame for this hit
   if (timeInROF < 0) {
     timeInROF = 0.;
@@ -290,7 +311,7 @@ void Digitizer::processHit(const o2::itsmft::Hit& hit, uint32_t& maxFr, int evID
     maxFr = roFrameMax; // if signal extends beyond current maxFrame, increase the latter
   }
 
-  // here we start stepping in the depth of the sensor to generate charge diffision
+  // here we start stepping in the depth of the sensor to generate charge diffusion
   float nStepsInv = mParams.getNSimStepsInv();
   int nSteps = mParams.getNSimSteps();
   const auto& matrix = mGeometry->getMatrixL2G(hit.GetDetectorID());