fix: an untagged bios serves every region

scripts/region.py

@@ -255,16 +255,25 @@ def fallback_groups(
     index: dict[str, dict],
     requested: list[str],
 ) -> list[str]:
-    """Group IDs where no candidate matched, so the whole group was kept."""
+    """Group IDs where no candidate matched, so the whole group was kept.
+
+    A world file, or one that declares no region at all, answers every
+    region: the C64 kernal VICE loads by default is untagged, and a group
+    holding it beside a Japanese and a Swedish kernal does have a BIOS for
+    North America.
+    """
     if not requested:
         return []
     out: list[str] = []
     for group_id, members in groups.items():
         ranked = _competing_ranks(members, index, requested)
-        has_world = any(
+        served = any(
-            WORLD in lookup_regions(index, destination, name)
+            not regions or WORLD in regions
-            for destination, name in members
+            for regions in (
+                lookup_regions(index, destination, name)
+                for destination, name in members
+            )
         )
-        if ranked and not has_world and min(r for r, _ in ranked) == len(requested):
+        if ranked and not served and min(r for r, _ in ranked) == len(requested):
             out.append(group_id)
     return sorted(out)

tests/test_region.py

@@ -355,6 +355,12 @@ class TestFallbackGroups(unittest.TestCase):
             region.fallback_groups(groups, self.index, ["north-america"]), ["fds"]
         )
 
+    def test_an_untagged_bios_answers_every_region(self):
+        """VICE's generic kernal is untagged beside its Japanese one: the C64
+        was reported as having no BIOS for North America."""
+        groups = {"fds": [("disksys.rom", "disksys.rom"), ("generic.rom", "generic.rom")]}
+        self.assertEqual(region.fallback_groups(groups, self.index, ["north-america"]), [])
+
     def test_no_requested_regions_reports_nothing(self):
         groups = {"fds": [("disksys.rom", "disksys.rom")]}
         self.assertEqual(region.fallback_groups(groups, self.index, []), [])