Make `HistCutMatrix::Init' be aware of groups. (#4115)

* Add checks for group size.
* Simple docs.
* Search group index during hist cut matrix initialization.

Co-authored-by: Jiaming Yuan <jm.yuan@outlook.com>
Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>

tests/cpp/common/test_hist_util.cc

@@ -0,0 +1,51 @@
+#include <gtest/gtest.h>
+#include <vector>
+#include <string>
+#include <utility>
+
+#include "../../../src/common/hist_util.h"
+#include "../helpers.h"
+
+namespace xgboost {
+namespace common {
+
+class HistCutMatrixMock : public HistCutMatrix {
+ public:
+  size_t SearchGroupIndFromBaseRow(
+      std::vector<bst_uint> const& group_ptr, size_t const base_rowid) {
+    return HistCutMatrix::SearchGroupIndFromBaseRow(group_ptr, base_rowid);
+  }
+};
+
+TEST(HistCutMatrix, SearchGroupInd) {
+  size_t constexpr kNumGroups = 4;
+  size_t constexpr kNumRows = 17;
+  size_t constexpr kNumCols = 15;
+
+  auto pp_mat = CreateDMatrix(kNumRows, kNumCols, 0);
+
+  auto& p_mat = *pp_mat;
+  std::vector<bst_int> group(kNumGroups);
+  group[0] = 2;
+  group[1] = 3;
+  group[2] = 7;
+  group[3] = 5;
+
+  p_mat->Info().SetInfo(
+      "group", group.data(), DataType::kUInt32, kNumGroups);
+
+  HistCutMatrixMock hmat;
+
+  size_t group_ind = hmat.SearchGroupIndFromBaseRow(p_mat->Info().group_ptr_, 0);
+  ASSERT_EQ(group_ind, 0);
+
+  group_ind = hmat.SearchGroupIndFromBaseRow(p_mat->Info().group_ptr_, 5);
+  ASSERT_EQ(group_ind, 2);
+
+  EXPECT_ANY_THROW(hmat.SearchGroupIndFromBaseRow(p_mat->Info().group_ptr_, 17));
+
+  delete pp_mat;
+}
+
+}  // namespace common
+}  // namespace xgboost