Expand categorical node. (#6028)

Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>

src/common/categorical.h

@@ -0,0 +1,50 @@
+/*!
+ * Copyright 2020 by XGBoost Contributors
+ * \file categorical.h
+ */
+#ifndef XGBOOST_COMMON_CATEGORICAL_H_
+#define XGBOOST_COMMON_CATEGORICAL_H_
+
+#include "xgboost/base.h"
+#include "xgboost/data.h"
+#include "xgboost/span.h"
+#include "xgboost/parameter.h"
+#include "bitfield.h"
+
+namespace xgboost {
+namespace common {
+// Cast the categorical type.
+template <typename T>
+XGBOOST_DEVICE bst_cat_t AsCat(T const& v) {
+  return static_cast<bst_cat_t>(v);
+}
+
+/* \brief Whether is fidx a categorical feature.
+ *
+ * \param ft   Feature type for all features.
+ * \param fidx Feature index.
+ * \return Whether feature pointed by fidx is categorical feature.
+ */
+inline XGBOOST_DEVICE bool IsCat(Span<FeatureType const> ft, bst_feature_t fidx) {
+  return !ft.empty() && ft[fidx] == FeatureType::kCategorical;
+}
+
+/* \brief Whether should it traverse to left branch of a tree.
+ *
+ *  For one hot split, go to left if it's NOT the matching category.
+ */
+inline XGBOOST_DEVICE bool Decision(common::Span<uint32_t const> cats, bst_cat_t cat) {
+  auto pos = CLBitField32::ToBitPos(cat);
+  if (pos.int_pos >= cats.size()) {
+    return true;
+  }
+  CLBitField32 const s_cats(cats);
+  return !s_cats.Check(cat);
+}
+
+using CatBitField = LBitField32;
+using KCatBitField = CLBitField32;
+}  // namespace common
+}  // namespace xgboost
+
+#endif  // XGBOOST_COMMON_CATEGORICAL_H_