Support categorical split in tree model dump. (#7036)

tests/cpp/tree/test_tree_model.cc

@@ -241,6 +241,65 @@ RegTree ConstructTree() {
       /*right_sum=*/0.0f);
   return tree;
 }
+
+RegTree ConstructTreeCat(std::vector<bst_cat_t>* cond) {
+  RegTree tree;
+  std::vector<uint32_t> cats_storage(common::CatBitField::ComputeStorageSize(33), 0);
+  common::CatBitField split_cats(cats_storage);
+  split_cats.Set(0);
+  split_cats.Set(14);
+  split_cats.Set(32);
+
+  cond->push_back(0);
+  cond->push_back(14);
+  cond->push_back(32);
+
+  tree.ExpandCategorical(0, /*split_index=*/0, cats_storage, true, 0.0f, 2.0,
+                         3.00, 11.0, 2.0, 3.0, 4.0);
+  auto left = tree[0].LeftChild();
+  auto right = tree[0].RightChild();
+  tree.ExpandNode(
+      /*nid=*/left, /*split_index=*/1, /*split_value=*/1.0f,
+      /*default_left=*/false, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, /*left_sum=*/0.0f,
+      /*right_sum=*/0.0f);
+  tree.ExpandCategorical(right, /*split_index=*/0, cats_storage, true, 0.0f,
+                         2.0, 3.00, 11.0, 2.0, 3.0, 4.0);
+  return tree;
+}
+
+void TestCategoricalTreeDump(std::string format, std::string sep) {
+  std::vector<bst_cat_t> cond;
+  auto tree = ConstructTreeCat(&cond);
+
+  FeatureMap fmap;
+  auto str = tree.DumpModel(fmap, true, format);
+  std::string cond_str;
+  for (size_t c = 0; c < cond.size(); ++c) {
+    cond_str += std::to_string(cond[c]);
+    if (c != cond.size() - 1) {
+      cond_str += sep;
+    }
+  }
+  auto pos = str.find(cond_str);
+  ASSERT_NE(pos, std::string::npos);
+  pos = str.find(cond_str, pos + 1);
+  ASSERT_NE(pos, std::string::npos);
+
+  fmap.PushBack(0, "feat_0", "categorical");
+  fmap.PushBack(1, "feat_1", "q");
+  fmap.PushBack(2, "feat_2", "int");
+
+  str = tree.DumpModel(fmap, true, format);
+  pos = str.find(cond_str);
+  ASSERT_NE(pos, std::string::npos);
+  pos = str.find(cond_str, pos + 1);
+  ASSERT_NE(pos, std::string::npos);
+
+  if (format == "json") {
+    // Make sure it's valid JSON
+    Json::Load(StringView{str});
+  }
+}
 }  // anonymous namespace
 
 TEST(Tree, DumpJson) {
@@ -278,6 +337,10 @@ TEST(Tree, DumpJson) {
   ASSERT_EQ(get<Array>(j_tree["children"]).size(), 2ul);
 }
 
+TEST(Tree, DumpJsonCategorical) {
+  TestCategoricalTreeDump("json", ", ");
+}
+
 TEST(Tree, DumpText) {
   auto tree = ConstructTree();
   FeatureMap fmap;
@@ -313,6 +376,10 @@ TEST(Tree, DumpText) {
   ASSERT_EQ(str.find("cover"), std::string::npos);
 }
 
+TEST(Tree, DumpTextCategorical) {
+  TestCategoricalTreeDump("text", ",");
+}
+
 TEST(Tree, DumpDot) {
   auto tree = ConstructTree();
   FeatureMap fmap;
@@ -350,6 +417,10 @@ TEST(Tree, DumpDot) {
   ASSERT_NE(str.find(R"(1 -> 4 [label="no, missing")"), std::string::npos);
 }
 
+TEST(Tree, DumpDotCategorical) {
+  TestCategoricalTreeDump("dot", ",");
+}
+
 TEST(Tree, JsonIO) {
   RegTree tree;
   tree.ExpandNode(0, 0, 0.0f, false, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f,