Add approx partitioner. (#7467)

tests/cpp/tree/test_approx.cc

@@ -0,0 +1,76 @@
+/*!
+ * Copyright 2021 XGBoost contributors
+ */
+#include <gtest/gtest.h>
+
+#include "../../../src/tree/updater_approx.h"
+#include "../helpers.h"
+
+namespace xgboost {
+namespace tree {
+TEST(Approx, Partitioner) {
+  size_t n_samples = 1024, n_features = 1, base_rowid = 0;
+  ApproxRowPartitioner partitioner{n_samples, base_rowid};
+  ASSERT_EQ(partitioner.base_rowid, base_rowid);
+  ASSERT_EQ(partitioner.Size(), 1);
+  ASSERT_EQ(partitioner.Partitions()[0].Size(), n_samples);
+
+  auto Xy = RandomDataGenerator{n_samples, n_features, 0}.GenerateDMatrix(true);
+  GenericParameter ctx;
+  ctx.InitAllowUnknown(Args{});
+  std::vector<CPUExpandEntry> candidates{{0, 0, 0.4}};
+
+  for (auto const &page : Xy->GetBatches<GHistIndexMatrix>({GenericParameter::kCpuId, 64})) {
+    bst_feature_t split_ind = 0;
+    {
+      auto min_value = page.cut.MinValues()[split_ind];
+      RegTree tree;
+      tree.ExpandNode(
+          /*nid=*/0, /*split_index=*/0, /*split_value=*/min_value,
+          /*default_left=*/true, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f,
+          /*left_sum=*/0.0f,
+          /*right_sum=*/0.0f);
+      ApproxRowPartitioner partitioner{n_samples, base_rowid};
+      candidates.front().split.split_value = min_value;
+      candidates.front().split.sindex = 0;
+      candidates.front().split.sindex |= (1U << 31);
+      partitioner.UpdatePosition(&ctx, page, candidates, &tree);
+      ASSERT_EQ(partitioner.Size(), 3);
+      ASSERT_EQ(partitioner[1].Size(), 0);
+      ASSERT_EQ(partitioner[2].Size(), n_samples);
+    }
+    {
+      ApproxRowPartitioner partitioner{n_samples, base_rowid};
+      auto ptr = page.cut.Ptrs()[split_ind + 1];
+      float split_value = page.cut.Values().at(ptr / 2);
+      RegTree tree;
+      tree.ExpandNode(
+          /*nid=*/RegTree::kRoot, /*split_index=*/split_ind,
+          /*split_value=*/split_value,
+          /*default_left=*/true, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f,
+          /*left_sum=*/0.0f,
+          /*right_sum=*/0.0f);
+      auto left_nidx = tree[RegTree::kRoot].LeftChild();
+      candidates.front().split.split_value = split_value;
+      candidates.front().split.sindex = 0;
+      candidates.front().split.sindex |= (1U << 31);
+      partitioner.UpdatePosition(&ctx, page, candidates, &tree);
+
+      auto elem = partitioner[left_nidx];
+      ASSERT_LT(elem.Size(), n_samples);
+      ASSERT_GT(elem.Size(), 1);
+      for (auto it = elem.begin; it != elem.end; ++it) {
+        auto value = page.cut.Values().at(page.index[*it]);
+        ASSERT_LE(value, split_value);
+      }
+      auto right_nidx = tree[RegTree::kRoot].RightChild();
+      elem = partitioner[right_nidx];
+      for (auto it = elem.begin; it != elem.end; ++it) {
+        auto value = page.cut.Values().at(page.index[*it]);
+        ASSERT_GT(value, split_value) << *it;
+      }
+    }
+  }
+}
+}  // namespace tree
+}  // namespace xgboost