[HS3] Add structured interpolation support for HistFactory modifiers

roofit/histfactory/inc/RooStats/HistFactory/PiecewiseInterpolation.h

@@ -33,6 +33,9 @@ class PiecewiseInterpolation : public RooAbsReal {
   PiecewiseInterpolation() ;
   PiecewiseInterpolation(const char *name, const char *title, const RooAbsReal &nominal, const RooArgList &lowSet,
                          const RooArgList &highSet, const RooArgList &paramSet);
+  PiecewiseInterpolation(const char *name, const char *title, const RooAbsReal &nominal, const RooArgList &lowSet,
+                         const RooArgList &highSet, const RooArgList &paramSet,
+                         const std::vector<int> &interpolationCodes);
   ~PiecewiseInterpolation() override ;
 
   PiecewiseInterpolation(const PiecewiseInterpolation& other, const char *name = nullptr);

roofit/histfactory/src/PiecewiseInterpolation.cxx

@@ -131,7 +131,32 @@ PiecewiseInterpolation::PiecewiseInterpolation(const char *name, const char *tit
   TRACE_CREATE;
 }
 
-
+////////////////////////////////////////////////////////////////////////////////
+/// Construct a new interpolation and set the interpolation code for each
+/// parameter by position.
+/// \param name Name of the object.
+/// \param title Title (for e.g. plotting).
+/// \param nominal Nominal value of the function.
+/// \param lowSet Set of down variations.
+/// \param highSet Set of up variations.
+/// \param paramSet Parameters that control the interpolation.
+/// \param interpolationCodes Interpolation code for each parameter.
+PiecewiseInterpolation::PiecewiseInterpolation(const char *name, const char *title, const RooAbsReal &nominal,
+                                               const RooArgList &lowSet, const RooArgList &highSet,
+                                               const RooArgList &paramSet, const std::vector<int> &interpolationCodes)
+   : PiecewiseInterpolation(name, title, nominal, lowSet, highSet, paramSet)
+{
+   if (interpolationCodes.size() != _paramSet.size()) {
+      coutE(InputArguments) << "PiecewiseInterpolation::ctor(" << GetName()
+                            << ") ERROR: interpolation code vector should have the same length as the parameter list"
+                            << std::endl;
+      RooErrorHandler::softAbort();
+      return;
+   }
+   for (std::size_t i = 0; i < interpolationCodes.size(); ++i) {
+      setInterpCodeForParam(i, interpolationCodes[i]);
+   }
+}
 
 ////////////////////////////////////////////////////////////////////////////////
 /// Copy constructor

roofit/histfactory/test/testPiecewiseInterpolation.cxx

@@ -101,3 +101,22 @@ TEST(PiecewiseInterpolation, AdditiveOrMultiplicative)
       }
    }
 }
+
+TEST(PiecewiseInterpolation, ConstructWithPerParameterCodes)
+{
+   RooRealVar nominal{"nominal", "nominal", 1.0};
+   RooRealVar parameter{"parameter", "parameter", 0.0, -2.0, 2.0};
+   RooRealVar low1{"low1", "low1", 0.8};
+   RooRealVar low2{"low2", "low2", 0.7};
+   RooRealVar high1{"high1", "high1", 1.2};
+   RooRealVar high2{"high2", "high2", 1.4};
+
+   // Repeating a parameter is supported by PiecewiseInterpolation, so codes
+   // need to be assigned by position rather than by looking up the parameter.
+   RooArgList parameters{parameter, parameter};
+   RooArgList lows{low1, low2};
+   RooArgList highs{high1, high2};
+   PiecewiseInterpolation interpolation{"interpolation", "", nominal, lows, highs, parameters, {0, 2}};
+
+   EXPECT_EQ(interpolation.interpolationCodes(), (std::vector<int>{0, 2}));
+}