[hist] Implement additional RHistEngine operations

hist/histv7/inc/ROOT/RAxes.hxx

@@ -55,6 +55,7 @@ public:
    const std::vector<RAxisVariant> &Get() const { return fAxes; }
 
    friend bool operator==(const RAxes &lhs, const RAxes &rhs) { return lhs.fAxes == rhs.fAxes; }
+   friend bool operator!=(const RAxes &lhs, const RAxes &rhs) { return !(lhs == rhs); }
 
    /// Compute the total number of bins for all axes.
    ///

hist/histv7/inc/ROOT/RHistEngine.hxx

@@ -82,11 +82,26 @@ public:
    {
    }
 
-   // Copy constructor and assignment operator are deleted to avoid surprises.
+   /// The copy constructor is deleted.
+   ///
+   /// Copying all bin contents can be an expensive operation, depending on the number of bins. If required, users can
+   /// explicitly call Clone().
    RHistEngine(const RHistEngine<BinContentType> &) = delete;
+   /// Efficiently move construct a histogram engine.
+   ///
+   /// After this operation, the moved-from object is invalid.
    RHistEngine(RHistEngine<BinContentType> &&) = default;
+
+   /// The copy assignment operator is deleted.
+   ///
+   /// Copying all bin contents can be an expensive operation, depending on the number of bins. If required, users can
+   /// explicitly call Clone().
    RHistEngine<BinContentType> &operator=(const RHistEngine<BinContentType> &) = delete;
+   /// Efficiently move a histogram engine.
+   ///
+   /// After this operation, the moved-from object is invalid.
    RHistEngine<BinContentType> &operator=(RHistEngine<BinContentType> &&) = default;
+
    ~RHistEngine() = default;
 
    const std::vector<RAxisVariant> &GetAxes() const { return fAxes.Get(); }
@@ -153,6 +168,43 @@ public:
       return GetBinContent(indices);
    }
 
+   /// Add all bin contents of another histogram.
+   ///
+   /// Throws an exception if the axes configurations are not identical.
+   ///
+   /// \param[in] other another histogram
+   void Add(const RHistEngine<BinContentType> &other)
+   {
+      if (fAxes != other.fAxes) {
+         throw std::invalid_argument("axes configurations not identical in Add");
+      }
+      for (std::size_t i = 0; i < fBinContents.size(); i++) {
+         fBinContents[i] += other.fBinContents[i];
+      }
+   }
+
+   /// Clear all bin contents.
+   void Clear()
+   {
+      for (std::size_t i = 0; i < fBinContents.size(); i++) {
+         fBinContents[i] = {};
+      }
+   }
+
+   /// Clone this histogram engine.
+   ///
+   /// Copying all bin contents can be an expensive operation, depending on the number of bins.
+   ///
+   /// \return the cloned object
+   RHistEngine<BinContentType> Clone() const
+   {
+      RHistEngine<BinContentType> h(fAxes.Get());
+      for (std::size_t i = 0; i < fBinContents.size(); i++) {
+         h.fBinContents[i] = fBinContents[i];
+      }
+      return h;
+   }
+
    /// Whether this histogram engine type supported weighted filling.
    static constexpr bool SupportsWeightedFilling = std::is_floating_point_v<BinContentType>;
 

hist/histv7/test/hist_engine.cxx

@@ -63,6 +63,104 @@ TEST(RHistEngine, GetBinContentNotFound)
    EXPECT_THROW(engine.GetBinContent(Bins), std::invalid_argument);
 }
 
+TEST(RHistEngine, Add)
+{
+   static constexpr std::size_t Bins = 20;
+   const RRegularAxis axis(Bins, 0, Bins);
+   RHistEngine<int> engineA({axis});
+   RHistEngine<int> engineB({axis});
+   RHistEngine<int> engineC({axis});
+
+   engineA.Fill(-100);
+   for (std::size_t i = 0; i < Bins; i++) {
+      engineA.Fill(i);
+      engineA.Fill(i);
+      engineB.Fill(i);
+   }
+   engineB.Fill(100);
+
+   engineC.Add(engineA);
+   engineC.Add(engineB);
+
+   engineA.Add(engineB);
+
+   EXPECT_EQ(engineA.GetBinContent(RBinIndex::Underflow()), 1);
+   EXPECT_EQ(engineB.GetBinContent(RBinIndex::Underflow()), 0);
+   EXPECT_EQ(engineC.GetBinContent(RBinIndex::Underflow()), 1);
+   for (auto index : axis.GetNormalRange()) {
+      EXPECT_EQ(engineA.GetBinContent(index), 3);
+      EXPECT_EQ(engineB.GetBinContent(index), 1);
+      EXPECT_EQ(engineC.GetBinContent(index), 3);
+   }
+   EXPECT_EQ(engineA.GetBinContent(RBinIndex::Overflow()), 1);
+   EXPECT_EQ(engineB.GetBinContent(RBinIndex::Overflow()), 1);
+   EXPECT_EQ(engineC.GetBinContent(RBinIndex::Overflow()), 1);
+}
+
+TEST(RHistEngine, AddDifferent)
+{
+   // The equality operators of RAxes and the axis objects are already unit-tested separately, so here we only check one
+   // case with different the number of bins.
+   RHistEngine<int> engineA(10, 0, 1);
+   RHistEngine<int> engineB(20, 0, 1);
+
+   EXPECT_THROW(engineA.Add(engineB), std::invalid_argument);
+}
+
+TEST(RHistEngine, Clear)
+{
+   static constexpr std::size_t Bins = 20;
+   const RRegularAxis axis(Bins, 0, Bins);
+   RHistEngine<int> engine({axis});
+
+   engine.Fill(-100);
+   for (std::size_t i = 0; i < Bins; i++) {
+      engine.Fill(i);
+   }
+   engine.Fill(100);
+
+   engine.Clear();
+
+   EXPECT_EQ(engine.GetBinContent(RBinIndex::Underflow()), 0);
+   for (auto index : axis.GetNormalRange()) {
+      EXPECT_EQ(engine.GetBinContent(index), 0);
+   }
+   EXPECT_EQ(engine.GetBinContent(RBinIndex::Overflow()), 0);
+}
+
+TEST(RHistEngine, Clone)
+{
+   static constexpr std::size_t Bins = 20;
+   const RRegularAxis axis(Bins, 0, Bins);
+   RHistEngine<int> engineA({axis});
+
+   engineA.Fill(-100);
+   for (std::size_t i = 0; i < Bins; i++) {
+      engineA.Fill(i);
+   }
+   engineA.Fill(100);
+
+   RHistEngine<int> engineB = engineA.Clone();
+   ASSERT_EQ(engineB.GetNDimensions(), 1);
+   ASSERT_EQ(engineB.GetTotalNBins(), Bins + 2);
+
+   EXPECT_EQ(engineB.GetBinContent(RBinIndex::Underflow()), 1);
+   for (auto index : axis.GetNormalRange()) {
+      EXPECT_EQ(engineB.GetBinContent(index), 1);
+   }
+   EXPECT_EQ(engineB.GetBinContent(RBinIndex::Overflow()), 1);
+
+   // Check that we can continue filling the clone.
+   for (std::size_t i = 0; i < Bins; i++) {
+      engineB.Fill(i);
+   }
+
+   for (auto index : axis.GetNormalRange()) {
+      EXPECT_EQ(engineA.GetBinContent(index), 1);
+      EXPECT_EQ(engineB.GetBinContent(index), 2);
+   }
+}
+
 TEST(RHistEngine, Fill)
 {
    static constexpr std::size_t Bins = 20;