Refactor out row partitioning logic from gpu_hist, introduce caching device vectors (#4554)

tests/cpp/tree/gpu_hist/test_row_partitioner.cu

@@ -0,0 +1,125 @@
+#include <gtest/gtest.h>
+#include <vector>
+
+#include <thrust/device_vector.h>
+#include <thrust/sequence.h>
+#include "../../../../src/tree/gpu_hist/row_partitioner.cuh"
+#include "../../helpers.h"
+
+namespace xgboost {
+namespace tree {
+
+void TestSortPosition(const std::vector<int>& position_in, int left_idx,
+                      int right_idx) {
+  std::vector<int64_t> left_count = {
+      std::count(position_in.begin(), position_in.end(), left_idx)};
+  thrust::device_vector<int64_t> d_left_count = left_count;
+  thrust::device_vector<int> position = position_in;
+  thrust::device_vector<int> position_out(position.size());
+
+  thrust::device_vector<RowPartitioner::RowIndexT> ridx(position.size());
+  thrust::sequence(ridx.begin(), ridx.end());
+  thrust::device_vector<RowPartitioner::RowIndexT> ridx_out(ridx.size());
+  RowPartitioner rp(0,10);
+  rp.SortPosition(
+      common::Span<int>(position.data().get(), position.size()),
+      common::Span<int>(position_out.data().get(), position_out.size()),
+      common::Span<RowPartitioner::RowIndexT>(ridx.data().get(), ridx.size()),
+      common::Span<RowPartitioner::RowIndexT>(ridx_out.data().get(), ridx_out.size()), left_idx,
+      right_idx, d_left_count.data().get(), nullptr);
+  thrust::host_vector<int> position_result = position_out;
+  thrust::host_vector<int> ridx_result = ridx_out;
+
+  // Check position is sorted
+  EXPECT_TRUE(std::is_sorted(position_result.begin(), position_result.end()));
+  // Check row indices are sorted inside left and right segment
+  EXPECT_TRUE(
+      std::is_sorted(ridx_result.begin(), ridx_result.begin() + left_count[0]));
+  EXPECT_TRUE(
+      std::is_sorted(ridx_result.begin() + left_count[0], ridx_result.end()));
+
+  // Check key value pairs are the same
+  for (auto i = 0ull; i < ridx_result.size(); i++) {
+    EXPECT_EQ(position_result[i], position_in[ridx_result[i]]);
+  }
+}
+TEST(GpuHist, SortPosition) {
+  TestSortPosition({1, 2, 1, 2, 1}, 1, 2);
+  TestSortPosition({1, 1, 1, 1}, 1, 2);
+  TestSortPosition({2, 2, 2, 2}, 1, 2);
+  TestSortPosition({1, 2, 1, 2, 3}, 1, 2);
+}
+
+void TestUpdatePosition() {
+  const int kNumRows = 10;
+  RowPartitioner rp(0, kNumRows);
+  auto rows = rp.GetRowsHost(0);
+  EXPECT_EQ(rows.size(), kNumRows);
+  for (auto i = 0ull; i < kNumRows; i++) {
+    EXPECT_EQ(rows[i], i);
+  }
+  // Send the first five training instances to the right node
+  // and the second 5 to the left node
+  rp.UpdatePosition(0, 1, 2,
+    [=] __device__(RowPartitioner::RowIndexT ridx) {
+    if (ridx > 4) {
+      return 1;
+    }
+    else {
+      return 2;
+    }
+  });
+  rows = rp.GetRowsHost(1);
+  for (auto r : rows) {
+    EXPECT_GT(r, 4);
+  }
+  rows = rp.GetRowsHost(2);
+  for (auto r : rows) {
+    EXPECT_LT(r, 5);
+  }
+
+  // Split the left node again
+  rp.UpdatePosition(1, 3, 4, [=]__device__(RowPartitioner::RowIndexT ridx)
+  {
+    if (ridx < 7) {
+      return 3
+        ;
+    }
+    return 4;
+  });
+  EXPECT_EQ(rp.GetRows(3).size(), 2);
+  EXPECT_EQ(rp.GetRows(4).size(), 3);
+  // Check position is as expected
+  EXPECT_EQ(rp.GetPositionHost(), std::vector<RowPartitioner::TreePositionT>({3,3,4,4,4,2,2,2,2,2}));
+}
+
+TEST(RowPartitioner, Basic) { TestUpdatePosition(); }
+  
+void TestFinalise() {
+  const int kNumRows = 10;
+  RowPartitioner rp(0, kNumRows);
+  rp.FinalisePosition([=]__device__(RowPartitioner::RowIndexT ridx, int position)
+  {
+    return 7;
+  });
+  auto position = rp.GetPositionHost();
+  for(auto p:position)
+  {
+    EXPECT_EQ(p, 7);
+  }
+}
+TEST(RowPartitioner, Finalise) { TestFinalise(); }
+
+void TestIncorrectRow() {
+  RowPartitioner rp(0, 1);
+  rp.UpdatePosition(0, 1, 2, [=]__device__ (RowPartitioner::RowIndexT ridx)
+  {
+    return 4; // This is not the left branch or the right branch
+  });
+}
+
+TEST(RowPartitioner, IncorrectRow) {
+  ASSERT_DEATH({ TestIncorrectRow(); },".*");
+}
+}  // namespace tree
+}  // namespace xgboost