fix: rank only held files in a slot

scripts/generate_pack.py

@@ -555,6 +555,7 @@ def _select_variants(
         slot_drops, slot_undecidable = slot_mod.resolve_slot_drops(
             slot_groups,
             slot_mod.platform_slot_index(config, emu_profiles or {}, target_cores),
+            held,
         )
         region_drops = region_drops | slot_drops
         if slot_undecidable:

scripts/slot.py

@@ -106,12 +106,14 @@ def lookup_slot(index: dict[str, dict], destination: str, name: str) -> dict | N
 def resolve_slot_drops(
     groups: dict[str, list[tuple[str, str]]],
     index: dict[str, dict],
+    held: set[str] | None = None,
 ) -> tuple[set[str], list[str]]:
     """Destinations to skip, and the groups no declared order can decide.
 
     Returns (drops, undecidable). A group is decided only when every candidate
     declares a priority and exactly one holds the lowest; anything else keeps
-    every candidate.
+    every candidate. ``held`` names the destinations the collection holds: a
+    candidate it lacks ships nothing, so it neither wins nor displaces one.
     """
     keep: set[str] = set()
     drop: set[str] = set()
@@ -123,6 +125,8 @@ def resolve_slot_drops(
     tiers: dict[tuple[str, tuple[str, ...]], list[tuple[str, int | None]]] = {}
     for group_id, members in groups.items():
         for destination, name in members:
+            if held is not None and destination not in held:
+                continue
             entry = lookup_slot(index, destination, name)
             if entry is None:
                 keep.add(destination)

tests/test_slot.py

@@ -297,5 +297,35 @@ class AnUnrankedDeclarationIsADisagreement(unittest.TestCase):
         self.assertEqual(drops, {"bios/b.bin"})
 
 
+
+class AnAbsentRankOneDisplacesNothing(unittest.TestCase):
+    """--one-per-slot received no `held`: a rank-1 file the collection lacks
+    dropped the rank-2 and rank-3 files it holds, leaving the slot empty."""
+
+    def test_the_best_held_file_wins(self):
+        from slot import build_slot_index, resolve_slot_drops
+
+        index = build_slot_index({"picodrive": {"files": [
+            {"name": "us_scd2_9306.bin", "priority": 1},
+            {"name": "SegaCDBIOS9303.bin", "priority": 2},
+            {"name": "us_scd1_9210.bin", "priority": 3},
+        ]}})
+        group = {"sega-mega-cd": [
+            ("bios/us_scd2_9306.bin", "us_scd2_9306.bin"),
+            ("bios/SegaCDBIOS9303.bin", "SegaCDBIOS9303.bin"),
+            ("bios/us_scd1_9210.bin", "us_scd1_9210.bin"),
+        ]}
+        held = {"bios/SegaCDBIOS9303.bin", "bios/us_scd1_9210.bin"}
+        drops, _ = resolve_slot_drops(group, index, held)
+        self.assertEqual(drops, {"bios/us_scd1_9210.bin"})
+
+    def test_the_builder_passes_held(self):
+        from pathlib import Path
+
+        source = (Path(__file__).resolve().parents[1] / "scripts" / "generate_pack.py").read_text()
+        compact = " ".join(source.split())
+        self.assertRegex(compact, r"resolve_slot_drops\( slot_groups, slot_mod\.platform_slot_index\([^)]*\), held, \)")
+
+
 if __name__ == "__main__":
     unittest.main()