Support categorical data in ellpack. (#6140)

tests/cpp/data/test_ellpack_page.cu

@@ -1,5 +1,5 @@
 /*!
- * Copyright 2019 XGBoost contributors
+ * Copyright 2019-2020 XGBoost contributors
  */
 #include <xgboost/base.h>
 
@@ -9,6 +9,7 @@
 #include "../histogram_helpers.h"
 #include "gtest/gtest.h"
 
+#include "../../../src/common/categorical.h"
 #include "../../../src/common/hist_util.h"
 #include "../../../src/data/ellpack_page.cuh"
 
@@ -77,6 +78,45 @@ TEST(EllpackPage, BuildGidxSparse) {
   }
 }
 
+TEST(EllpackPage, FromCategoricalBasic) {
+  using common::AsCat;
+  size_t constexpr kRows = 1000, kCats = 13, kCols = 1;
+  size_t max_bins = 8;
+  auto x = GenerateRandomCategoricalSingleColumn(kRows, kCats);
+  auto m = GetDMatrixFromData(x, kRows, 1);
+  auto& h_ft = m->Info().feature_types.HostVector();
+  h_ft.resize(kCols, FeatureType::kCategorical);
+
+  BatchParam p(0, max_bins);
+  auto ellpack = EllpackPage(m.get(), p);
+  auto accessor = ellpack.Impl()->GetDeviceAccessor(0);
+  ASSERT_EQ(kCats, accessor.NumBins());
+
+  auto x_copy = x;
+  std::sort(x_copy.begin(), x_copy.end());
+  auto n_uniques = std::unique(x_copy.begin(), x_copy.end()) - x_copy.begin();
+  ASSERT_EQ(n_uniques, kCats);
+
+  std::vector<uint32_t> h_cuts_ptr(accessor.feature_segments.size());
+  dh::CopyDeviceSpanToVector(&h_cuts_ptr, accessor.feature_segments);
+  std::vector<float> h_cuts_values(accessor.gidx_fvalue_map.size());
+  dh::CopyDeviceSpanToVector(&h_cuts_values, accessor.gidx_fvalue_map);
+
+  ASSERT_EQ(h_cuts_ptr.size(), 2);
+  ASSERT_EQ(h_cuts_values.size(), kCats);
+
+  std::vector<common::CompressedByteT> const &h_gidx_buffer =
+      ellpack.Impl()->gidx_buffer.HostVector();
+  auto h_gidx_iter = common::CompressedIterator<uint32_t>(
+      h_gidx_buffer.data(), accessor.NumSymbols());
+
+  for (size_t i = 0; i < x.size(); ++i) {
+    auto bin = h_gidx_iter[i];
+    auto bin_value = h_cuts_values.at(bin);
+    ASSERT_EQ(AsCat(x[i]), AsCat(bin_value));
+  }
+}
+
 struct ReadRowFunction {
   EllpackDeviceAccessor matrix;
   int row;