/*!
 * Copyright 2014-2020 by Contributors
 * \file gbtree.cc
 * \brief gradient boosted tree implementation.
 * \author Tianqi Chen
 */
#include <dmlc/omp.h>
#include <dmlc/parameter.h>

#include <vector>
#include <memory>
#include <utility>
#include <string>
#include <limits>
#include <algorithm>

#include "xgboost/data.h"
#include "xgboost/gbm.h"
#include "xgboost/logging.h"
#include "xgboost/json.h"
#include "xgboost/predictor.h"
#include "xgboost/tree_updater.h"
#include "xgboost/host_device_vector.h"

#include "gbtree.h"
#include "gbtree_model.h"
#include "../common/common.h"
#include "../common/random.h"
#include "../common/timer.h"

namespace xgboost {
namespace gbm {

DMLC_REGISTRY_FILE_TAG(gbtree);

void GBTree::Configure(const Args& cfg) {
  this->cfg_ = cfg;
  std::string updater_seq = tparam_.updater_seq;
  tparam_.UpdateAllowUnknown(cfg);

  model_.Configure(cfg);

  // for the 'update' process_type, move trees into trees_to_update
  if (tparam_.process_type == TreeProcessType::kUpdate) {
    model_.InitTreesToUpdate();
  }

  // configure predictors
  if (!cpu_predictor_) {
    cpu_predictor_ = std::unique_ptr<Predictor>(
        Predictor::Create("cpu_predictor", this->generic_param_));
  }
  cpu_predictor_->Configure(cfg);
#if defined(XGBOOST_USE_CUDA)
  auto n_gpus = common::AllVisibleGPUs();
  if (!gpu_predictor_ && n_gpus != 0) {
    gpu_predictor_ = std::unique_ptr<Predictor>(
        Predictor::Create("gpu_predictor", this->generic_param_));
  }
  if (n_gpus != 0) {
    gpu_predictor_->Configure(cfg);
  }
#endif  // defined(XGBOOST_USE_CUDA)

#if defined(XGBOOST_USE_ONEAPI)
  if (!oneapi_predictor_) {
    oneapi_predictor_ = std::unique_ptr<Predictor>(
        Predictor::Create("oneapi_predictor", this->generic_param_));
  }
  oneapi_predictor_->Configure(cfg);
#endif  // defined(XGBOOST_USE_ONEAPI)

  monitor_.Init("GBTree");

  specified_updater_ = std::any_of(cfg.cbegin(), cfg.cend(),
                   [](std::pair<std::string, std::string> const& arg) {
                     return arg.first == "updater";
                   });

  if (specified_updater_ && !showed_updater_warning_) {
    LOG(WARNING) << "DANGER AHEAD: You have manually specified `updater` "
        "parameter. The `tree_method` parameter will be ignored. "
        "Incorrect sequence of updaters will produce undefined "
        "behavior. For common uses, we recommend using "
        "`tree_method` parameter instead.";
    // Don't drive users to silent XGBOost.
    showed_updater_warning_ = true;
  }

  this->ConfigureUpdaters();
  if (updater_seq != tparam_.updater_seq) {
    updaters_.clear();
    this->InitUpdater(cfg);
  } else {
    for (auto &up : updaters_) {
      up->Configure(cfg);
    }
  }

  configured_ = true;
}

// FIXME(trivialfis): This handles updaters.  Because the choice of updaters depends on
// whether external memory is used and how large is dataset.  We can remove the dependency
// on DMatrix once `hist` tree method can handle external memory so that we can make it
// default.
void GBTree::ConfigureWithKnownData(Args const& cfg, DMatrix* fmat) {
  CHECK(this->configured_);
  std::string updater_seq = tparam_.updater_seq;
  CHECK(tparam_.GetInitialised());

  tparam_.UpdateAllowUnknown(cfg);

  this->PerformTreeMethodHeuristic(fmat);
  this->ConfigureUpdaters();

  // initialize the updaters only when needed.
  if (updater_seq != tparam_.updater_seq) {
    LOG(DEBUG) << "Using updaters: " << tparam_.updater_seq;
    this->updaters_.clear();
    this->InitUpdater(cfg);
  }
}

void GBTree::PerformTreeMethodHeuristic(DMatrix* fmat) {
  if (specified_updater_) {
    // This method is disabled when `updater` parameter is explicitly
    // set, since only experts are expected to do so.
    return;
  }
  // tparam_ is set before calling this function.
  if (tparam_.tree_method != TreeMethod::kAuto) {
    return;
  }

  if (rabit::IsDistributed()) {
    LOG(INFO) << "Tree method is automatically selected to be 'approx' "
                 "for distributed training.";
    tparam_.tree_method = TreeMethod::kApprox;
  } else if (!fmat->SingleColBlock()) {
    LOG(INFO) << "Tree method is automatically set to 'approx' "
                 "since external-memory data matrix is used.";
    tparam_.tree_method = TreeMethod::kApprox;
  } else if (fmat->Info().num_row_ >= (4UL << 20UL)) {
    /* Choose tree_method='approx' automatically for large data matrix */
    LOG(INFO) << "Tree method is automatically selected to be "
                 "'approx' for faster speed. To use old behavior "
                 "(exact greedy algorithm on single machine), "
                 "set tree_method to 'exact'.";
    tparam_.tree_method = TreeMethod::kApprox;
  } else {
    tparam_.tree_method = TreeMethod::kExact;
  }
  LOG(DEBUG) << "Using tree method: " << static_cast<int>(tparam_.tree_method);
}

void GBTree::ConfigureUpdaters() {
  if (specified_updater_) {
    return;
  }
  // `updater` parameter was manually specified
  /* Choose updaters according to tree_method parameters */
  switch (tparam_.tree_method) {
    case TreeMethod::kAuto:
      // Use heuristic to choose between 'exact' and 'approx' This
      // choice is carried out in PerformTreeMethodHeuristic() before
      // calling this function.
      break;
    case TreeMethod::kApprox:
      tparam_.updater_seq = "grow_histmaker,prune";
      break;
    case TreeMethod::kExact:
      tparam_.updater_seq = "grow_colmaker,prune";
      break;
    case TreeMethod::kHist:
      LOG(INFO) <<
          "Tree method is selected to be 'hist', which uses a "
          "single updater grow_quantile_histmaker.";
      tparam_.updater_seq = "grow_quantile_histmaker";
      break;
    case TreeMethod::kGPUHist: {
      common::AssertGPUSupport();
      tparam_.updater_seq = "grow_gpu_hist";
      break;
    }
    default:
      LOG(FATAL) << "Unknown tree_method ("
                 << static_cast<int>(tparam_.tree_method) << ") detected";
  }
}

void GBTree::DoBoost(DMatrix* p_fmat,
                     HostDeviceVector<GradientPair>* in_gpair,
                     PredictionCacheEntry* predt) {
  std::vector<std::vector<std::unique_ptr<RegTree> > > new_trees;
  const int ngroup = model_.learner_model_param->num_output_group;
  ConfigureWithKnownData(this->cfg_, p_fmat);
  monitor_.Start("BoostNewTrees");
  auto* out = &predt->predictions;
  CHECK_NE(ngroup, 0);
  if (ngroup == 1) {
    std::vector<std::unique_ptr<RegTree> > ret;
    BoostNewTrees(in_gpair, p_fmat, 0, &ret);
    const size_t num_new_trees = ret.size();
    new_trees.push_back(std::move(ret));
    if (updaters_.size() > 0 && num_new_trees == 1 && out->Size() > 0 &&
        updaters_.back()->UpdatePredictionCache(p_fmat, out)) {
      predt->Update(1);
    }
  } else {
    CHECK_EQ(in_gpair->Size() % ngroup, 0U)
        << "must have exactly ngroup * nrow gpairs";
    // TODO(canonizer): perform this on GPU if HostDeviceVector has device set.
    HostDeviceVector<GradientPair> tmp(in_gpair->Size() / ngroup,
                                       GradientPair(),
                                       in_gpair->DeviceIdx());
    const auto& gpair_h = in_gpair->ConstHostVector();
    auto nsize = static_cast<bst_omp_uint>(tmp.Size());
    bool update_predict = true;
    for (int gid = 0; gid < ngroup; ++gid) {
      std::vector<GradientPair>& tmp_h = tmp.HostVector();
#pragma omp parallel for schedule(static)
      for (bst_omp_uint i = 0; i < nsize; ++i) {
        tmp_h[i] = gpair_h[i * ngroup + gid];
      }
      std::vector<std::unique_ptr<RegTree> > ret;
      BoostNewTrees(&tmp, p_fmat, gid, &ret);
      const size_t num_new_trees = ret.size();
      new_trees.push_back(std::move(ret));
      auto* out = &predt->predictions;
      if (!(updaters_.size() > 0 && out->Size() > 0 && num_new_trees == 1 &&
          updaters_.back()->UpdatePredictionCacheMulticlass(p_fmat, out, gid, ngroup))) {
        update_predict = false;
      }
    }
    if (update_predict) {
      predt->Update(1);
    }
  }
  monitor_.Stop("BoostNewTrees");
  this->CommitModel(std::move(new_trees), p_fmat, predt);
}

void GBTree::InitUpdater(Args const& cfg) {
  std::string tval = tparam_.updater_seq;
  std::vector<std::string> ups = common::Split(tval, ',');

  if (updaters_.size() != 0) {
    // Assert we have a valid set of updaters.
    CHECK_EQ(ups.size(), updaters_.size());
    for (auto const& up : updaters_) {
      bool contains = std::any_of(ups.cbegin(), ups.cend(),
                        [&up](std::string const& name) {
                          return name == up->Name();
                        });
      if (!contains) {
        std::stringstream ss;
        ss << "Internal Error: " << " mismatched updater sequence.\n";
        ss << "Specified updaters: ";
        std::for_each(ups.cbegin(), ups.cend(),
                      [&ss](std::string const& name){
                        ss << name << " ";
                      });
        ss << "\n" << "Actual updaters: ";
        std::for_each(updaters_.cbegin(), updaters_.cend(),
                      [&ss](std::unique_ptr<TreeUpdater> const& updater){
                        ss << updater->Name() << " ";
                      });
        LOG(FATAL) << ss.str();
      }
    }
    // Do not push new updater in.
    return;
  }

  // create new updaters
  for (const std::string& pstr : ups) {
    std::unique_ptr<TreeUpdater> up(TreeUpdater::Create(pstr.c_str(), generic_param_));
    up->Configure(cfg);
    updaters_.push_back(std::move(up));
  }
}

void GBTree::BoostNewTrees(HostDeviceVector<GradientPair>* gpair,
                           DMatrix *p_fmat,
                           int bst_group,
                           std::vector<std::unique_ptr<RegTree> >* ret) {
  std::vector<RegTree*> new_trees;
  ret->clear();
  // create the trees
  for (int i = 0; i < tparam_.num_parallel_tree; ++i) {
    if (tparam_.process_type == TreeProcessType::kDefault) {
      CHECK(!updaters_.front()->CanModifyTree())
          << "Updater: `" << updaters_.front()->Name() << "` "
          << "can not be used to create new trees. "
          << "Set `process_type` to `update` if you want to update existing "
             "trees.";
      // create new tree
      std::unique_ptr<RegTree> ptr(new RegTree());
      ptr->param.UpdateAllowUnknown(this->cfg_);
      new_trees.push_back(ptr.get());
      ret->push_back(std::move(ptr));
    } else if (tparam_.process_type == TreeProcessType::kUpdate) {
      for (auto const& up : updaters_) {
        CHECK(up->CanModifyTree())
          << "Updater: `" << up->Name() << "` "
          << "can not be used to modify existing trees. "
          << "Set `process_type` to `default` if you want to build new trees.";
      }
      CHECK_LT(model_.trees.size(), model_.trees_to_update.size())
          << "No more tree left for updating.  For updating existing trees, "
          << "boosting rounds can not exceed previous training rounds";
      // move an existing tree from trees_to_update
      auto t = std::move(model_.trees_to_update[model_.trees.size() +
                                                bst_group * tparam_.num_parallel_tree + i]);
      new_trees.push_back(t.get());
      ret->push_back(std::move(t));
    }
  }
  // update the trees
  CHECK_EQ(gpair->Size(), p_fmat->Info().num_row_)
      << "Mismatching size between number of rows from input data and size of "
         "gradient vector.";
  for (auto& up : updaters_) {
    up->Update(gpair, p_fmat, new_trees);
  }
}

void GBTree::CommitModel(std::vector<std::vector<std::unique_ptr<RegTree>>>&& new_trees,
                         DMatrix* m,
                         PredictionCacheEntry* predts) {
  monitor_.Start("CommitModel");
  for (uint32_t gid = 0; gid < model_.learner_model_param->num_output_group; ++gid) {
    model_.CommitModel(std::move(new_trees[gid]), gid);
  }
  monitor_.Stop("CommitModel");
}

void GBTree::LoadConfig(Json const& in) {
  CHECK_EQ(get<String>(in["name"]), "gbtree");
  FromJson(in["gbtree_train_param"], &tparam_);
  // Process type cannot be kUpdate from loaded model
  // This would cause all trees to be pushed to trees_to_update
  // e.g. updating a model, then saving and loading it would result in an empty model
  tparam_.process_type = TreeProcessType::kDefault;
  int32_t const n_gpus = xgboost::common::AllVisibleGPUs();
  if (n_gpus == 0 && tparam_.predictor == PredictorType::kGPUPredictor) {
    LOG(WARNING)
        << "Loading from a raw memory buffer on CPU only machine.  "
           "Changing predictor to auto.";
    tparam_.UpdateAllowUnknown(Args{{"predictor", "auto"}});
  }
  if (n_gpus == 0 && tparam_.tree_method == TreeMethod::kGPUHist) {
    tparam_.UpdateAllowUnknown(Args{{"tree_method", "hist"}});
    LOG(WARNING)
        << "Loading from a raw memory buffer on CPU only machine.  "
           "Changing tree_method to hist.";
  }

  auto const& j_updaters = get<Object const>(in["updater"]);
  updaters_.clear();
  for (auto const& kv : j_updaters) {
    std::unique_ptr<TreeUpdater> up(TreeUpdater::Create(kv.first, generic_param_));
    up->LoadConfig(kv.second);
    updaters_.push_back(std::move(up));
  }

  specified_updater_ = get<Boolean>(in["specified_updater"]);
}

void GBTree::SaveConfig(Json* p_out) const {
  auto& out = *p_out;
  out["name"] = String("gbtree");
  out["gbtree_train_param"] = ToJson(tparam_);

  // Process type cannot be kUpdate from loaded model
  // This would cause all trees to be pushed to trees_to_update
  // e.g. updating a model, then saving and loading it would result in an empty
  // model
  out["gbtree_train_param"]["process_type"] = String("default");

  out["updater"] = Object();

  auto& j_updaters = out["updater"];
  for (auto const& up : updaters_) {
    j_updaters[up->Name()] = Object();
    auto& j_up = j_updaters[up->Name()];
    up->SaveConfig(&j_up);
  }
  out["specified_updater"] = Boolean{specified_updater_};
}

void GBTree::LoadModel(Json const& in) {
  CHECK_EQ(get<String>(in["name"]), "gbtree");
  model_.LoadModel(in["model"]);
}

void GBTree::SaveModel(Json* p_out) const {
  auto& out = *p_out;
  out["name"] = String("gbtree");
  out["model"] = Object();
  auto& model = out["model"];
  model_.SaveModel(&model);
}

void GBTree::Slice(int32_t layer_begin, int32_t layer_end, int32_t step,
                   GradientBooster *out, bool* out_of_bound) const {
  CHECK(configured_);
  CHECK(out);

  auto p_gbtree = dynamic_cast<GBTree *>(out);
  CHECK(p_gbtree);
  GBTreeModel &out_model = p_gbtree->model_;
  auto layer_trees = this->LayerTrees();

  layer_end = layer_end == 0 ? model_.trees.size() / layer_trees : layer_end;
  CHECK_GE(layer_end, layer_begin);
  CHECK_GE(step, 1);
  int32_t n_layers = (layer_end - layer_begin) / step;
  std::vector<std::unique_ptr<RegTree>> &out_trees = out_model.trees;
  out_trees.resize(layer_trees * n_layers);
  std::vector<int32_t> &out_trees_info = out_model.tree_info;
  out_trees_info.resize(layer_trees * n_layers);
  out_model.param.num_trees = out_model.trees.size();
  CHECK(this->model_.trees_to_update.empty());

  *out_of_bound = detail::SliceTrees(
      layer_begin, layer_end, step, this->model_, tparam_, layer_trees,
      [&](auto const &in_it, auto const &out_it) {
        auto new_tree =
            std::make_unique<RegTree>(*this->model_.trees.at(in_it));
        bst_group_t group = this->model_.tree_info[in_it];
        out_trees.at(out_it) = std::move(new_tree);
        out_trees_info.at(out_it) = group;
      });
}

void GBTree::PredictBatch(DMatrix* p_fmat,
                          PredictionCacheEntry* out_preds,
                          bool,
                          unsigned ntree_limit) {
  CHECK(configured_);
  GetPredictor(&out_preds->predictions, p_fmat)
      ->PredictBatch(p_fmat, out_preds, model_, 0, ntree_limit);
}

std::unique_ptr<Predictor> const &
GBTree::GetPredictor(HostDeviceVector<float> const *out_pred,
                     DMatrix *f_dmat) const {
  CHECK(configured_);
  if (tparam_.predictor != PredictorType::kAuto) {
    if (tparam_.predictor == PredictorType::kGPUPredictor) {
#if defined(XGBOOST_USE_CUDA)
      CHECK_GE(common::AllVisibleGPUs(), 1) << "No visible GPU is found for XGBoost.";
      CHECK(gpu_predictor_);
      return gpu_predictor_;
#else
      common::AssertGPUSupport();
#endif  // defined(XGBOOST_USE_CUDA)
    }
    if (tparam_.predictor == PredictorType::kOneAPIPredictor) {
#if defined(XGBOOST_USE_ONEAPI)
      CHECK(oneapi_predictor_);
      return oneapi_predictor_;
#else
      common::AssertOneAPISupport();
#endif  // defined(XGBOOST_USE_ONEAPI)
    }
    CHECK(cpu_predictor_);
    return cpu_predictor_;
  }

  // Data comes from Device DMatrix.
  auto is_ellpack = f_dmat && f_dmat->PageExists<EllpackPage>() &&
                    !f_dmat->PageExists<SparsePage>();
  // Data comes from device memory, like CuDF or CuPy.
  auto is_from_device =
      f_dmat && f_dmat->PageExists<SparsePage>() &&
      (*(f_dmat->GetBatches<SparsePage>().begin())).data.DeviceCanRead();
  auto on_device = is_ellpack || is_from_device;

  // Use GPU Predictor if data is already on device and gpu_id is set.
  if (on_device && generic_param_->gpu_id >= 0) {
#if defined(XGBOOST_USE_CUDA)
    CHECK_GE(common::AllVisibleGPUs(), 1) << "No visible GPU is found for XGBoost.";
    CHECK(gpu_predictor_);
    return gpu_predictor_;
#else
    LOG(FATAL) << "Data is on CUDA device, but XGBoost is not compiled with "
                  "CUDA support.";
    return cpu_predictor_;
#endif  // defined(XGBOOST_USE_CUDA)
  }

  // GPU_Hist by default has prediction cache calculated from quantile values,
  // so GPU Predictor is not used for training dataset.  But when XGBoost
  // performs continue training with an existing model, the prediction cache is
  // not availbale and number of trees doesn't equal zero, the whole training
  // dataset got copied into GPU for precise prediction.  This condition tries
  // to avoid such copy by calling CPU Predictor instead.
  if ((out_pred && out_pred->Size() == 0) && (model_.param.num_trees != 0) &&
      // FIXME(trivialfis): Implement a better method for testing whether data
      // is on device after DMatrix refactoring is done.
      !on_device) {
    CHECK(cpu_predictor_);
    return cpu_predictor_;
  }

  if (tparam_.tree_method == TreeMethod::kGPUHist) {
#if defined(XGBOOST_USE_CUDA)
    CHECK_GE(common::AllVisibleGPUs(), 1) << "No visible GPU is found for XGBoost.";
    CHECK(gpu_predictor_);
    return gpu_predictor_;
#else
    common::AssertGPUSupport();
    return cpu_predictor_;
#endif  // defined(XGBOOST_USE_CUDA)
  }

  CHECK(cpu_predictor_);
  return cpu_predictor_;
}

class Dart : public GBTree {
 public:
  explicit Dart(LearnerModelParam const* booster_config) :
      GBTree(booster_config) {}

  void Configure(const Args& cfg) override {
    GBTree::Configure(cfg);
    dparam_.UpdateAllowUnknown(cfg);
  }

  void Slice(int32_t layer_begin, int32_t layer_end, int32_t step,
             GradientBooster *out, bool* out_of_bound) const final {
    GBTree::Slice(layer_begin, layer_end, step, out, out_of_bound);
    if (*out_of_bound) {
      return;
    }
    auto p_dart = dynamic_cast<Dart*>(out);
    CHECK(p_dart);
    CHECK(p_dart->weight_drop_.empty());
    detail::SliceTrees(
        layer_begin, layer_end, step, model_, tparam_, this->LayerTrees(),
        [&](auto const& in_it, auto const&) {
          p_dart->weight_drop_.push_back(this->weight_drop_.at(in_it));
        });
  }

  void SaveModel(Json *p_out) const override {
    auto &out = *p_out;
    out["name"] = String("dart");
    out["gbtree"] = Object();
    GBTree::SaveModel(&(out["gbtree"]));

    std::vector<Json> j_weight_drop(weight_drop_.size());
    for (size_t i = 0; i < weight_drop_.size(); ++i) {
      j_weight_drop[i] = Number(weight_drop_[i]);
    }
    out["weight_drop"] = Array(std::move(j_weight_drop));
  }
  void LoadModel(Json const& in) override {
    CHECK_EQ(get<String>(in["name"]), "dart");
    auto const& gbtree = in["gbtree"];
    GBTree::LoadModel(gbtree);

    auto const& j_weight_drop = get<Array>(in["weight_drop"]);
    weight_drop_.resize(j_weight_drop.size());
    for (size_t i = 0; i < weight_drop_.size(); ++i) {
      weight_drop_[i] = get<Number const>(j_weight_drop[i]);
    }
  }

  void Load(dmlc::Stream* fi) override {
    GBTree::Load(fi);
    weight_drop_.resize(model_.param.num_trees);
    if (model_.param.num_trees != 0) {
      fi->Read(&weight_drop_);
    }
  }
  void Save(dmlc::Stream* fo) const override {
    GBTree::Save(fo);
    if (weight_drop_.size() != 0) {
      fo->Write(weight_drop_);
    }
  }

  void LoadConfig(Json const& in) override {
    CHECK_EQ(get<String>(in["name"]), "dart");
    auto const& gbtree = in["gbtree"];
    GBTree::LoadConfig(gbtree);
    FromJson(in["dart_train_param"], &dparam_);
  }
  void SaveConfig(Json* p_out) const override {
    auto& out = *p_out;
    out["name"] = String("dart");
    out["gbtree"] = Object();
    auto& gbtree = out["gbtree"];
    GBTree::SaveConfig(&gbtree);
    out["dart_train_param"] = ToJson(dparam_);
  }

  void PredictBatch(DMatrix* p_fmat,
                    PredictionCacheEntry* p_out_preds,
                    bool training,
                    unsigned ntree_limit) override {
    DropTrees(training);
    int num_group = model_.learner_model_param->num_output_group;
    ntree_limit *= num_group;
    if (ntree_limit == 0 || ntree_limit > model_.trees.size()) {
      ntree_limit = static_cast<unsigned>(model_.trees.size());
    }
    size_t n = num_group * p_fmat->Info().num_row_;
    const auto &base_margin = p_fmat->Info().base_margin_.ConstHostVector();
    auto& out_preds = p_out_preds->predictions.HostVector();
    out_preds.resize(n);
    if (base_margin.size() != 0) {
      CHECK_EQ(out_preds.size(), n);
      std::copy(base_margin.begin(), base_margin.end(), out_preds.begin());
    } else {
      std::fill(out_preds.begin(), out_preds.end(),
                model_.learner_model_param->base_score);
    }
    const int nthread = omp_get_max_threads();
    InitThreadTemp(nthread);
    PredLoopSpecalize(p_fmat, &out_preds, num_group, 0, ntree_limit);
  }

  void PredictInstance(const SparsePage::Inst &inst,
                       std::vector<bst_float> *out_preds,
                       unsigned ntree_limit) override {
    DropTrees(false);
    if (thread_temp_.size() == 0) {
      thread_temp_.resize(1, RegTree::FVec());
      thread_temp_[0].Init(model_.learner_model_param->num_feature);
    }
    out_preds->resize(model_.learner_model_param->num_output_group);
    ntree_limit *= model_.learner_model_param->num_output_group;
    if (ntree_limit == 0 || ntree_limit > model_.trees.size()) {
      ntree_limit = static_cast<unsigned>(model_.trees.size());
    }
    // loop over output groups
    for (uint32_t gid = 0; gid < model_.learner_model_param->num_output_group; ++gid) {
      (*out_preds)[gid] =
          PredValue(inst, gid, &thread_temp_[0], 0, ntree_limit) +
          model_.learner_model_param->base_score;
    }
  }

  bool UseGPU() const override {
    return GBTree::UseGPU();
  }

  void PredictContribution(DMatrix* p_fmat,
                           HostDeviceVector<bst_float>* out_contribs,
                           unsigned ntree_limit, bool approximate, int,
                           unsigned) override {
    CHECK(configured_);
    cpu_predictor_->PredictContribution(p_fmat, out_contribs, model_,
                                        ntree_limit, &weight_drop_, approximate);
  }

  void PredictInteractionContributions(DMatrix* p_fmat,
                                       HostDeviceVector<bst_float>* out_contribs,
                                       unsigned ntree_limit, bool approximate) override {
    CHECK(configured_);
    cpu_predictor_->PredictInteractionContributions(p_fmat, out_contribs, model_,
                                                    ntree_limit, &weight_drop_, approximate);
  }


 protected:
  inline void PredLoopSpecalize(
      DMatrix* p_fmat,
      std::vector<bst_float>* out_preds,
      int num_group,
      unsigned tree_begin,
      unsigned tree_end) {
    CHECK_EQ(num_group, model_.learner_model_param->num_output_group);
    std::vector<bst_float>& preds = *out_preds;
    CHECK_EQ(model_.param.size_leaf_vector, 0)
        << "size_leaf_vector is enforced to 0 so far";
    CHECK_EQ(preds.size(), p_fmat->Info().num_row_ * num_group);
    // start collecting the prediction
    for (const auto &batch : p_fmat->GetBatches<SparsePage>()) {
      auto page = batch.GetView();
      constexpr int kUnroll = 8;
      const auto nsize = static_cast<bst_omp_uint>(batch.Size());
      const bst_omp_uint rest = nsize % kUnroll;
      if (nsize >= kUnroll) {
#pragma omp parallel for schedule(static)
        for (bst_omp_uint i = 0; i < nsize - rest; i += kUnroll) {
          const int tid = omp_get_thread_num();
          RegTree::FVec& feats = thread_temp_[tid];
          int64_t ridx[kUnroll];
          SparsePage::Inst inst[kUnroll];
          for (int k = 0; k < kUnroll; ++k) {
            ridx[k] = static_cast<int64_t>(batch.base_rowid + i + k);
          }
          for (int k = 0; k < kUnroll; ++k) {
            inst[k] = page[i + k];
          }
          for (int k = 0; k < kUnroll; ++k) {
            for (int gid = 0; gid < num_group; ++gid) {
              const size_t offset = ridx[k] * num_group + gid;
              preds[offset] +=
                  this->PredValue(inst[k], gid, &feats, tree_begin, tree_end);
            }
          }
        }
      }

      for (bst_omp_uint i = nsize - rest; i < nsize; ++i) {
        RegTree::FVec& feats = thread_temp_[0];
        const auto ridx = static_cast<int64_t>(batch.base_rowid + i);
        const SparsePage::Inst inst = page[i];
        for (int gid = 0; gid < num_group; ++gid) {
          const size_t offset = ridx * num_group + gid;
          preds[offset] +=
              this->PredValue(inst, gid,
                              &feats, tree_begin, tree_end);
        }
      }
    }
  }

  // commit new trees all at once
  void
  CommitModel(std::vector<std::vector<std::unique_ptr<RegTree>>>&& new_trees,
              DMatrix*, PredictionCacheEntry*) override {
    int num_new_trees = 0;
    for (uint32_t gid = 0; gid < model_.learner_model_param->num_output_group; ++gid) {
      num_new_trees += new_trees[gid].size();
      model_.CommitModel(std::move(new_trees[gid]), gid);
    }
    size_t num_drop = NormalizeTrees(num_new_trees);
    LOG(INFO) << "drop " << num_drop << " trees, "
              << "weight = " << weight_drop_.back();
  }

  // predict the leaf scores without dropped trees
  bst_float PredValue(const SparsePage::Inst &inst, int bst_group,
                      RegTree::FVec *p_feats, unsigned tree_begin,
                      unsigned tree_end) const {
    bst_float psum = 0.0f;
    p_feats->Fill(inst);
    for (size_t i = tree_begin; i < tree_end; ++i) {
      if (model_.tree_info[i] == bst_group) {
        bool drop = std::binary_search(idx_drop_.begin(), idx_drop_.end(), i);
        if (!drop) {
          int tid = model_.trees[i]->GetLeafIndex(*p_feats);
          psum += weight_drop_[i] * (*model_.trees[i])[tid].LeafValue();
        }
      }
    }
    p_feats->Drop(inst);
    return psum;
  }

  // select which trees to drop
  // passing clear=True will clear selection
  inline void DropTrees(bool is_training) {
    idx_drop_.clear();
    if (!is_training) {
      return;
    }

    std::uniform_real_distribution<> runif(0.0, 1.0);
    auto& rnd = common::GlobalRandom();
    bool skip = false;
    if (dparam_.skip_drop > 0.0) skip = (runif(rnd) < dparam_.skip_drop);
    // sample some trees to drop
    if (!skip) {
      if (dparam_.sample_type == 1) {
        bst_float sum_weight = 0.0;
        for (auto elem : weight_drop_) {
          sum_weight += elem;
        }
        for (size_t i = 0; i < weight_drop_.size(); ++i) {
          if (runif(rnd) < dparam_.rate_drop * weight_drop_.size() * weight_drop_[i] / sum_weight) {
            idx_drop_.push_back(i);
          }
        }
        if (dparam_.one_drop && idx_drop_.empty() && !weight_drop_.empty()) {
          // the expression below is an ugly but MSVC2013-friendly equivalent of
          // size_t i = std::discrete_distribution<size_t>(weight_drop.begin(),
          //                                               weight_drop.end())(rnd);
          size_t i = std::discrete_distribution<size_t>(
            weight_drop_.size(), 0., static_cast<double>(weight_drop_.size()),
            [this](double x) -> double {
              return weight_drop_[static_cast<size_t>(x)];
            })(rnd);
          idx_drop_.push_back(i);
        }
      } else {
        for (size_t i = 0; i < weight_drop_.size(); ++i) {
          if (runif(rnd) < dparam_.rate_drop) {
            idx_drop_.push_back(i);
          }
        }
        if (dparam_.one_drop && idx_drop_.empty() && !weight_drop_.empty()) {
          size_t i = std::uniform_int_distribution<size_t>(0, weight_drop_.size() - 1)(rnd);
          idx_drop_.push_back(i);
        }
      }
    }
  }

  // set normalization factors
  inline size_t NormalizeTrees(size_t size_new_trees) {
    float lr = 1.0 * dparam_.learning_rate / size_new_trees;
    size_t num_drop = idx_drop_.size();
    if (num_drop == 0) {
      for (size_t i = 0; i < size_new_trees; ++i) {
        weight_drop_.push_back(1.0);
      }
    } else {
      if (dparam_.normalize_type == 1) {
        // normalize_type 1
        float factor = 1.0 / (1.0 + lr);
        for (auto i : idx_drop_) {
          weight_drop_[i] *= factor;
        }
        for (size_t i = 0; i < size_new_trees; ++i) {
          weight_drop_.push_back(factor);
        }
      } else {
        // normalize_type 0
        float factor = 1.0 * num_drop / (num_drop + lr);
        for (auto i : idx_drop_) {
          weight_drop_[i] *= factor;
        }
        for (size_t i = 0; i < size_new_trees; ++i) {
          weight_drop_.push_back(1.0 / (num_drop + lr));
        }
      }
    }
    // reset
    idx_drop_.clear();
    return num_drop;
  }

  // init thread buffers
  inline void InitThreadTemp(int nthread) {
    int prev_thread_temp_size = thread_temp_.size();
    if (prev_thread_temp_size < nthread) {
      thread_temp_.resize(nthread, RegTree::FVec());
      for (int i = prev_thread_temp_size; i < nthread; ++i) {
        thread_temp_[i].Init(model_.learner_model_param->num_feature);
      }
    }
  }

  // --- data structure ---
  // training parameter
  DartTrainParam dparam_;
  /*! \brief prediction buffer */
  std::vector<bst_float> weight_drop_;
  // indexes of dropped trees
  std::vector<size_t> idx_drop_;
  // temporal storage for per thread
  std::vector<RegTree::FVec> thread_temp_;
};

// register the objective functions
DMLC_REGISTER_PARAMETER(GBTreeModelParam);
DMLC_REGISTER_PARAMETER(GBTreeTrainParam);
DMLC_REGISTER_PARAMETER(DartTrainParam);

XGBOOST_REGISTER_GBM(GBTree, "gbtree")
.describe("Tree booster, gradient boosted trees.")
.set_body([](LearnerModelParam const* booster_config) {
    auto* p = new GBTree(booster_config);
    return p;
  });
XGBOOST_REGISTER_GBM(Dart, "dart")
.describe("Tree booster, dart.")
.set_body([](LearnerModelParam const* booster_config) {
    GBTree* p = new Dart(booster_config);
    return p;
  });
}  // namespace gbm
}  // namespace xgboost