Add tests for tree grow policy.

tests/cpp/tree/test_tree_policy.cc

@@ -0,0 +1,80 @@
+/*!
+ * Copyright 2021 XGBoost contributors
+ */
+#include <gtest/gtest.h>
+#include <xgboost/base.h>
+#include <xgboost/tree_model.h>
+#include "../helpers.h"
+
+namespace xgboost {
+class TestGrowPolicy : public ::testing::Test {
+ protected:
+  std::shared_ptr<DMatrix> Xy_;
+  size_t n_samples_ = 4096, n_features_ = 13;
+  float sparsity_ = 0.5;
+
+ protected:
+  void SetUp() override {
+    Xy_ =
+        RandomDataGenerator{n_samples_, n_features_, sparsity_}.GenerateDMatrix(
+            true);
+  }
+
+  void TestTreeGrowPolicy(std::string tree_method, std::string policy) {
+    {
+      std::unique_ptr<Learner> learner{Learner::Create({this->Xy_})};
+      learner->SetParam("tree_method", tree_method);
+      learner->SetParam("max_leaves", "16");
+      learner->SetParam("grow_policy", policy);
+      learner->Configure();
+
+      learner->UpdateOneIter(0, Xy_);
+      Json model{Object{}};
+      learner->SaveModel(&model);
+
+      auto j_tree = model["learner"]["gradient_booster"]["model"]["trees"][0];
+      RegTree tree;
+      tree.LoadModel(j_tree);
+      ASSERT_EQ(tree.GetNumLeaves(), 16);
+    }
+    {
+      std::unique_ptr<Learner> learner{Learner::Create({this->Xy_})};
+      learner->SetParam("tree_method", tree_method);
+      learner->SetParam("max_depth", "3");
+      learner->SetParam("grow_policy", policy);
+      learner->Configure();
+
+      learner->UpdateOneIter(0, Xy_);
+      Json model{Object{}};
+      learner->SaveModel(&model);
+
+      auto j_tree = model["learner"]["gradient_booster"]["model"]["trees"][0];
+      RegTree tree;
+      tree.LoadModel(j_tree);
+      bst_node_t depth = 0;
+      tree.WalkTree([&](bst_node_t nidx) {
+        depth = std::max(tree.GetDepth(nidx), depth);
+        return true;
+      });
+      ASSERT_EQ(depth, 3);
+    }
+  }
+};
+
+TEST_F(TestGrowPolicy, DISABLED_Approx) {
+  this->TestTreeGrowPolicy("approx", "depthwise");
+  this->TestTreeGrowPolicy("approx", "lossguide");
+}
+
+TEST_F(TestGrowPolicy, Hist) {
+  this->TestTreeGrowPolicy("hist", "depthwise");
+  this->TestTreeGrowPolicy("hist", "lossguide");
+}
+
+#if defined(XGBOOST_USE_CUDA)
+TEST_F(TestGrowPolicy, GpuHist) {
+  this->TestTreeGrowPolicy("gpu_hist", "depthwise");
+  this->TestTreeGrowPolicy("gpu_hist", "lossguide");
+}
+#endif  // defined(XGBOOST_USE_CUDA)
+}  // namespace xgboost