[RF] Add flag to globally enable or disable all RooBinWidthFunctions

With the (very useful) introduction of the RooBinWidthFunction, the bin
width correction moved from the "coefficient" branch of the
RooRealSumPdf to the "function" branch of the RooRealSumPdf in
HistFactory. This can cause some distress for users who meddle with
histfactory models by hand, because bin correction can now appear at
places that are different from what they were originally.

This commit adds a flag to globally disable all bin width corrections of
RooBinWidthFunction, allowing the user to switch an entire model from
density-mode to eventcount-mode.

Author: cburgard

roofit/roofitcore/inc/RooBinWidthFunction.h

@@ -25,7 +25,13 @@
 namespace BatchHelpers { struct RunContext; }
 
 class RooBinWidthFunction : public RooAbsReal {
-public:
+  static bool _enabled;
+  
+public:  
+  static void enableClass();
+  static void disableClass();
+  static bool isClassEnabled();    
+   
   /// Create an empty instance.
   RooBinWidthFunction() :
     _histFunc("HistFuncForBinWidth", "Handle to a RooHistFunc, whose bin volumes should be returned.", this,

roofit/roofitcore/src/RooBinWidthFunction.cxx

@@ -29,11 +29,28 @@
 #include "RooDataHist.h"
 #include "RunContext.h"
 
+bool RooBinWidthFunction::_enabled = true;
+
+/// Globally enable bin-width corrections by this class.
+void RooBinWidthFunction::enableClass() {
+  _enabled = true;
+}
+
+/// Returns `true` if bin-width corrections by this class are globally enabled, `false` otherwise.
+bool RooBinWidthFunction::isClassEnabled() {
+  return _enabled;
+}
+
+/// Globally disable bin-width corrections by this class.
+void RooBinWidthFunction::disableClass() {
+  _enabled = false;
+}
 
 /// Compute current bin of observable, and return its volume or inverse volume, depending
 /// on configuration chosen in the constructor.
 /// If the bin is not valid, return a volume of 1.
 double RooBinWidthFunction::evaluate() const {
+  if(!_enabled) return 1.;
   const RooDataHist& dataHist = _histFunc->dataHist();
   const auto idx = _histFunc->getBin();
   auto volumes = dataHist.binVolumes(0, dataHist.numEntries());
@@ -51,13 +68,19 @@ void RooBinWidthFunction::computeBatch(cudaStream_t*, double* output, size_t, Ro
   std::vector<Int_t> bins = _histFunc->getBins(dataMap);
   auto volumes = dataHist.binVolumes(0, dataHist.numEntries());
 
-  if (_divideByBinWidth) {
+  if(!_enabled){
     for (std::size_t i=0; i < bins.size(); ++i) {
-      output[i] = bins[i] >= 0 ? 1./volumes[bins[i]] : 1.;
+      output[i] = 1.;
     }
   } else {
-    for (std::size_t i=0; i < bins.size(); ++i) {
-      output[i] = bins[i] >= 0 ? volumes[bins[i]] : 1.;
+    if (_divideByBinWidth) {
+      for (std::size_t i=0; i < bins.size(); ++i) {
+	output[i] = bins[i] >= 0 ? 1./volumes[bins[i]] : 1.;
+      }
+    } else {
+      for (std::size_t i=0; i < bins.size(); ++i) {
+	output[i] = bins[i] >= 0 ? volumes[bins[i]] : 1.;
+      }
     }
   }
 }