improve(CompositeArray): performance gain by flattening binary tree

The vtkCompositeArray performance was lagging due to the internal binary
tree structure. Flattening it out allowed for a considerable reduction
in the overhead of the composite array.

Common/ImplicitArrays/Testing/Cxx/TestCompositeImplicitBackend.cxx

@@ -37,7 +37,7 @@ int TestCompositeImplicitBackend(int, char*[])
   std::iota(rightRange.begin(), rightRange.end(), 10);
 
   // Make structure
-  vtkCompositeImplicitBackend<int> composite(left, right);
+  vtkCompositeImplicitBackend<int> composite(std::vector<vtkDataArray*>({ left, right }));
 
   // Do checks on structure
   for (int i = 0; i < 20; ++i)
@@ -62,7 +62,8 @@ int TestCompositeImplicitBackend(int, char*[])
   auto rightMultiRange = vtk::DataArrayValueRange<3>(rightMulti);
   std::iota(rightMultiRange.begin(), rightMultiRange.end(), 30);
 
-  vtkCompositeImplicitBackend<int> compositeMulti(leftMulti, rightMulti);
+  vtkCompositeImplicitBackend<int> compositeMulti(
+    std::vector<vtkDataArray*>({ leftMulti, rightMulti }));
 
   for (int i = 0; i < 60; ++i)
   {

Common/ImplicitArrays/vtkCompositeArray.h

@@ -48,8 +48,9 @@
  * rightArr->SetNumberOfComponents(1);
  * rightArr->SetNumberOfTuples(1);
  * rightArr->SetValue(0, 1);
+ * std::vector<vtkDataArray*> arrays({leftArr, rightArr});
  * vtkNew<vtkCompositeArray<int>> compositeArr;
- * compositeArr->SetBackend(std::make_shared<vtkCompositeImplicitBackend<int>>(leftArr, rightArr));
+ * compositeArr->SetBackend(std::make_shared<vtkCompositeImplicitBackend<int>>(arrays));
  * compositeArr->SetNumberOfComponents(1);
  * compositeArr->SetNumberOfTuples(2);
  * CHECK(compositArr->GetValue(1) == 1);

Common/ImplicitArrays/vtkCompositeArray.txx

@@ -14,14 +14,6 @@ vtkSmartPointer<vtkCompositeArray<T>> ConcatenateDataArrays(
   {
     return nullptr;
   }
-  if (arrays.size() == 1)
-  {
-    vtkNew<vtkCompositeArray<T>> composite;
-    composite->SetBackend(std::make_shared<vtkCompositeImplicitBackend<T>>(arrays[0], nullptr));
-    composite->SetNumberOfComponents(arrays[0]->GetNumberOfComponents());
-    composite->SetNumberOfTuples(arrays[0]->GetNumberOfTuples());
-    return composite;
-  }
   vtkIdType nComps = arrays[0]->GetNumberOfComponents();
   for (auto arr : arrays)
   {
@@ -31,35 +23,14 @@ vtkSmartPointer<vtkCompositeArray<T>> ConcatenateDataArrays(
       return nullptr;
     }
   }
-  std::vector<vtkSmartPointer<vtkDataArray>> lifetimeBuffer;
-  lifetimeBuffer.assign(arrays.begin(), arrays.end());
-  std::vector<vtkSmartPointer<vtkCompositeArray<T>>> newComps;
-  while (lifetimeBuffer.size() != 1)
-  {
-    newComps.clear();
-    for (int i = 0; i < static_cast<int>(lifetimeBuffer.size() - 1); i += 2)
-    {
-      vtkNew<vtkCompositeArray<T>> composite;
-      composite->SetBackend(
-        std::make_shared<vtkCompositeImplicitBackend<T>>(lifetimeBuffer[i], lifetimeBuffer[i + 1]));
-      composite->SetNumberOfComponents(lifetimeBuffer[i]->GetNumberOfComponents());
-      composite->SetNumberOfTuples(
-        lifetimeBuffer[i]->GetNumberOfTuples() + lifetimeBuffer[i + 1]->GetNumberOfTuples());
-      newComps.emplace_back(composite);
-    }
-    if (lifetimeBuffer.size() % 2 != 0)
-    {
-      vtkNew<vtkCompositeArray<T>> composite;
-      composite->SetBackend(
-        std::make_shared<vtkCompositeImplicitBackend<T>>(newComps.back(), lifetimeBuffer.back()));
-      composite->SetNumberOfComponents(lifetimeBuffer.back()->GetNumberOfComponents());
-      composite->SetNumberOfTuples(
-        newComps.back()->GetNumberOfTuples() + lifetimeBuffer.back()->GetNumberOfTuples());
-      newComps.back() = composite;
-    }
-    lifetimeBuffer.assign(newComps.begin(), newComps.end());
-  }
-  return newComps[0];
+  vtkNew<vtkCompositeArray<T>> composite;
+  composite->SetBackend(std::make_shared<vtkCompositeImplicitBackend<T>>(arrays));
+  composite->SetNumberOfComponents(nComps);
+  int nTuples = 0;
+  std::for_each(arrays.begin(), arrays.end(),
+    [&nTuples](vtkDataArray* arr) { nTuples += arr->GetNumberOfTuples(); });
+  composite->SetNumberOfTuples(nTuples);
+  return composite;
 }
 VTK_ABI_NAMESPACE_END
 }

Common/ImplicitArrays/vtkCompositeImplicitBackend.h

@@ -23,9 +23,9 @@
  * This structure can be classified as a closure and can be called using syntax similar to a
  * function call.
  *
- * The class implements a binary tree like structure for facilitating fast access.
+ * The class implements a flat binary tree like structure for facilitating fast access.
  *
- * At construction it takes two arrays in order to represent their concatenation.
+ * At construction it takes an array arrays in order to represent their concatenation.
  *
  * An example of potential usage in a vtkImplicitArray
  * ```
@@ -39,7 +39,8 @@
  * rightArr->SetValue(0, 1);
  * vtkNew<vtkImplicitArray<vtkCompositeImplicitBackend<int>>> compositeArr; // easier with
  * `vtkNew<vtkCompositeArray<int>> compositeArr;` if applicable
- * compositeArr->SetBackend(std::make_shared<vtkCompositeImplicitBackend<int>>(leftArr, rightArr));
+ * std::vector<vtkDataArray*> arrays({leftArr, rightArr});
+ * compositeArr->SetBackend(std::make_shared<vtkCompositeImplicitBackend<int>>(arrays));
  * CHECK(compositArr->GetValue(1) == 1);
  * ```
  *
@@ -48,7 +49,9 @@
  * > information.
  */
 #include "vtkCommonImplicitArraysModule.h"
+
 #include <memory>
+#include <vector>
 
 VTK_ABI_NAMESPACE_BEGIN
 class vtkDataArray;
@@ -58,11 +61,10 @@ class vtkCompositeImplicitBackend
 public:
   /**
    * Constructor for the backend
-   * @param leftArr the array starting the composite at index 0
-   * @param rightArr the array following the leftArr and starting at index
+   * @param arrays std::vector of arrays to composite together
    * leftArr->GetNumberOfTuples()
    */
-  vtkCompositeImplicitBackend(vtkDataArray* leftArr, vtkDataArray* rightArr);
+  vtkCompositeImplicitBackend(const std::vector<vtkDataArray*>& arrays);
   ~vtkCompositeImplicitBackend();
 
   /**

Common/ImplicitArrays/vtkCompositeImplicitBackend.txx

@@ -26,41 +26,37 @@ VTK_ABI_NAMESPACE_BEGIN
 template <typename ValueType>
 struct vtkCompositeImplicitBackend<ValueType>::Internals
 {
-  Internals(vtkDataArray* leftArr, vtkDataArray* rightArr)
-    : Left(leftArr)
-    , Right(rightArr)
+  template <class Iterator>
+  Internals(Iterator first, Iterator last)
   {
-    if (!this->Left && !this->Right)
+    this->Arrays.assign(first, last);
+    if (this->Arrays.size() > 0)
     {
-      vtkWarningWithObjectMacro(nullptr, "Creating composite array with two nullptrs");
+      this->Ranges.resize(this->Arrays.size());
+      std::transform(first, last, this->Ranges.begin(),
+        [](vtkDataArray* arr) { return vtk::DataArrayValueRange(arr); });
+      this->Offsets.resize(this->Arrays.size() - 1);
+      std::size_t runningSum = 0;
+      std::transform(this->Ranges.begin(), this->Ranges.end() - 1, this->Offsets.begin(),
+        [&runningSum](vtk::detail::SelectValueRange<vtkDataArray*,
+          vtk::detail::DynamicTupleSize>::type& range) {
+          runningSum += range.size();
+          return runningSum;
+        });
     }
-    auto checkNullRectify = [](vtkSmartPointer<vtkDataArray>& arr) {
-      if (!arr)
-      {
-        arr = vtkSmartPointer<vtkAOSDataArrayTemplate<ValueType>>::New();
-        arr->SetNumberOfComponents(1);
-        arr->SetNumberOfTuples(0);
-      }
-    };
-    checkNullRectify(this->Left);
-    checkNullRectify(this->Right);
-    this->LeftRange = vtk::DataArrayValueRange(this->Left);
-    this->RightRange = vtk::DataArrayValueRange(this->Right);
-    this->Offset = this->LeftRange.size();
   }
 
-  vtkSmartPointer<vtkDataArray> Left;
-  vtk::detail::SelectValueRange<vtkDataArray*, vtk::detail::DynamicTupleSize>::type LeftRange;
-  vtkSmartPointer<vtkDataArray> Right;
-  vtk::detail::SelectValueRange<vtkDataArray*, vtk::detail::DynamicTupleSize>::type RightRange;
-  int Offset = -1;
+  std::vector<vtkSmartPointer<vtkDataArray>> Arrays;
+  std::vector<vtk::detail::SelectValueRange<vtkDataArray*, vtk::detail::DynamicTupleSize>::type>
+    Ranges;
+  std::vector<std::size_t> Offsets;
 };
 
 //-----------------------------------------------------------------------
 template <typename ValueType>
 vtkCompositeImplicitBackend<ValueType>::vtkCompositeImplicitBackend(
-  vtkDataArray* leftArr, vtkDataArray* rightArr)
-  : Internal(std::unique_ptr<Internals>(new Internals(leftArr, rightArr)))
+  const std::vector<vtkDataArray*>& arrays)
+  : Internal(std::unique_ptr<Internals>(new Internals(arrays.begin(), arrays.end())))
 {
 }
 
@@ -72,8 +68,9 @@ vtkCompositeImplicitBackend<ValueType>::~vtkCompositeImplicitBackend() = default
 template <typename ValueType>
 ValueType vtkCompositeImplicitBackend<ValueType>::operator()(int idx) const
 {
-  return static_cast<ValueType>((idx < this->Internal->Offset)
-      ? this->Internal->LeftRange[idx]
-      : this->Internal->RightRange[idx - this->Internal->Offset]);
+  auto itPos =
+    std::upper_bound(this->Internal->Offsets.begin(), this->Internal->Offsets.end(), idx);
+  int locIdx = itPos == this->Internal->Offsets.begin() ? idx : idx - *(itPos - 1);
+  return this->Internal->Ranges[std::distance(this->Internal->Offsets.begin(), itPos)][locIdx];
 }
 VTK_ABI_NAMESPACE_END