[hist] Implement initial RProfile

hist/histv7/doc/CodeArchitecture.md

@@ -50,6 +50,10 @@ It also keeps statistics of the unbinned values for each dimension.
 This class combines `RHistEngine`, with its axes and bin contents, and `RHistStats`.
 During `Fill`, it delegates to `RHistEngine::Fill` but also updates the histogram statistics.
 
+### `RProfile`
+
+This class builds on `RHistEngine`, but requires an additional value during `Fill`.
+
 ## Classes for Weighted Filling
 
 ### `RBinWithError`

hist/histv7/headers.cmake

@@ -14,6 +14,7 @@ set(histv7_headers
     ROOT/RHistStats.hxx
     ROOT/RHistUtils.hxx
     ROOT/RLinearizedIndex.hxx
+    ROOT/RProfile.hxx
     ROOT/RRegularAxis.hxx
     ROOT/RSliceBinIndexMapper.hxx
     ROOT/RSliceSpec.hxx

hist/histv7/inc/LinkDef.h

@@ -20,6 +20,10 @@
 #pragma link C++ class ROOT::Experimental::RHistEngine<double>-;
 #pragma link C++ class ROOT::Experimental::RHistEngine<ROOT::Experimental::RBinWithError>-;
 
+#pragma link C++ class ROOT::Experimental::RProfile::RProfileBin-;
+#pragma link C++ class ROOT::Experimental::RHistEngine<ROOT::Experimental::RProfile::RProfileBin>-;
+#pragma link C++ class ROOT::Experimental::RProfile-;
+
 #pragma link C++ class ROOT::Experimental::RAxisVariant-;
 #pragma link C++ class ROOT::Experimental::RCategoricalAxis-;
 #pragma link C++ class ROOT::Experimental::RHistStats-;

hist/histv7/inc/ROOT/RHist.hxx

@@ -8,7 +8,6 @@
 #include "RAxisVariant.hxx"
 #include "RBinIndex.hxx"
 #include "RBinIndexMultiDimRange.hxx"
-#include "RCategoricalAxis.hxx"
 #include "RHistEngine.hxx"
 #include "RHistStats.hxx"
 #include "RRegularAxis.hxx"

hist/histv7/inc/ROOT/RHistEngine.hxx

@@ -75,6 +75,8 @@ class RHistEngine final {
    // For slicing, RHist needs to call SliceImpl.
    friend class RHist<BinContentType>;
 
+   friend class RProfile;
+
    /// The axis configuration for this histogram. Relevant methods are forwarded from the public interface.
    Internal::RAxes fAxes;
    /// The bin contents for this histogram
@@ -407,6 +409,25 @@ public:
       }
    }
 
+   /// \}
+   // End the group to ensure that all contained member functions are public.
+
+private:
+   // Also used by RProfile::Fill(const A &... args)
+   template <std::size_t N, typename... A, typename W>
+   void FillImpl(const std::tuple<A...> &args, const W &weight)
+   {
+      RLinearizedIndex index = fAxes.ComputeGlobalIndexImpl<N>(args);
+      if (index.fValid) {
+         assert(index.fIndex < fBinContents.size());
+         fBinContents[index.fIndex] += weight;
+      }
+   }
+
+public:
+   /// \name Filling
+   /// \{
+
    /// Fill an entry into the histogram with a user-defined weight.
    ///
    /// This overload is only available for user-defined bin content types.
@@ -430,11 +451,7 @@ public:
       if (sizeof...(A) != GetNDimensions()) {
          throw std::invalid_argument("invalid number of arguments to Fill");
       }
-      RLinearizedIndex index = fAxes.ComputeGlobalIndexImpl<sizeof...(A)>(args);
-      if (index.fValid) {
-         assert(index.fIndex < fBinContents.size());
-         fBinContents[index.fIndex] += weight;
-      }
+      FillImpl<sizeof...(A)>(args, weight);
    }
 
    /// Fill an entry into the histogram.
@@ -470,7 +487,7 @@ public:
          if constexpr (std::is_same_v<typename Internal::LastType<A...>::type, RWeight>) {
             static_assert(SupportsWeightedFilling, "weighted filling is not supported for integral bin content types");
             static constexpr std::size_t N = sizeof...(A) - 1;
-            if (N != fAxes.GetNDimensions()) {
+            if (N != GetNDimensions()) {
                throw std::invalid_argument("invalid number of arguments to Fill");
             }
             RWeight weight = std::get<N>(t);
@@ -566,7 +583,7 @@ public:
          if constexpr (std::is_same_v<typename Internal::LastType<A...>::type, RWeight>) {
             static_assert(SupportsWeightedFilling, "weighted filling is not supported for integral bin content types");
             static constexpr std::size_t N = sizeof...(A) - 1;
-            if (N != fAxes.GetNDimensions()) {
+            if (N != GetNDimensions()) {
                throw std::invalid_argument("invalid number of arguments to Fill");
             }
             RWeight weight = std::get<N>(t);

hist/histv7/inc/ROOT/RHistUtils.hxx

@@ -5,7 +5,9 @@
 #ifndef ROOT_RHistUtils
 #define ROOT_RHistUtils
 
+#include <tuple>
 #include <type_traits>
+#include <utility>
 
 #ifdef _MSC_VER
 #include <atomic>
@@ -25,6 +27,19 @@ struct LastType<T> {
    using type = T;
 };
 
+template <typename... Ts, std::size_t... I, typename A>
+std::tuple<const Ts &..., const A &>
+AppendReferenceImpl(const std::tuple<Ts...> &t, std::index_sequence<I...>, const A &a)
+{
+   return std::tuple<const Ts &..., const A &>(std::get<I>(t)..., a);
+}
+
+template <typename... Ts, typename A>
+std::tuple<const Ts &..., const A &> AppendReference(const std::tuple<Ts...> &t, const A &a)
+{
+   return AppendReferenceImpl(t, std::make_index_sequence<sizeof...(Ts)>(), a);
+}
+
 #ifdef _MSC_VER
 namespace MSVC {
 template <std::size_t N>