[RF] Don't randomize num. events when creating Asimov from Simultaneous

Closes JIRA ticket https://its.cern.ch/jira/browse/ROOT-7499

Implement also a unit test, inspired by the reproducer posted on JIRA.

Author: guitargeek

roofit/roofitcore/inc/RooSimSplitGenContext.h

@@ -54,6 +54,7 @@ class RooSimSplitGenContext : public RooAbsGenContext {
   std::vector<int>               _gcIndex ; ///< Index value corresponding to component
   TString _idxCatName ;                     ///< Name of index category
   Int_t _numPdf ;                           ///< Number of generated PDFs
+  bool _expectedData = false;               ///< Asimov?
 
   RooArgSet _allVarsPdf ; ///< All pdf variables
 

roofit/roofitcore/src/RooSimSplitGenContext.cxx

@@ -191,7 +191,11 @@ RooDataSet* RooSimSplitGenContext::generate(double nEvents, bool skipInit, bool
     nExpectedTotal += nExpected.back();
   }
 
-  if (extendedMode ) {
+  // We don't randomize events in two cases:
+  //  1. When the generation is extended, each component pdf will already
+  //     randomize the expected number of events so we don't need to do it here.
+  //  2. If we want to create an expected Asimov dataset.
+  if (extendedMode || _expectedData) {
     nGen = nExpected;
   } else {
     // Determine from that total number of events to be generated for each component
@@ -237,6 +241,7 @@ RooDataSet* RooSimSplitGenContext::generate(double nEvents, bool skipInit, bool
 
 void RooSimSplitGenContext::setExpectedData(bool flag)
 {
+  _expectedData = flag;
   for(RooAbsGenContext *elem : _gcList) {
     elem->setExpectedData(flag) ;
   }

roofit/roofitcore/test/testRooSimultaneous.cxx

@@ -1,6 +1,7 @@
 // Tests for the RooSimultaneous
 // Authors: Jonas Rembser, CERN  06/2021
 
+#include <Roo1DTable.h>
 #include <RooAddition.h>
 #include <RooCategory.h>
 #include <RooConstVar.h>
@@ -541,3 +542,35 @@ TEST(RooSimultaneous, PlotProjWData)
    // sum of data entries in the center.
    EXPECT_DOUBLE_EQ(frame->getCurve()->interpolate(0.), combData.sumEntries());
 }
+
+/// JIRA ticket https://its.cern.ch/jira/browse/ROOT-7499
+/// Check that we can also generate Asimov datasets with non-integer weights
+/// via RooSimultaneous.
+TEST(RooSimultaneous, ExpectedDataWithNonIntegerWeights)
+{
+
+   RooWorkspace ws{"ws"};
+   ws.factory("dummy_obs_a[0,1]");
+   ws.factory("dummy_obs_b[0,1]");
+   ws.factory("Uniform::uniform_a(dummy_obs_a)");
+   ws.factory("Uniform::uniform_b(dummy_obs_b)");
+   ws.factory("SUM::model_a(coeff_a[3.5]*uniform_a)");
+   ws.factory("SUM::model_b(coeff_b[6.5]*uniform_b)");
+
+   RooRealVar &dummy_obs_a = *ws.var("dummy_obs_a");
+   RooRealVar &dummy_obs_b = *ws.var("dummy_obs_b");
+
+   ws.factory("dummy_cat[a]");
+   ws.factory("SIMUL::sim_model(dummy_cat, a = model_a, b = model_b)");
+   RooAbsCategory &dummy_cat = *ws.cat("dummy_cat");
+
+   // std::cout << "simultaneous expected = " << ws.pdf("sim_model")->expectedEvents(dummy_obs) << std::endl;
+   RooDataSet *data = ws.pdf("sim_model")->generate({dummy_obs_a, dummy_obs_b, dummy_cat}, RooFit::ExpectedData());
+
+   std::unique_ptr<Roo1DTable> tab{data->table(dummy_cat)};
+
+   // Check that the sum of entries for each category is as expected, matching
+   // the coefficients from the RooAddPdf.
+   EXPECT_FLOAT_EQ(tab->get("a"), ws.var("coeff_a")->getVal());
+   EXPECT_FLOAT_EQ(tab->get("b"), ws.var("coeff_b")->getVal());
+}