leaf_splits.hpp

/*!
 * Copyright (c) 2016 Microsoft Corporation. All rights reserved.
 * Licensed under the MIT License. See LICENSE file in the project root for license information.
 */
#ifndef LIGHTGBM_TREELEARNER_LEAF_SPLITS_HPP_
#define LIGHTGBM_TREELEARNER_LEAF_SPLITS_HPP_

#include <LightGBM/meta.h>
#include <LightGBM/utils/threading.h>

#include <limits>
#include <vector>

#include "data_partition.hpp"

namespace LightGBM {

/*!
* \brief used to find split candidates for a leaf
*/
class LeafSplits {
 public:
  explicit LeafSplits(data_size_t num_data)
    :num_data_in_leaf_(num_data), num_data_(num_data),
    data_indices_(nullptr), weight_(0) {
  }
  void ResetNumData(data_size_t num_data) {
    num_data_ = num_data;
    num_data_in_leaf_ = num_data;
  }
  ~LeafSplits() {
  }

  /*!
  * \brief Init split on current leaf on partial data. 
  * \param leaf Index of current leaf
  * \param data_partition current data partition
  * \param sum_gradients
  * \param sum_hessians
  */
  void Init(int leaf, const DataPartition* data_partition, double sum_gradients,
            double sum_hessians, double weight) {
    leaf_index_ = leaf;
    data_indices_ = data_partition->GetIndexOnLeaf(leaf, &num_data_in_leaf_);
    sum_gradients_ = sum_gradients;
    sum_hessians_ = sum_hessians;
    weight_ = weight;
  }

  /*!
  * \brief Init split on current leaf on partial data.
  * \param leaf Index of current leaf
  * \param sum_gradients
  * \param sum_hessians
  */
  void Init(int leaf, double sum_gradients, double sum_hessians) {
    leaf_index_ = leaf;
    sum_gradients_ = sum_gradients;
    sum_hessians_ = sum_hessians;
  }

  /*!
   * \brief Init splits on the current leaf, it will traverse all data to sum up the results
   * \param gradients
   * \param hessians
   */
  void Init(const score_t* gradients, const score_t* hessians) {
    num_data_in_leaf_ = num_data_;
    leaf_index_ = 0;
    data_indices_ = nullptr;
    if (num_data_in_leaf_ < 4096) {
      sum_gradients_ = 0.0f;
      sum_hessians_ = 0.0f;
      for (data_size_t i = 0; i < num_data_in_leaf_; ++i) {
        sum_gradients_ += gradients[i];
        sum_hessians_ += hessians[i];
      }
    } else {
      Threading::SumReduction<data_size_t, double, double>(
          0, num_data_in_leaf_, 2048,
          [=](int, data_size_t start, data_size_t end, double* s1, double* s2) {
            for (data_size_t i = start; i < end; ++i) {
              *s1 += gradients[i];
              *s2 += hessians[i];
            }
          },
          &sum_gradients_, &sum_hessians_);
    }
  }

  /*!
   * \brief Init splits on current leaf of partial data.
   * \param leaf Index of current leaf
   * \param data_partition current data partition
   * \param gradients
   * \param hessians
   */
  void Init(int leaf, const DataPartition* data_partition,
            const score_t* gradients, const score_t* hessians) {
    leaf_index_ = leaf;
    data_indices_ = data_partition->GetIndexOnLeaf(leaf, &num_data_in_leaf_);
    if (num_data_in_leaf_ < 4096) {
      sum_gradients_ = 0.0f;
      sum_hessians_ = 0.0f;
      for (data_size_t i = 0; i < num_data_in_leaf_; ++i) {
        const data_size_t idx = data_indices_[i];
        sum_gradients_ += gradients[idx];
        sum_hessians_ += hessians[idx];
      }
    } else {
      Threading::SumReduction<data_size_t, double, double>(
          0, num_data_in_leaf_, 2048,
          [=](int, data_size_t start, data_size_t end, double* s1, double* s2) {
            for (data_size_t i = start; i < end; ++i) {
              const data_size_t idx = data_indices_[i];
              *s1 += gradients[idx];
              *s2 += hessians[idx];
            }
          },
          &sum_gradients_, &sum_hessians_);
    }
  }


  /*!
  * \brief Init splits on current leaf, only update sum_gradients and sum_hessians
  * \param sum_gradients
  * \param sum_hessians
  */
  void Init(double sum_gradients, double sum_hessians) {
    leaf_index_ = 0;
    sum_gradients_ = sum_gradients;
    sum_hessians_ = sum_hessians;
  }

  /*!
  * \brief Init splits on current leaf
  */
  void Init() {
    leaf_index_ = -1;
    data_indices_ = nullptr;
    num_data_in_leaf_ = 0;
  }


  /*! \brief Get current leaf index */
  int leaf_index() const { return leaf_index_; }

  /*! \brief Get numer of data in current leaf */
  data_size_t num_data_in_leaf() const { return num_data_in_leaf_; }

  /*! \brief Get sum of gradients of current leaf */
  double sum_gradients() const { return sum_gradients_; }

  /*! \brief Get sum of hessians of current leaf */
  double sum_hessians() const { return sum_hessians_; }

  /*! \brief Get indices of data of current leaf */
  const data_size_t* data_indices() const { return data_indices_; }

  /*! \brief Get weight of current leaf */
  double weight() const { return weight_; }



 private:
  /*! \brief current leaf index */
  int leaf_index_;
  /*! \brief number of data on current leaf */
  data_size_t num_data_in_leaf_;
  /*! \brief number of all training data */
  data_size_t num_data_;
  /*! \brief sum of gradients of current leaf */
  double sum_gradients_;
  /*! \brief sum of hessians of current leaf */
  double sum_hessians_;
  /*! \brief indices of data of current leaf */
  const data_size_t* data_indices_;
  /*! \brief weight of current leaf */
  double weight_;
};

}  // namespace LightGBM
#endif   // LightGBM_TREELEARNER_LEAF_SPLITS_HPP_