ENH: Add itkImageRandomConstIteratorWithIndexGTest unit test

Tests that `ImageRandomConstIteratorWithIndex` is deterministic when the initial
seed _and_ the next seed of the global `MersenneTwisterRandomVariateGenerator`
are reset.

Modules/Core/Common/test/CMakeLists.txt

@@ -1727,6 +1727,7 @@ set(ITKCommonGTests
     itkImageBufferRangeGTest.cxx
     itkImageRegionRangeGTest.cxx
     itkImageIORegionGTest.cxx
+    itkImageRandomConstIteratorWithIndexGTest.cxx
     itkImageRegionGTest.cxx
     itkIndexGTest.cxx
     itkIndexRangeGTest.cxx

Modules/Core/Common/test/itkImageRandomConstIteratorWithIndexGTest.cxx

@@ -0,0 +1,76 @@
+/*=========================================================================
+ *
+ *  Copyright NumFOCUS
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *         https://www.apache.org/licenses/LICENSE-2.0.txt
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *=========================================================================*/
+
+// First include the header file to be tested:
+#include "itkImageRandomConstIteratorWithIndex.h"
+#include "itkImage.h"
+#include "itkImageBufferRange.h"
+#include "itkMersenneTwisterRandomVariateGenerator.h"
+
+#include <gtest/gtest.h> // For iota.
+
+#include <numeric>
+
+
+// Tests that ImageRandomConstIteratorWithIndex is deterministic, when the initial seed and the next seed of the global
+// MersenneTwisterRandomVariateGenerator are reset before creating the iterator.
+TEST(ImageRandomConstIteratorWithIndex, IsDeterministicWhenGlobalRandomSeedsAreReset)
+{
+  using PixelType = int;
+  const unsigned int Dimension{ 2 };
+  using ImageType = itk::Image<PixelType, Dimension>;
+
+  // Create just a small test image, but still having enough pixels for the iterator to pick different samples from.
+  const auto                        image = ImageType::New();
+  const itk::ImageRegion<Dimension> imageRegion(ImageType::SizeType::Filled(3));
+  image->SetRegions(imageRegion);
+  image->Allocate();
+
+  // Make sure that each pixel has a different value.
+  const itk::ImageBufferRange imageBufferRange(*image);
+  std::iota(imageBufferRange.begin(), imageBufferRange.end(), 0);
+
+  using itk::Statistics::MersenneTwisterRandomVariateGenerator;
+
+  for (const MersenneTwisterRandomVariateGenerator::IntegerType initialSeed : { 0, 1 })
+  {
+    const auto generateSamples = [initialSeed, image]() {
+      // Reset both the initial seed and the next seed of the global random variate generator.
+      MersenneTwisterRandomVariateGenerator::GetInstance()->SetSeed(initialSeed);
+      MersenneTwisterRandomVariateGenerator::ResetNextSeed();
+
+      itk::ImageRandomConstIteratorWithIndex<ImageType> iterator(image, image->GetRequestedRegion());
+
+      // Request just a small number of samples, for performance reasons. Note that even for this small number of
+      // samples, it is important to have the global seeds reset, in order to have a deterministic iteration.
+      iterator.SetNumberOfSamples(3);
+
+      std::vector<std::pair<PixelType, ImageType::IndexType>> samples{};
+
+      while (!iterator.IsAtEnd())
+      {
+        samples.push_back({ iterator.Get(), iterator.GetIndex() });
+        ++iterator;
+      }
+      return samples;
+    };
+
+    // Expect the same samples when generating them twice with the same initial seed.
+    EXPECT_EQ(generateSamples(), generateSamples());
+  }
+}