Share tree kernel: hoist group criterion base + unify builders (#945)

causalml/inference/tree/_tree/_group_builder.pxd

@@ -0,0 +1,43 @@
+# cython: cdivision=True
+# cython: boundscheck=False
+# cython: wraparound=False
+# cython: language_level=3
+
+# See _group_builder.pyx for implementation details.
+from ._tree cimport Node, Tree, TreeBuilder
+from ._splitter cimport Splitter, SplitRecord
+from ._typedefs cimport intp_t, int32_t, int64_t, float32_t, float64_t
+from ._tree cimport FrontierRecord, StackRecord
+from ._tree cimport ParentInfo, _init_parent_record
+
+
+cdef class GroupDepthFirstTreeBuilder(TreeBuilder):
+    # Shared depth-first skeleton for treatment-group trees (causal + uplift).
+    # Family-specific behavior is injected through the hooks below; all default
+    # to no-ops so the base builder reproduces the stock sklearn traversal.
+    cdef void _before_build(self, Tree tree) noexcept nogil
+    cdef void _before_node_split(self, Splitter splitter, intp_t parent) noexcept nogil
+    cdef void _after_node_value(self, Splitter splitter, intp_t node_id) noexcept nogil
+    cdef bint _node_is_leaf_extra(self, Splitter splitter) noexcept nogil
+
+
+cdef class GroupBestFirstTreeBuilder(TreeBuilder):
+    cdef intp_t max_leaf_nodes
+
+    cdef void _before_build(self, Tree tree) noexcept nogil
+    cdef void _before_node_split(self, Splitter splitter, intp_t parent) noexcept nogil
+    cdef void _after_node_value(self, Splitter splitter, intp_t node_id) noexcept nogil
+    cdef bint _node_is_leaf_extra(self, Splitter splitter) noexcept nogil
+    cdef int _add_split_node(
+        self,
+        Splitter splitter,
+        Tree tree,
+        intp_t start,
+        intp_t end,
+        bint is_first,
+        bint is_left,
+        Node* parent,
+        intp_t depth,
+        ParentInfo* parent_record,
+        FrontierRecord* res
+    ) except -1 nogil