Allow different splitting rules (#96)

* Added an option to specify different splitting rules for each dimension

* Allow batch sampling in tree.predict, speeding up _sample_posterior by 60x

* Addressed most issues flagged in PR comments

* Fix the reference to flexBART

Co-authored-by: Osvaldo A Martin <aloctavodia@gmail.com>

* Added tests for split rules

* Fix mypy annotations

* Fix linter issues

---------

Co-authored-by: Osvaldo A Martin <aloctavodia@gmail.com>

Author: velochy

pymc_bart/__init__.py

@@ -15,6 +15,7 @@
 
 from pymc_bart.bart import BART
 from pymc_bart.pgbart import PGBART
+from pymc_bart.split_rules import ContinuousSplitRule, OneHotSplitRule, SubsetSplitRule
 from pymc_bart.utils import (
     plot_convergence,
     plot_pdp,

pymc_bart/bart.py

@@ -27,6 +27,7 @@
 
 from .tree import Tree
 from .utils import TensorLike, _sample_posterior
+from .split_rules import SplitRule
 
 __all__ = ["BART"]
 
@@ -91,6 +92,10 @@ class BART(Distribution):
         Each element of split_prior should be in the [0, 1] interval and the elements should sum to
         1. Otherwise they will be normalized.
         Defaults to 0, i.e. all covariates have the same prior probability to be selected.
+    split_rules : Optional[SplitRule], default None
+        List of SplitRule objects, one per column in input data.
+        Allows using different split rules for different columns. Default is ContinuousSplitRule.
+        Other options are OneHotSplitRule and SubsetSplitRule, both meant for categorical variables.
 
     Notes
     -----
@@ -109,6 +114,7 @@ def __new__(
         beta: float = 2.0,
         response: str = "constant",
         split_prior: Optional[List[float]] = None,
+        split_rules: Optional[SplitRule] = None,
         **kwargs,
     ):
         manager = Manager()
@@ -134,6 +140,7 @@ def __new__(
                 alpha=alpha,
                 beta=beta,
                 split_prior=split_prior,
+                split_rules=split_rules,
             ),
         )()
 

pymc_bart/pgbart.py

@@ -26,6 +26,7 @@
 
 from pymc_bart.bart import BARTRV
 from pymc_bart.tree import Node, Tree, get_idx_left_child, get_idx_right_child, get_depth
+from pymc_bart.split_rules import ContinuousSplitRule
 
 
 class ParticleTree:
@@ -141,6 +142,11 @@ def __init__(
         else:
             self.alpha_vec = np.ones(self.X.shape[1], dtype=np.int32)
 
+        if self.bart.split_rules:
+            self.split_rules = self.bart.split_rules
+        else:
+            self.split_rules = [ContinuousSplitRule] * self.X.shape[1]
+
         init_mean = self.bart.Y.mean()
         self.num_observations = self.X.shape[0]
         self.num_variates = self.X.shape[1]
@@ -164,6 +170,7 @@ def __init__(
             idx_data_points=np.arange(self.num_observations, dtype="int32"),
             num_observations=self.num_observations,
             shape=self.shape,
+            split_rules=self.split_rules,
         )
 
         self.normal = NormalSampler(1, self.shape)
@@ -443,13 +450,17 @@ def grow_tree(
     idx_data_points, available_splitting_values = filter_missing_values(
         X[idx_data_points, selected_predictor], idx_data_points, missing_data
     )
-    split_value = get_split_value(available_splitting_values)
+
+    split_rule = tree.split_rules[selected_predictor]
+
+    split_value = split_rule.get_split_value(available_splitting_values)
 
     if split_value is None:
         return None
-    new_idx_data_points = get_new_idx_data_points(
-        available_splitting_values, split_value, idx_data_points
-    )
+
+    to_left = split_rule.divide(available_splitting_values, split_value)
+    new_idx_data_points = idx_data_points[to_left], idx_data_points[~to_left]
+
     current_node_children = (
         get_idx_left_child(index_leaf_node),
         get_idx_right_child(index_leaf_node),
@@ -481,12 +492,6 @@ def grow_tree(
     return current_node_children
 
 
-@njit
-def get_new_idx_data_points(available_splitting_values, split_value, idx_data_points):
-    split_idx = available_splitting_values <= split_value
-    return idx_data_points[split_idx], idx_data_points[~split_idx]
-
-
 def filter_missing_values(available_splitting_values, idx_data_points, missing_data):
     if missing_data:
         mask = ~np.isnan(available_splitting_values)
@@ -495,14 +500,6 @@ def filter_missing_values(available_splitting_values, idx_data_points, missing_d
     return idx_data_points, available_splitting_values
 
 
-def get_split_value(available_splitting_values):
-    split_value = None
-    if available_splitting_values.size > 0:
-        idx_selected_splitting_values = discrete_uniform_sampler(len(available_splitting_values))
-        split_value = available_splitting_values[idx_selected_splitting_values]
-    return split_value
-
-
 def draw_leaf_value(
     y_mu_pred: npt.NDArray[np.float_],
     x_mu: npt.NDArray[np.float_],

pymc_bart/split_rules.py

@@ -0,0 +1,103 @@
+#   Copyright 2022 The PyMC Developers
+#
+#   Licensed under the Apache License, Version 2.0 (the "License");
+#   you may not use this file except in compliance with the License.
+#   You may obtain a copy of the License at
+#
+#       http://www.apache.org/licenses/LICENSE-2.0
+#
+#   Unless required by applicable law or agreed to in writing, software
+#   distributed under the License is distributed on an "AS IS" BASIS,
+#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#   See the License for the specific language governing permissions and
+#   limitations under the License.
+
+from abc import abstractmethod
+from numba import njit
+import numpy as np
+
+
+class SplitRule:
+    """
+    Abstract template class for a split rule
+    """
+
+    @staticmethod
+    @abstractmethod
+    def get_split_value(available_splitting_values):
+        pass
+
+    @staticmethod
+    @abstractmethod
+    def divide(available_splitting_values, split_value):
+        pass
+
+
+class ContinuousSplitRule(SplitRule):
+    """
+    Standard continuous split rule: pick a pivot value and split
+    depending on if variable is smaller or greater than the value picked.
+    """
+
+    @staticmethod
+    def get_split_value(available_splitting_values):
+        split_value = None
+        if available_splitting_values.size > 1:
+            idx_selected_splitting_values = int(
+                np.random.random() * len(available_splitting_values)
+            )
+            split_value = available_splitting_values[idx_selected_splitting_values]
+        return split_value
+
+    @staticmethod
+    @njit
+    def divide(available_splitting_values, split_value):
+        return available_splitting_values <= split_value
+
+
+class OneHotSplitRule(SplitRule):
+    """Choose a single categorical value and branch on if the variable is that value or not"""
+
+    @staticmethod
+    def get_split_value(available_splitting_values):
+        split_value = None
+        if available_splitting_values.size > 1 and not np.all(
+            available_splitting_values == available_splitting_values[0]
+        ):
+            idx_selected_splitting_values = int(
+                np.random.random() * len(available_splitting_values)
+            )
+            split_value = available_splitting_values[idx_selected_splitting_values]
+        return split_value
+
+    @staticmethod
+    @njit
+    def divide(available_splitting_values, split_value):
+        return available_splitting_values == split_value
+
+
+class SubsetSplitRule(SplitRule):
+    """
+    Choose a random subset of the categorical values and branch on belonging to that set.
+    This is the approach taken by  Sameer K. Deshpande.
+    flexBART: Flexible Bayesian regression trees with categorical predictors. arXiv,
+    `link <https://arxiv.org/abs/2211.04459>`__
+    """
+
+    @staticmethod
+    def get_split_value(available_splitting_values):
+        split_value = None
+        if available_splitting_values.size > 1 and not np.all(
+            available_splitting_values == available_splitting_values[0]
+        ):
+            unique_values = np.unique(available_splitting_values)
+            while True:
+                sample = np.random.randint(0, 2, size=len(unique_values)).astype(bool)
+                if np.any(sample):
+                    break
+            split_value = unique_values[sample]
+        return split_value
+
+    @staticmethod
+    def divide(available_splitting_values, split_value):
+        return np.isin(available_splitting_values, split_value)