Fix and test file_sort_key

mkdocs_gen_files/editor.py

@@ -13,9 +13,11 @@ def _normpath(*path: str):
     return os.path.normpath(os.path.join(*path)).replace(os.sep, "/")
 
 
-def _file_sort_key(f: File):
+def file_sort_key(f: File):
     parts = pathlib.PurePath(f.src_path).parts
-    return tuple(chr(i != len(parts) - 1) + chr(f.name != "index") + p for i, p in enumerate(parts))
+    return tuple(
+        chr(f.name != "index" if i == len(parts) - 1 else 2) + p for i, p in enumerate(parts)
+    )
 
 
 class FilesEditor:
@@ -111,5 +113,5 @@ def files(self) -> Files:
 
         [Files]: https://github.com/mkdocs/mkdocs/blob/master/mkdocs/structure/files.py
         """
-        files = sorted(self._files.values(), key=_file_sort_key)
+        files = sorted(self._files.values(), key=file_sort_key)
         return Files(files)

tests/test_editor.py

@@ -0,0 +1,20 @@
+import pytest
+from mkdocs.structure.files import File
+
+from mkdocs_gen_files import editor
+
+
+@pytest.mark.parametrize(
+    "names",
+    [
+        ["a/b.md", "b/index.md", "b/a.md"],
+        ["SUMMARY.md", "foo/a.md", "foo/bar/index.md", "foo/bar/SUMMARY.md"],
+    ],
+)
+@pytest.mark.parametrize("use_directory_urls", [False, True])
+def test_file_sort_key(use_directory_urls, names):
+    files = [
+        File(name, src_dir="", dest_dir="", use_directory_urls=use_directory_urls) for name in names
+    ]
+    sorted_names = [f.src_path for f in sorted(files, key=editor.file_sort_key)]
+    assert sorted_names == names