tree_model.h

/*!
 * Copyright 2014-2022 by Contributors
 * \file tree_model.h
 * \brief model structure for tree
 * \author Tianqi Chen
 */
#ifndef XGBOOST_TREE_MODEL_H_
#define XGBOOST_TREE_MODEL_H_

#include <dmlc/io.h>
#include <dmlc/parameter.h>

#include <xgboost/base.h>
#include <xgboost/data.h>
#include <xgboost/logging.h>
#include <xgboost/feature_map.h>
#include <xgboost/model.h>

#include <limits>
#include <vector>
#include <string>
#include <cstring>
#include <algorithm>
#include <tuple>
#include <stack>

namespace xgboost {

struct PathElement;  // forward declaration

class Json;
// FIXME(trivialfis): Once binary IO is gone, make this parameter internal as it should
// not be configured by users.
/*! \brief meta parameters of the tree */
struct TreeParam : public dmlc::Parameter<TreeParam> {
  /*! \brief (Deprecated) number of start root */
  int deprecated_num_roots;
  /*! \brief total number of nodes */
  int num_nodes;
  /*!\brief number of deleted nodes */
  int num_deleted;
  /*! \brief maximum depth, this is a statistics of the tree */
  int deprecated_max_depth;
  /*! \brief number of features used for tree construction */
  bst_feature_t num_feature;
  /*!
   * \brief leaf vector size, used for vector tree
   * used to store more than one dimensional information in tree
   */
  int size_leaf_vector;
  /*! \brief reserved part, make sure alignment works for 64bit */
  int reserved[31];
  /*! \brief constructor */
  TreeParam() {
    // assert compact alignment
    static_assert(sizeof(TreeParam) == (31 + 6) * sizeof(int),
                  "TreeParam: 64 bit align");
    std::memset(this, 0, sizeof(TreeParam));
    num_nodes = 1;
    deprecated_num_roots = 1;
  }

  // Swap byte order for all fields. Useful for transporting models between machines with different
  // endianness (big endian vs little endian)
  inline TreeParam ByteSwap() const {
    TreeParam x = *this;
    dmlc::ByteSwap(&x.deprecated_num_roots, sizeof(x.deprecated_num_roots), 1);
    dmlc::ByteSwap(&x.num_nodes, sizeof(x.num_nodes), 1);
    dmlc::ByteSwap(&x.num_deleted, sizeof(x.num_deleted), 1);
    dmlc::ByteSwap(&x.deprecated_max_depth, sizeof(x.deprecated_max_depth), 1);
    dmlc::ByteSwap(&x.num_feature, sizeof(x.num_feature), 1);
    dmlc::ByteSwap(&x.size_leaf_vector, sizeof(x.size_leaf_vector), 1);
    dmlc::ByteSwap(x.reserved, sizeof(x.reserved[0]), sizeof(x.reserved) / sizeof(x.reserved[0]));
    return x;
  }

  // declare the parameters
  DMLC_DECLARE_PARAMETER(TreeParam) {
    // only declare the parameters that can be set by the user.
    // other arguments are set by the algorithm.
    DMLC_DECLARE_FIELD(num_nodes).set_lower_bound(1).set_default(1);
    DMLC_DECLARE_FIELD(num_feature)
        .describe("Number of features used in tree construction.");
    DMLC_DECLARE_FIELD(num_deleted);
    DMLC_DECLARE_FIELD(size_leaf_vector).set_lower_bound(0).set_default(0)
        .describe("Size of leaf vector, reserved for vector tree");
  }

  bool operator==(const TreeParam& b) const {
    return num_nodes == b.num_nodes &&
           num_deleted == b.num_deleted &&
           num_feature == b.num_feature &&
           size_leaf_vector == b.size_leaf_vector;
  }
};

/*! \brief node statistics used in regression tree */
struct RTreeNodeStat {
  /*! \brief loss change caused by current split */
  bst_float loss_chg;
  /*! \brief sum of hessian values, used to measure coverage of data */
  bst_float sum_hess;
  /*! \brief weight of current node */
  bst_float base_weight;
  /*! \brief number of child that is leaf node known up to now */
  int leaf_child_cnt {0};

  RTreeNodeStat() = default;
  RTreeNodeStat(float loss_chg, float sum_hess, float weight) :
      loss_chg{loss_chg}, sum_hess{sum_hess}, base_weight{weight} {}
  bool operator==(const RTreeNodeStat& b) const {
    return loss_chg == b.loss_chg && sum_hess == b.sum_hess &&
           base_weight == b.base_weight && leaf_child_cnt == b.leaf_child_cnt;
  }
  // Swap byte order for all fields. Useful for transporting models between machines with different
  // endianness (big endian vs little endian)
  inline RTreeNodeStat ByteSwap() const {
    RTreeNodeStat x = *this;
    dmlc::ByteSwap(&x.loss_chg, sizeof(x.loss_chg), 1);
    dmlc::ByteSwap(&x.sum_hess, sizeof(x.sum_hess), 1);
    dmlc::ByteSwap(&x.base_weight, sizeof(x.base_weight), 1);
    dmlc::ByteSwap(&x.leaf_child_cnt, sizeof(x.leaf_child_cnt), 1);
    return x;
  }
};

/*!
 * \brief define regression tree to be the most common tree model.
 *  This is the data structure used in xgboost's major tree models.
 */
class RegTree : public Model {
 public:
  using SplitCondT = bst_float;
  static constexpr bst_node_t kInvalidNodeId {-1};
  static constexpr uint32_t kDeletedNodeMarker = std::numeric_limits<uint32_t>::max();
  static constexpr bst_node_t kRoot { 0 };

  /*! \brief tree node */
  class Node {
   public:
    XGBOOST_DEVICE Node()  {
      // assert compact alignment
      static_assert(sizeof(Node) == 4 * sizeof(int) + sizeof(Info),
                    "Node: 64 bit align");
    }
    Node(int32_t cleft, int32_t cright, int32_t parent,
         uint32_t split_ind, float split_cond, bool default_left) :
        parent_{parent}, cleft_{cleft}, cright_{cright} {
      this->SetParent(parent_);
      this->SetSplit(split_ind, split_cond, default_left);
    }

    /*! \brief index of left child */
    XGBOOST_DEVICE int LeftChild() const {
      return this->cleft_;
    }
    /*! \brief index of right child */
    XGBOOST_DEVICE int RightChild() const {
      return this->cright_;
    }
    /*! \brief index of default child when feature is missing */
    XGBOOST_DEVICE int DefaultChild() const {
      return this->DefaultLeft() ? this->LeftChild() : this->RightChild();
    }
    /*! \brief feature index of split condition */
    XGBOOST_DEVICE unsigned SplitIndex() const {
      return sindex_ & ((1U << 31) - 1U);
    }
    /*! \brief when feature is unknown, whether goes to left child */
    XGBOOST_DEVICE bool DefaultLeft() const {
      return (sindex_ >> 31) != 0;
    }
    /*! \brief whether current node is leaf node */
    XGBOOST_DEVICE bool IsLeaf() const {
      return cleft_ == kInvalidNodeId;
    }
    /*! \return get leaf value of leaf node */
    XGBOOST_DEVICE bst_float LeafValue() const {
      return (this->info_).leaf_value;
    }
    /*! \return get split condition of the node */
    XGBOOST_DEVICE SplitCondT SplitCond() const {
      return (this->info_).split_cond;
    }
    /*! \brief get parent of the node */
    XGBOOST_DEVICE int Parent() const {
      return parent_ & ((1U << 31) - 1);
    }
    /*! \brief whether current node is left child */
    XGBOOST_DEVICE bool IsLeftChild() const {
      return (parent_ & (1U << 31)) != 0;
    }
    /*! \brief whether this node is deleted */
    XGBOOST_DEVICE bool IsDeleted() const {
      return sindex_ == kDeletedNodeMarker;
    }
    /*! \brief whether current node is root */
    XGBOOST_DEVICE bool IsRoot() const { return parent_ == kInvalidNodeId; }
    /*!
     * \brief set the left child
     * \param nid node id to right child
     */
    XGBOOST_DEVICE void SetLeftChild(int nid) {
      this->cleft_ = nid;
    }
    /*!
     * \brief set the right child
     * \param nid node id to right child
     */
    XGBOOST_DEVICE void SetRightChild(int nid) {
      this->cright_ = nid;
    }
    /*!
     * \brief set split condition of current node
     * \param split_index feature index to split
     * \param split_cond  split condition
     * \param default_left the default direction when feature is unknown
     */
    XGBOOST_DEVICE void SetSplit(unsigned split_index, SplitCondT split_cond,
                          bool default_left = false) {
      if (default_left) split_index |= (1U << 31);
      this->sindex_ = split_index;
      (this->info_).split_cond = split_cond;
    }
    /*!
     * \brief set the leaf value of the node
     * \param value leaf value
     * \param right right index, could be used to store
     *        additional information
     */
    XGBOOST_DEVICE void SetLeaf(bst_float value, int right = kInvalidNodeId) {
      (this->info_).leaf_value = value;
      this->cleft_ = kInvalidNodeId;
      this->cright_ = right;
    }
    /*! \brief mark that this node is deleted */
    XGBOOST_DEVICE void MarkDelete() {
      this->sindex_ = kDeletedNodeMarker;
    }
    /*! \brief Reuse this deleted node. */
    XGBOOST_DEVICE void Reuse() {
      this->sindex_ = 0;
    }
    // set parent
    XGBOOST_DEVICE void SetParent(int pidx, bool is_left_child = true) {
      if (is_left_child) pidx |= (1U << 31);
      this->parent_ = pidx;
    }
    bool operator==(const Node& b) const {
      return parent_ == b.parent_ && cleft_ == b.cleft_ &&
             cright_ == b.cright_ && sindex_ == b.sindex_ &&
             info_.leaf_value == b.info_.leaf_value;
    }

    inline Node ByteSwap() const {
      Node x = *this;
      dmlc::ByteSwap(&x.parent_, sizeof(x.parent_), 1);
      dmlc::ByteSwap(&x.cleft_, sizeof(x.cleft_), 1);
      dmlc::ByteSwap(&x.cright_, sizeof(x.cright_), 1);
      dmlc::ByteSwap(&x.sindex_, sizeof(x.sindex_), 1);
      dmlc::ByteSwap(&x.info_, sizeof(x.info_), 1);
      return x;
    }

   private:
    /*!
     * \brief in leaf node, we have weights, in non-leaf nodes,
     *        we have split condition
     */
    union Info{
      bst_float leaf_value;
      SplitCondT split_cond;
    };
    // pointer to parent, highest bit is used to
    // indicate whether it's a left child or not
    int32_t parent_{kInvalidNodeId};
    // pointer to left, right
    int32_t cleft_{kInvalidNodeId}, cright_{kInvalidNodeId};
    // split feature index, left split or right split depends on the highest bit
    uint32_t sindex_{0};
    // extra info
    Info info_;
  };

  /*!
   * \brief change a non leaf node to a leaf node, delete its children
   * \param rid node id of the node
   * \param value new leaf value
   */
  void ChangeToLeaf(int rid, bst_float value) {
    CHECK(nodes_[nodes_[rid].LeftChild() ].IsLeaf());
    CHECK(nodes_[nodes_[rid].RightChild()].IsLeaf());
    this->DeleteNode(nodes_[rid].LeftChild());
    this->DeleteNode(nodes_[rid].RightChild());
    nodes_[rid].SetLeaf(value);
  }
  /*!
   * \brief collapse a non leaf node to a leaf node, delete its children
   * \param rid node id of the node
   * \param value new leaf value
   */
  void CollapseToLeaf(int rid, bst_float value) {
    if (nodes_[rid].IsLeaf()) return;
    if (!nodes_[nodes_[rid].LeftChild() ].IsLeaf()) {
      CollapseToLeaf(nodes_[rid].LeftChild(), 0.0f);
    }
    if (!nodes_[nodes_[rid].RightChild() ].IsLeaf()) {
      CollapseToLeaf(nodes_[rid].RightChild(), 0.0f);
    }
    this->ChangeToLeaf(rid, value);
  }

  /*! \brief model parameter */
  TreeParam param;
  /*! \brief constructor */
  RegTree() {
    param.num_nodes = 1;
    param.num_deleted = 0;
    nodes_.resize(param.num_nodes);
    stats_.resize(param.num_nodes);
    split_types_.resize(param.num_nodes, FeatureType::kNumerical);
    split_categories_segments_.resize(param.num_nodes);
    for (int i = 0; i < param.num_nodes; i ++) {
      nodes_[i].SetLeaf(0.0f);
      nodes_[i].SetParent(kInvalidNodeId);
    }
  }
  /*! \brief get node given nid */
  Node& operator[](int nid) {
    return nodes_[nid];
  }
  /*! \brief get node given nid */
  const Node& operator[](int nid) const {
    return nodes_[nid];
  }

  /*! \brief get const reference to nodes */
  const std::vector<Node>& GetNodes() const { return nodes_; }

  /*! \brief get const reference to stats */
  const std::vector<RTreeNodeStat>& GetStats() const { return stats_; }

  /*! \brief get node statistics given nid */
  RTreeNodeStat& Stat(int nid) {
    return stats_[nid];
  }
  /*! \brief get node statistics given nid */
  const RTreeNodeStat& Stat(int nid) const {
    return stats_[nid];
  }

  /*!
   * \brief load model from stream
   * \param fi input stream
   */
  void Load(dmlc::Stream* fi);
  /*!
   * \brief save model to stream
   * \param fo output stream
   */
  void Save(dmlc::Stream* fo) const;

  void LoadModel(Json const& in) override;
  void SaveModel(Json* out) const override;

  bool operator==(const RegTree& b) const {
    return nodes_ == b.nodes_ && stats_ == b.stats_ &&
           deleted_nodes_ == b.deleted_nodes_ && param == b.param;
  }
  /* \brief Iterate through all nodes in this tree.
   *
   * \param Function that accepts a node index, and returns false when iteration should
   *        stop, otherwise returns true.
   */
  template <typename Func> void WalkTree(Func func) const {
    std::stack<bst_node_t> nodes;
    nodes.push(kRoot);
    auto &self = *this;
    while (!nodes.empty()) {
      auto nidx = nodes.top();
      nodes.pop();
      if (!func(nidx)) {
        return;
      }
      auto left = self[nidx].LeftChild();
      auto right = self[nidx].RightChild();
      if (left != RegTree::kInvalidNodeId) {
        nodes.push(left);
      }
      if (right != RegTree::kInvalidNodeId) {
        nodes.push(right);
      }
    }
  }
  /*!
   * \brief Compares whether 2 trees are equal from a user's perspective.  The equality
   *        compares only non-deleted nodes.
   *
   * \param b The other tree.
   */
  bool Equal(const RegTree& b) const;

  /**
   * \brief Expands a leaf node into two additional leaf nodes.
   *
   * \param nid               The node index to expand.
   * \param split_index       Feature index of the split.
   * \param split_value       The split condition.
   * \param default_left      True to default left.
   * \param base_weight       The base weight, before learning rate.
   * \param left_leaf_weight  The left leaf weight for prediction, modified by learning rate.
   * \param right_leaf_weight The right leaf weight for prediction, modified by learning rate.
   * \param loss_change       The loss change.
   * \param sum_hess          The sum hess.
   * \param left_sum          The sum hess of left leaf.
   * \param right_sum         The sum hess of right leaf.
   * \param leaf_right_child  The right child index of leaf, by default kInvalidNodeId,
   *                          some updaters use the right child index of leaf as a marker
   */
  void ExpandNode(bst_node_t nid, unsigned split_index, bst_float split_value,
                  bool default_left, bst_float base_weight,
                  bst_float left_leaf_weight, bst_float right_leaf_weight,
                  bst_float loss_change, float sum_hess, float left_sum,
                  float right_sum,
                  bst_node_t leaf_right_child = kInvalidNodeId);

  /**
   * \brief Expands a leaf node with categories
   *
   * \param nid               The node index to expand.
   * \param split_index       Feature index of the split.
   * \param split_cat         The bitset containing categories
   * \param default_left      True to default left.
   * \param base_weight       The base weight, before learning rate.
   * \param left_leaf_weight  The left leaf weight for prediction, modified by learning rate.
   * \param right_leaf_weight The right leaf weight for prediction, modified by learning rate.
   * \param loss_change       The loss change.
   * \param sum_hess          The sum hess.
   * \param left_sum          The sum hess of left leaf.
   * \param right_sum         The sum hess of right leaf.
   */
  void ExpandCategorical(bst_node_t nid, unsigned split_index,
                         common::Span<const uint32_t> split_cat, bool default_left,
                         bst_float base_weight, bst_float left_leaf_weight,
                         bst_float right_leaf_weight, bst_float loss_change, float sum_hess,
                         float left_sum, float right_sum);

  bool HasCategoricalSplit() const {
    return !split_categories_.empty();
  }

  /*!
   * \brief get current depth
   * \param nid node id
   */
  int GetDepth(int nid) const {
    int depth = 0;
    while (!nodes_[nid].IsRoot()) {
      ++depth;
      nid = nodes_[nid].Parent();
    }
    return depth;
  }

  /*!
   * \brief get maximum depth
   * \param nid node id
   */
  int MaxDepth(int nid) const {
    if (nodes_[nid].IsLeaf()) return 0;
    return std::max(MaxDepth(nodes_[nid].LeftChild())+1,
                     MaxDepth(nodes_[nid].RightChild())+1);
  }

  /*!
   * \brief get maximum depth
   */
  int MaxDepth() {
    return MaxDepth(0);
  }

  /*! \brief number of extra nodes besides the root */
  int NumExtraNodes() const {
    return param.num_nodes - 1 - param.num_deleted;
  }

  /* \brief Count number of leaves in tree. */
  bst_node_t GetNumLeaves() const;
  bst_node_t GetNumSplitNodes() const;

  /*!
   * \brief dense feature vector that can be taken by RegTree
   * and can be construct from sparse feature vector.
   */
  struct FVec {
    /*!
     * \brief initialize the vector with size vector
     * \param size The size of the feature vector.
     */
    void Init(size_t size);
    /*!
     * \brief fill the vector with sparse vector
     * \param inst The sparse instance to fill.
     */
    void Fill(const SparsePage::Inst& inst);

    /*!
     * \brief drop the trace after fill, must be called after fill.
     * \param inst The sparse instance to drop.
     */
    void Drop(const SparsePage::Inst& inst);
    /*!
     * \brief returns the size of the feature vector
     * \return the size of the feature vector
     */
    size_t Size() const;
    /*!
     * \brief get ith value
     * \param i feature index.
     * \return the i-th feature value
     */
    bst_float GetFvalue(size_t i) const;
    /*!
     * \brief check whether i-th entry is missing
     * \param i feature index.
     * \return whether i-th value is missing.
     */
    bool IsMissing(size_t i) const;
    bool HasMissing() const;


   private:
    /*!
     * \brief a union value of value and flag
     *  when flag == -1, this indicate the value is missing
     */
    union Entry {
      bst_float fvalue;
      int flag;
    };
    std::vector<Entry> data_;
    bool has_missing_;
  };

  /*!
   * \brief calculate the feature contributions (https://arxiv.org/abs/1706.06060) for the tree
   * \param feat dense feature vector, if the feature is missing the field is set to NaN
   * \param out_contribs output vector to hold the contributions
   * \param condition fix one feature to either off (-1) on (1) or not fixed (0 default)
   * \param condition_feature the index of the feature to fix
   */
  void CalculateContributions(const RegTree::FVec& feat,
                              std::vector<float>* mean_values,
                              bst_float* out_contribs, int condition = 0,
                              unsigned condition_feature = 0) const;
  /*!
   * \brief Recursive function that computes the feature attributions for a single tree.
   * \param feat dense feature vector, if the feature is missing the field is set to NaN
   * \param phi dense output vector of feature attributions
   * \param node_index the index of the current node in the tree
   * \param unique_depth how many unique features are above the current node in the tree
   * \param parent_unique_path a vector of statistics about our current path through the tree
   * \param parent_zero_fraction what fraction of the parent path weight is coming as 0 (integrated)
   * \param parent_one_fraction what fraction of the parent path weight is coming as 1 (fixed)
   * \param parent_feature_index what feature the parent node used to split
   * \param condition fix one feature to either off (-1) on (1) or not fixed (0 default)
   * \param condition_feature the index of the feature to fix
   * \param condition_fraction what fraction of the current weight matches our conditioning feature
   */
  void TreeShap(const RegTree::FVec& feat, bst_float* phi, bst_node_t node_index,
                unsigned unique_depth, PathElement* parent_unique_path,
                bst_float parent_zero_fraction, bst_float parent_one_fraction,
                int parent_feature_index, int condition,
                unsigned condition_feature, bst_float condition_fraction) const;

  /*!
   * \brief calculate the approximate feature contributions for the given root
   * \param feat dense feature vector, if the feature is missing the field is set to NaN
   * \param out_contribs output vector to hold the contributions
   */
  void CalculateContributionsApprox(const RegTree::FVec& feat,
                                    std::vector<float>* mean_values,
                                    bst_float* out_contribs) const;
  /*!
   * \brief dump the model in the requested format as a text string
   * \param fmap feature map that may help give interpretations of feature
   * \param with_stats whether dump out statistics as well
   * \param format the format to dump the model in
   * \return the string of dumped model
   */
  std::string DumpModel(const FeatureMap& fmap,
                        bool with_stats,
                        std::string format) const;
  /*!
   * \brief Get split type for a node.
   * \param nidx Index of node.
   * \return The type of this split.  For leaf node it's always kNumerical.
   */
  FeatureType NodeSplitType(bst_node_t nidx) const {
    return split_types_.at(nidx);
  }
  /*!
   * \brief Get split types for all nodes.
   */
  std::vector<FeatureType> const &GetSplitTypes() const { return split_types_; }
  common::Span<uint32_t const> GetSplitCategories() const { return split_categories_; }
  /*!
   * \brief Get the bit storage for categories
   */
  common::Span<uint32_t const> NodeCats(bst_node_t nidx) const {
    auto node_ptr = GetCategoriesMatrix().node_ptr;
    auto categories = GetCategoriesMatrix().categories;
    auto segment = node_ptr[nidx];
    auto node_cats = categories.subspan(segment.beg, segment.size);
    return node_cats;
  }
  auto const& GetSplitCategoriesPtr() const { return split_categories_segments_; }

  // The fields of split_categories_segments_[i] are set such that
  // the range split_categories_[beg:(beg+size)] stores the bitset for
  // the matching categories for the i-th node.
  struct Segment {
    size_t beg {0};
    size_t size {0};
  };

  struct CategoricalSplitMatrix {
    common::Span<FeatureType const> split_type;
    common::Span<uint32_t const> categories;
    common::Span<Segment const> node_ptr;
  };

  CategoricalSplitMatrix GetCategoriesMatrix() const {
    CategoricalSplitMatrix view;
    view.split_type = common::Span<FeatureType const>(this->GetSplitTypes());
    view.categories = this->GetSplitCategories();
    view.node_ptr = common::Span<Segment const>(split_categories_segments_);
    return view;
  }

 private:
  template <bool typed>
  void LoadCategoricalSplit(Json const& in);
  void SaveCategoricalSplit(Json* p_out) const;
  // vector of nodes
  std::vector<Node> nodes_;
  // free node space, used during training process
  std::vector<int>  deleted_nodes_;
  // stats of nodes
  std::vector<RTreeNodeStat> stats_;
  std::vector<FeatureType> split_types_;

  // Categories for each internal node.
  std::vector<uint32_t> split_categories_;
  // Ptr to split categories of each node.
  std::vector<Segment> split_categories_segments_;

  // allocate a new node,
  // !!!!!! NOTE: may cause BUG here, nodes.resize
  bst_node_t AllocNode() {
    if (param.num_deleted != 0) {
      int nid = deleted_nodes_.back();
      deleted_nodes_.pop_back();
      nodes_[nid].Reuse();
      --param.num_deleted;
      return nid;
    }
    int nd = param.num_nodes++;
    CHECK_LT(param.num_nodes, std::numeric_limits<int>::max())
        << "number of nodes in the tree exceed 2^31";
    nodes_.resize(param.num_nodes);
    stats_.resize(param.num_nodes);
    split_types_.resize(param.num_nodes, FeatureType::kNumerical);
    split_categories_segments_.resize(param.num_nodes);
    return nd;
  }
  // delete a tree node, keep the parent field to allow trace back
  void DeleteNode(int nid) {
    CHECK_GE(nid, 1);
    auto pid = (*this)[nid].Parent();
    if (nid == (*this)[pid].LeftChild()) {
      (*this)[pid].SetLeftChild(kInvalidNodeId);
    } else {
      (*this)[pid].SetRightChild(kInvalidNodeId);
    }

    deleted_nodes_.push_back(nid);
    nodes_[nid].MarkDelete();
    ++param.num_deleted;
  }
};

inline void RegTree::FVec::Init(size_t size) {
  Entry e; e.flag = -1;
  data_.resize(size);
  std::fill(data_.begin(), data_.end(), e);
  has_missing_ = true;
}

inline void RegTree::FVec::Fill(const SparsePage::Inst& inst) {
  size_t feature_count = 0;
  for (auto const& entry : inst) {
    if (entry.index >= data_.size()) {
      continue;
    }
    data_[entry.index].fvalue = entry.fvalue;
    ++feature_count;
  }
  has_missing_ = data_.size() != feature_count;
}

inline void RegTree::FVec::Drop(const SparsePage::Inst& inst) {
  for (auto const& entry : inst) {
    if (entry.index >= data_.size()) {
      continue;
    }
    data_[entry.index].flag = -1;
  }
  has_missing_ = true;
}

inline size_t RegTree::FVec::Size() const {
  return data_.size();
}

inline bst_float RegTree::FVec::GetFvalue(size_t i) const {
  return data_[i].fvalue;
}

inline bool RegTree::FVec::IsMissing(size_t i) const {
  return data_[i].flag == -1;
}

inline bool RegTree::FVec::HasMissing() const {
  return has_missing_;
}
}  // namespace xgboost
#endif  // XGBOOST_TREE_MODEL_H_