refactor: Move camel_to_snake to util.py to fix cyclic import

Author: koxudaxi

src/datamodel_code_generator/parser/base.py

@@ -67,8 +67,9 @@
 from datamodel_code_generator.parser import DefaultPutDict, LiteralType
 from datamodel_code_generator.parser._graph import stable_toposort
 from datamodel_code_generator.parser._scc import find_circular_sccs, strongly_connected_components
-from datamodel_code_generator.reference import ModelResolver, ModelType, Reference, camel_to_snake
+from datamodel_code_generator.reference import ModelResolver, ModelType, Reference
 from datamodel_code_generator.types import DataType, DataTypeManager, StrictTypes
+from datamodel_code_generator.util import camel_to_snake
 
 if TYPE_CHECKING:
     from collections.abc import Iterable, Iterator, Mapping, Sequence

src/datamodel_code_generator/reference.py

@@ -36,7 +36,7 @@
 from typing_extensions import TypeIs
 
 from datamodel_code_generator import Error
-from datamodel_code_generator.util import PYDANTIC_V2, ConfigDict, model_validator
+from datamodel_code_generator.util import PYDANTIC_V2, ConfigDict, camel_to_snake, model_validator
 
 if TYPE_CHECKING:
     from collections.abc import Generator, Iterator, Mapping, Sequence
@@ -221,17 +221,6 @@ def context_variable(setter: Callable[[T], None], current_value: T, new_value: T
         setter(previous_value)
 
 
-_UNDER_SCORE_1: Pattern[str] = re.compile(r"([^_])([A-Z][a-z]+)")
-_UNDER_SCORE_2: Pattern[str] = re.compile(r"([a-z0-9])([A-Z])")
-
-
-@lru_cache
-def camel_to_snake(string: str) -> str:
-    """Convert camelCase or PascalCase to snake_case."""
-    subbed = _UNDER_SCORE_1.sub(r"\1_\2", string)
-    return _UNDER_SCORE_2.sub(r"\1_\2", subbed).lower()
-
-
 class FieldNameResolver:
     """Converts schema field names to valid Python identifiers."""
 

src/datamodel_code_generator/util.py

@@ -7,6 +7,9 @@
 from __future__ import annotations
 
 import copy
+import re
+from functools import lru_cache
+from re import Pattern
 from typing import TYPE_CHECKING, Any, Callable, Literal, TypeVar, overload
 
 import pydantic
@@ -140,3 +143,14 @@ class BaseModel(_BaseModel):
 
     if PYDANTIC_V2:
         model_config = ConfigDict(strict=False)  # pyright: ignore[reportAssignmentType]
+
+
+_UNDER_SCORE_1: Pattern[str] = re.compile(r"([^_])([A-Z][a-z]+)")
+_UNDER_SCORE_2: Pattern[str] = re.compile(r"([a-z0-9])([A-Z])")
+
+
+@lru_cache
+def camel_to_snake(string: str) -> str:
+    """Convert camelCase or PascalCase to snake_case."""
+    subbed = _UNDER_SCORE_1.sub(r"\1_\2", string)
+    return _UNDER_SCORE_2.sub(r"\1_\2", subbed).lower()