fix(map): reject duplicate YAML keys via a SafeLoader duplicate-check

src/pf1e_simulator/map.py

@@ -12,10 +12,14 @@ from typing import TYPE_CHECKING
 
 import yaml
 from pydantic import BaseModel, ConfigDict, Field, ValidationError, field_validator
+from yaml.nodes import ScalarNode
 
 if TYPE_CHECKING:
+    from collections.abc import Hashable
     from pathlib import Path
 
+    from yaml.nodes import MappingNode
+
 Pos = tuple[int, int]  # (row, col), 0-indexed
 
 _ELEVATION_CHARS = frozenset("0123456789.")  # digit = height, '.' = ground level
@@ -38,6 +42,29 @@ class MapValidationError(Exception):
         self.reason = reason
 
 
+class _NoDuplicateLoader(yaml.SafeLoader):
+    """SafeLoader that rejects duplicate mapping keys (PyYAML is last-wins)."""
+
+
+def _construct_mapping(
+    loader: _NoDuplicateLoader, node: MappingNode, *, deep: bool = False
+) -> dict[Hashable, object]:
+    seen: set[str] = set()
+    for key_node, _ in node.value:
+        if isinstance(key_node, ScalarNode):
+            key = loader.construct_scalar(key_node)
+            if key in seen:
+                msg = f"duplicate mapping key: {key!r}"
+                raise yaml.constructor.ConstructorError(None, None, msg, key_node.start_mark)
+            seen.add(key)
+    return yaml.constructor.SafeConstructor.construct_mapping(loader, node, deep)
+
+
+_NoDuplicateLoader.add_constructor(
+    yaml.resolver.BaseResolver.DEFAULT_MAPPING_TAG, _construct_mapping
+)
+
+
 class TerrainType(BaseModel):
     """What one square means for movement and sight lines."""
 
@@ -177,7 +204,13 @@ def load_map(path: Path) -> MapSpec:
         msg = f"cannot read file: {exc}"
         raise MapValidationError(path, msg) from exc
     try:
-        data: object = yaml.safe_load(text)
+        # _NoDuplicateLoader subclasses SafeLoader: no arbitrary object
+        # instantiation, it only adds duplicate-key rejection (S506 is a
+        # false positive for this loader).
+        data: object = yaml.load(
+            text,
+            Loader=_NoDuplicateLoader,  # noqa: S506
+        )
     except yaml.YAMLError as exc:
         msg = f"invalid YAML: {exc}"
         raise MapValidationError(path, msg) from exc