fix: let a system-less extra compete with nothing

tests/test_region_groups.py

@@ -0,0 +1,67 @@
+"""A core extra competes for a region only with files of its own machine.
+
+CLK profiles 30 systems and names none per file. Its extras were filed in
+the region group of every one of them: its American MSX ROM made RetroDECK's
+Enterprise group drop the German Enterprise ROM under --region us.
+"""
+
+from __future__ import annotations
+
+import sys
+import unittest
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(REPO_ROOT / "scripts"))
+
+from packextras import extra_region_groups  # noqa: E402
+
+
+class TheGroupFollowsWhatTheProfileSays(unittest.TestCase):
+    def test_a_file_system(self):
+        self.assertEqual(
+            extra_region_groups({"source_system": "msx", "source_systems": ["msx", "vic20"]}),
+            ["msx"],
+        )
+
+    def test_a_single_system_profile(self):
+        self.assertEqual(extra_region_groups({"source_systems": ["sega-saturn"]}), ["sega-saturn"])
+
+    def test_a_multi_system_profile_without_a_file_system(self):
+        groups = extra_region_groups({
+            "source_emulator": "clk", "destination": "MSX/msx-american.rom",
+            "source_systems": ["msx", "enterprise-64-128", "vic20"],
+        })
+        self.assertEqual(groups, ["clk:file:MSX/msx-american.rom"])
+
+    def test_a_variant_group_stays_one_slot(self):
+        self.assertEqual(
+            extra_region_groups({"source_emulator": "picodrive", "variant_group": "mcd",
+                                 "source_systems": ["sega-segacd", "sega-mega-cd"]}),
+            ["picodrive:variant:mcd"],
+        )
+
+
+class TheEnterpriseKeepsItsRom(unittest.TestCase):
+    def test_retrodeck_keeps_brd_rom_under_a_region(self):
+        if not (REPO_ROOT / "database.json").exists():
+            self.skipTest("database.json is not built")
+        import region
+        from common import load_database, load_emulator_profiles, load_platform_config
+        from packextras import platform_region_groups
+
+        db = load_database(str(REPO_ROOT / "database.json"))
+        profiles = load_emulator_profiles(str(REPO_ROOT / "emulators"))
+        config = load_platform_config("retrodeck", str(REPO_ROOT / "platforms"))
+        groups, _ = platform_region_groups(
+            config, config["systems"], str(REPO_ROOT / "emulators"), db,
+            config.get("base_destination", ""), profiles,
+        )
+        drops = region.resolve_region_drops(
+            groups, region.build_region_index(profiles), ["north-america"]
+        )
+        self.assertNotIn("bios/brd.rom", drops)
+
+
+if __name__ == "__main__":
+    unittest.main()