fix: decide slots on the region pass grouping

tests/test_pack_grouping.py

@@ -0,0 +1,79 @@
+"""--one-per-slot and --region read one grouping of a pack.
+
+The slot pass put each core extra under every system of its profile. PicoDrive
+ranks its Mega CD BIOSes within sega-megacd; spread across sega-megadrive,
+sega-32x and the rest, they shared a tier with unranked files, the tier was
+ruled undecidable, and the keep overrode the drop.
+"""
+
+from __future__ import annotations
+
+import os
+import sys
+import unittest
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(REPO_ROOT / "scripts"))
+
+
+class SlotPassReadsTheRegionGrouping(unittest.TestCase):
+    def test_ranked_mega_cd_alternatives_are_decided(self):
+        if not (REPO_ROOT / "database.json").is_file():
+            self.skipTest("database.json not built")
+        import generate_pack as gp
+        from common import load_database, load_emulator_profiles, load_platform_config
+
+        previous = os.getcwd()
+        os.chdir(REPO_ROOT)
+        self.addCleanup(os.chdir, previous)
+        db = load_database("database.json")
+        profiles = load_emulator_profiles("emulators")
+        config = load_platform_config("retroarch", "platforms")
+        drops, _fallbacks, _undecided = gp._select_variants(
+            config, config["systems"], "emulators", db,
+            config.get("base_destination", ""), profiles, None, "full", None, True,
+        )
+        names = {os.path.basename(d) for d in drops}
+        for name in ("jp_mcd1_9111.bin", "eu_mcd1_9210.bin"):
+            self.assertIn(name, names)
+
+    def test_one_grouping_function(self):
+        source = (REPO_ROOT / "scripts" / "generate_pack.py").read_text(encoding="utf-8")
+        self.assertNotIn("def _pack_member_groups", source)
+
+
+class VariantGroupSpansSystems(unittest.TestCase):
+    def test_us_request_withdraws_the_other_mega_cd_lists(self):
+        """PicoDrive files US under sega-segacd and EU/JP under sega-megacd.
+
+        find_bios (platform/libretro/libretro.c:1296) fills one slot from one
+        of three lists. Grouped by system, sega-megacd had no US member and was
+        kept whole as a fallback.
+        """
+        if not (REPO_ROOT / "database.json").is_file():
+            self.skipTest("database.json not built")
+        import generate_pack as gp
+        import region as region_mod
+        from common import load_database, load_emulator_profiles, load_platform_config
+
+        previous = os.getcwd()
+        os.chdir(REPO_ROOT)
+        self.addCleanup(os.chdir, previous)
+        db = load_database("database.json")
+        profiles = load_emulator_profiles("emulators")
+        config = load_platform_config("retroarch", "platforms")
+        drops, _fallbacks, _undecided = gp._select_variants(
+            config, config["systems"], "emulators", db,
+            config.get("base_destination", ""), profiles, None, "full",
+            region_mod.parse_requested("us"), False,
+        )
+        names = {os.path.basename(d) for d in drops}
+        for name in ("jp_mcd1_9111.bin", "eu_mcd2_9306.bin"):
+            self.assertIn(name, names)
+        for name in ("us_scd2_9306.bin", "us_scd1_9210.bin"):
+            self.assertNotIn(name, names)
+
+
+if __name__ == "__main__":
+    unittest.main()