Predefined bin thresholds

docs/Parameters.rst

@@ -412,6 +412,14 @@ Learning Control Parameters
 
    -  see `this file <https://github.com/microsoft/LightGBM/tree/master/examples/binary_classification/forced_splits.json>`__ as an example
 
+-  ``forcedbins_filename`` :raw-html:`<a id="forcedbins_filename" title="Permalink to this parameter" href="#forcedbins_filename">&#x1F517;&#xFE0E;</a>`, default = ``""``, type = string
+
+   -  path to a ``.json`` file that specifies bin upper bounds for some or all features
+
+   -  ``.json`` file should contain an array of objects, each containing the word ``feature`` (integer feature index) and ``bin_upper_bound`` (array of thresholds for binning)
+
+   -  see `this file <https://github.com/microsoft/LightGBM/tree/master/examples/regression/forced_bins.json>`__ as an example
+
 -  ``refit_decay_rate`` :raw-html:`<a id="refit_decay_rate" title="Permalink to this parameter" href="#refit_decay_rate">&#x1F517;&#xFE0E;</a>`, default = ``0.9``, type = double, constraints: ``0.0 <= refit_decay_rate <= 1.0``
 
    -  decay rate of ``refit`` task, will use ``leaf_output = refit_decay_rate * old_leaf_output + (1.0 - refit_decay_rate) * new_leaf_output`` to refit trees

examples/regression/forced_bins.json

@@ -0,0 +1,10 @@
+[
+    {
+        "feature": 0,
+        "bin_upper_bound": [ 0.3, 0.35, 0.4 ]
+    },
+    {
+        "feature": 1,
+        "bin_upper_bound": [ -0.1, -0.15, -0.2 ]
+    }
+]

examples/regression/forced_bins2.json

@@ -0,0 +1,6 @@
+[
+    {
+        "feature": 0,
+        "bin_upper_bound": [ 0.19, 0.39, 0.59, 0.79 ]
+    }
+]

examples/regression/train.conf

@@ -29,6 +29,9 @@ is_training_metric = true
 # number of bins for feature bucket, 255 is a recommend setting, it can save memories, and also has good accuracy. 
 max_bin = 255
 
+# forced bin thresholds
+# forcedbins_filename = forced_bins.json
+
 # training data
 # if exsting weight file, should name to "regression.train.weight"
 # alias: train_data, train

include/LightGBM/bin.h

@@ -146,9 +146,10 @@ class BinMapper {
   * \param bin_type Type of this bin
   * \param use_missing True to enable missing value handle
   * \param zero_as_missing True to use zero as missing value
+  * \param forced_upper_bounds Vector of split points that must be used (if this has size less than max_bin, remaining splits are found by the algorithm)
   */
   void FindBin(double* values, int num_values, size_t total_sample_cnt, int max_bin, int min_data_in_bin, int min_split_data, BinType bin_type,
-               bool use_missing, bool zero_as_missing);
+               bool use_missing, bool zero_as_missing, std::vector<double> forced_upper_bounds);
 
   /*!
   * \brief Use specific number of bin to calculate the size of this class

include/LightGBM/config.h

@@ -408,6 +408,11 @@ struct Config {
   // desc = see `this file <https://github.com/microsoft/LightGBM/tree/master/examples/binary_classification/forced_splits.json>`__ as an example
   std::string forcedsplits_filename = "";
 
+  // desc = path to a ``.json`` file that specifies bin upper bounds for some or all features
+  // desc = ``.json`` file should contain an array of objects, each containing the word ``feature`` (integer feature index) and ``bin_upper_bound`` (array of thresholds for binning)
+  // desc = see `this file <https://github.com/microsoft/LightGBM/tree/master/examples/regression/forced_bins.json>`__ as an example
+  std::string forcedbins_filename = "";
+
   // check = >=0.0
   // check = <=1.0
   // desc = decay rate of ``refit`` task, will use ``leaf_output = refit_decay_rate * old_leaf_output + (1.0 - refit_decay_rate) * new_leaf_output`` to refit trees

include/LightGBM/dataset.h

@@ -596,6 +596,8 @@ class Dataset {
 
   void addFeaturesFrom(Dataset* other);
 
+  static std::vector<std::vector<double>> GetForcedBins(std::string forced_bins_path, int num_total_features);
+
  private:
   std::string data_filename_;
   /*! \brief Store used features */
@@ -630,6 +632,7 @@ class Dataset {
   bool is_finish_load_;
   int max_bin_;
   std::vector<int32_t> max_bin_by_feature_;
+  std::vector<std::vector<double>> forced_bin_bounds_;
   int bin_construct_sample_cnt_;
   int min_data_in_bin_;
   bool use_missing_;

src/io/bin.cpp

@@ -71,7 +71,7 @@ namespace LightGBM {
     return true;
   }
 
-  std::vector<double> GreedyFindBin(const double* distinct_values, const int* counts,
+  std::vector<double> GreedyFindBin(const double* distinct_values, const int* counts, 
     int num_distinct_values, int max_bin, size_t total_cnt, int min_data_in_bin) {
     std::vector<double> bin_upper_bound;
     CHECK(max_bin > 0);
@@ -149,8 +149,107 @@ namespace LightGBM {
     return bin_upper_bound;
   }
 
-  std::vector<double> FindBinWithZeroAsOneBin(const double* distinct_values, const int* counts,
-    int num_distinct_values, int max_bin, size_t total_sample_cnt, int min_data_in_bin) {
+  std::vector<double> FindBinWithPredefinedBin(const double* distinct_values, const int* counts,
+    int num_distinct_values, int max_bin, size_t total_sample_cnt, int min_data_in_bin, std::vector<double> forced_upper_bounds) {
+    std::vector<double> bin_upper_bound;
+
+    // get list of distinct values
+    int left_cnt_data = 0;
+    int cnt_zero = 0;
+    int right_cnt_data = 0;
+    for (int i = 0; i < num_distinct_values; ++i) {
+      if (distinct_values[i] <= -kZeroThreshold) {
+        left_cnt_data += counts[i];
+      } else if (distinct_values[i] > kZeroThreshold) {
+        right_cnt_data += counts[i];
+      } else {
+        cnt_zero += counts[i];
+      }
+    }
+
+    // get number of positive and negative distinct values
+    int left_cnt = -1;
+    for (int i = 0; i < num_distinct_values; ++i) {
+      if (distinct_values[i] > -kZeroThreshold) {
+        left_cnt = i;
+        break;
+      }
+    }
+    if (left_cnt < 0) {
+      left_cnt = num_distinct_values;
+    }
+    int right_start = -1;
+    for (int i = left_cnt; i < num_distinct_values; ++i) {
+      if (distinct_values[i] > kZeroThreshold) {
+        right_start = i;
+        break;
+      }
+    }
+
+    // include zero bounds and infinity bound
+    if (max_bin == 2) {
+      if (left_cnt == 0) {
+        bin_upper_bound.push_back(kZeroThreshold);
+      } else {
+        bin_upper_bound.push_back(-kZeroThreshold);
+      }
+    } else if (max_bin >= 3) {
+      if (left_cnt > 0) {
+        bin_upper_bound.push_back(-kZeroThreshold);
+      }
+      if (right_start >= 0) {
+        bin_upper_bound.push_back(kZeroThreshold);
+      }
+    }
+    bin_upper_bound.push_back(std::numeric_limits<double>::infinity());
+
+    // add forced bounds, excluding zeros since we have already added zero bounds
+    size_t i = 0;
+    while (i < forced_upper_bounds.size()) {
+      if (std::fabs(forced_upper_bounds[i]) <= kZeroThreshold) {
+        forced_upper_bounds.erase(forced_upper_bounds.begin() + i);
+      } else {
+        ++i;
+      }
+    }
+    int max_to_insert = max_bin - static_cast<int>(bin_upper_bound.size());
+    int num_to_insert = std::min(max_to_insert, static_cast<int>(forced_upper_bounds.size()));
+    if (num_to_insert > 0) {
+      bin_upper_bound.insert(bin_upper_bound.end(), forced_upper_bounds.begin(), forced_upper_bounds.begin() + num_to_insert);
+    }
+    std::stable_sort(bin_upper_bound.begin(), bin_upper_bound.end());
+
+    // find remaining bounds
+    int free_bins = max_bin - static_cast<int>(bin_upper_bound.size());
+    std::vector<double> bounds_to_add;
+    int value_ind = 0;
+    for (size_t i = 0; i < bin_upper_bound.size(); ++i) {
+      int cnt_in_bin = 0;
+      int distinct_cnt_in_bin = 0;
+      int bin_start = value_ind;
+      while ((value_ind < num_distinct_values) && (distinct_values[value_ind] < bin_upper_bound[i])) {
+        cnt_in_bin += counts[value_ind];
+        ++distinct_cnt_in_bin;
+        ++value_ind;
+      }
+      int bins_remaining = max_bin - static_cast<int>(bin_upper_bound.size()) - static_cast<int>(bounds_to_add.size());
+      int num_sub_bins = static_cast<int>(std::lround((static_cast<double>(cnt_in_bin) * free_bins / total_sample_cnt)));
+      num_sub_bins = std::min(num_sub_bins, bins_remaining) + 1;
+      if (i == bin_upper_bound.size() - 1) {
+        num_sub_bins = bins_remaining + 1;
+      }
+      std::vector<double> new_upper_bounds = GreedyFindBin(distinct_values + bin_start, counts + bin_start, distinct_cnt_in_bin,
+        num_sub_bins, cnt_in_bin, min_data_in_bin);
+      bounds_to_add.insert(bounds_to_add.end(), new_upper_bounds.begin(), new_upper_bounds.end() - 1);  // last bound is infinity
+    }
+    bin_upper_bound.insert(bin_upper_bound.end(), bounds_to_add.begin(), bounds_to_add.end());
+    std::stable_sort(bin_upper_bound.begin(), bin_upper_bound.end());
+    CHECK(bin_upper_bound.size() <= static_cast<size_t>(max_bin));
+    return bin_upper_bound;
+  }
+
+  std::vector<double> FindBinWithZeroAsOneBin(const double* distinct_values, const int* counts, int num_distinct_values, 
+    int max_bin, size_t total_sample_cnt, int min_data_in_bin) {
     std::vector<double> bin_upper_bound;
     int left_cnt_data = 0;
     int cnt_zero = 0;
@@ -207,8 +306,19 @@ namespace LightGBM {
     return bin_upper_bound;
   }
 
+  std::vector<double> FindBinWithZeroAsOneBin(const double* distinct_values, const int* counts, int num_distinct_values,
+    int max_bin, size_t total_sample_cnt, int min_data_in_bin, std::vector<double> forced_upper_bounds) {
+    if (forced_upper_bounds.empty()) {
+      return FindBinWithZeroAsOneBin(distinct_values, counts, num_distinct_values, max_bin, total_sample_cnt, min_data_in_bin);
+    } else {
+      return FindBinWithPredefinedBin(distinct_values, counts, num_distinct_values, max_bin, total_sample_cnt, min_data_in_bin,
+                                      forced_upper_bounds);
+    }
+  }
+
   void BinMapper::FindBin(double* values, int num_sample_values, size_t total_sample_cnt,
-    int max_bin, int min_data_in_bin, int min_split_data, BinType bin_type, bool use_missing, bool zero_as_missing) {
+    int max_bin, int min_data_in_bin, int min_split_data, BinType bin_type, bool use_missing, bool zero_as_missing, 
+    std::vector<double> forced_upper_bounds) {
     int na_cnt = 0;
     int tmp_num_sample_values = 0;
     for (int i = 0; i < num_sample_values; ++i) {
@@ -276,14 +386,17 @@ namespace LightGBM {
     int num_distinct_values = static_cast<int>(distinct_values.size());
     if (bin_type_ == BinType::NumericalBin) {
       if (missing_type_ == MissingType::Zero) {
-        bin_upper_bound_ = FindBinWithZeroAsOneBin(distinct_values.data(), counts.data(), num_distinct_values, max_bin, total_sample_cnt, min_data_in_bin);
+        bin_upper_bound_ = FindBinWithZeroAsOneBin(distinct_values.data(), counts.data(), num_distinct_values, max_bin, total_sample_cnt,
+                                                   min_data_in_bin, forced_upper_bounds);
         if (bin_upper_bound_.size() == 2) {
           missing_type_ = MissingType::None;
         }
       } else if (missing_type_ == MissingType::None) {
-        bin_upper_bound_ = FindBinWithZeroAsOneBin(distinct_values.data(), counts.data(), num_distinct_values, max_bin, total_sample_cnt, min_data_in_bin);
+        bin_upper_bound_ = FindBinWithZeroAsOneBin(distinct_values.data(), counts.data(), num_distinct_values, max_bin, total_sample_cnt,
+                                                   min_data_in_bin, forced_upper_bounds);
       } else {
-        bin_upper_bound_ = FindBinWithZeroAsOneBin(distinct_values.data(), counts.data(), num_distinct_values, max_bin - 1, total_sample_cnt - na_cnt, min_data_in_bin);
+        bin_upper_bound_ = FindBinWithZeroAsOneBin(distinct_values.data(), counts.data(), num_distinct_values, max_bin - 1, total_sample_cnt - na_cnt,
+                                                   min_data_in_bin, forced_upper_bounds);
         bin_upper_bound_.push_back(NaN);
       }
       num_bin_ = static_cast<int>(bin_upper_bound_.size());

src/io/config_auto.cpp

@@ -214,6 +214,7 @@ std::unordered_set<std::string> Config::parameter_set({
   "monotone_constraints",
   "feature_contri",
   "forcedsplits_filename",
+  "forcedbins_filename",
   "refit_decay_rate",
   "cegb_tradeoff",
   "cegb_penalty_split",
@@ -402,6 +403,8 @@ void Config::GetMembersFromString(const std::unordered_map<std::string, std::str
 
   GetString(params, "forcedsplits_filename", &forcedsplits_filename);
 
+  GetString(params, "forcedbins_filename", &forcedbins_filename);
+
   GetDouble(params, "refit_decay_rate", &refit_decay_rate);
   CHECK(refit_decay_rate >=0.0);
   CHECK(refit_decay_rate <=1.0);
@@ -617,6 +620,7 @@ std::string Config::SaveMembersToString() const {
   str_buf << "[monotone_constraints: " << Common::Join(Common::ArrayCast<int8_t, int>(monotone_constraints), ",") << "]\n";
   str_buf << "[feature_contri: " << Common::Join(feature_contri, ",") << "]\n";
   str_buf << "[forcedsplits_filename: " << forcedsplits_filename << "]\n";
+  str_buf << "[forcedbins_filename: " << forcedbins_filename << "]\n";
   str_buf << "[refit_decay_rate: " << refit_decay_rate << "]\n";
   str_buf << "[cegb_tradeoff: " << cegb_tradeoff << "]\n";
   str_buf << "[cegb_penalty_split: " << cegb_penalty_split << "]\n";