Common: add downscale factor to BC table for studies (#6282)

* Common: add downscale factor to BC table for studies

* Please consider the following formatting changes (#301)

* Fix megalinter error

* Fix (weird) megalinter complaint

* Megalinter...

---------

Co-authored-by: ALICE Builder <alibuild@users.noreply.github.com>

Author: ddobrigk

Common/TableProducer/multiplicityExtraTable.cxx

@@ -8,6 +8,8 @@
 // In applying this license CERN does not waive the privileges and immunities
 // granted to it by virtue of its status as an Intergovernmental Organization
 // or submit itself to any jurisdiction.
+#include <CCDB/BasicCCDBManager.h> // megalinter thinks this is a C header...
+#include <bitset>
 #include "Framework/ConfigParamSpec.h"
 #include "Framework/runDataProcessing.h"
 #include "Framework/AnalysisTask.h"
@@ -16,13 +18,10 @@
 #include "Common/DataModel/Multiplicity.h"
 #include "DataFormatsFIT/Triggers.h"
 #include "TableHelper.h"
-
 #include "CCDB/CcdbApi.h"
 #include "CommonDataFormat/BunchFilling.h"
-#include <CCDB/BasicCCDBManager.h>
 #include "DataFormatsParameters/GRPObject.h"
 #include "DataFormatsParameters/GRPLHCIFData.h"
-#include <bitset>
 
 using namespace o2;
 using namespace o2::framework;
@@ -34,6 +33,12 @@ struct MultiplicityExtraTable {
   Produces<aod::MultsBC> multBC;
   Produces<aod::MultNeighs> multNeigh;
 
+  // Allow for downscaling of BC table for less space use in derived data
+  Configurable<float> bcDownscaleFactor{"bcDownscaleFactor", 2, "Downscale factor for BC table (0: save nothing, 1: save all)"};
+
+  // needed for downscale
+  unsigned int randomSeed = 0;
+
   o2::ccdb::CcdbApi ccdbApi;
   Service<o2::ccdb::BasicCCDBManager> ccdb;
   BCPattern CollidingBunch;
@@ -43,6 +48,8 @@ struct MultiplicityExtraTable {
 
   void init(InitContext&)
   {
+    randomSeed = static_cast<unsigned int>(std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::system_clock::now().time_since_epoch()).count());
+
     ccdbApi.init("http://alice-ccdb.cern.ch");
     ccdb->setURL("http://alice-ccdb.cern.ch");
     ccdb->setCaching(true);
@@ -53,6 +60,11 @@ struct MultiplicityExtraTable {
 
   void processBCs(BCsWithRun3Matchings::iterator const& bc, aod::FV0As const&, aod::FT0s const&, aod::FDDs const&, aod::Zdcs const&)
   {
+    // downscale if requested to do so
+    if (bcDownscaleFactor < 1.f && (static_cast<float>(rand_r(&randomSeed)) / static_cast<float>(RAND_MAX)) > bcDownscaleFactor) {
+      return;
+    }
+
     bool Tvx = false;
     bool isFV0OrA = false;
     float multFT0C = 0.f;