feat: name doubled destinations in exports

A platform list that declares one path for two files fails one of the
two checks on every install. The export report lists each declaration
with its hash and the emulators whose profile reads that file.

tests/test_export_report.py

@@ -202,17 +202,23 @@ class TheRunReportStatesWhatTheFileCarries(unittest.TestCase):
             {"name": "a.bin", "destination": "a.bin", "native_system": "psx",
              "md5": "a" * 32},
         ]}}}
+        doubled = {"destination": "a.bin", "declarations": []}
         with tempfile.TemporaryDirectory(dir=ROOT / "tmp") as scratch, \
                 mock.patch.object(export_native, "load_inputs",
                                   return_value=(truth, scraped)), \
                 mock.patch.object(export_native, "unpinned", return_value=[]), \
-                mock.patch.object(export_native, "removal_candidates", return_value=[]):
+                mock.patch.object(export_native, "removal_candidates", return_value=[]), \
+                mock.patch.object(export_native, "doubled_in", return_value=[doubled]):
             ok, messages = export_native.export_platform(
                 "plat", Plain, Path(scratch), Path(scratch) / "out",
                 str(ROOT / "platforms"), Path(scratch) / "cache", False,
             )
             self.assertTrue(ok, messages)
             report = json.loads((Path(scratch) / "out" / "plat.json").read_text())
+        self.assertEqual(report["doubled"], [doubled])
+        self.assertIn(
+            "1 destination declared twice for two files, listed in the report", messages
+        )
         self.assertEqual(
             report["summary"],
             "1 kept, 1 file added, 1 hash updated to the emulator source",
@@ -542,5 +548,115 @@ class RemovalCandidatesNeedAReadingNoCoreAnswers(unittest.TestCase):
         self.assertIn('"removal_candidates": candidates,', body)
 
 
+class ADestinationDeclaredTwiceIsNamed(unittest.TestCase):
+    """RetroDECK aims a PC-88 disk ROM and a CoCo disk ROM at one
+    bios/disk.rom, each with its own md5. One path holds one file, so one
+    check fails on every install. The export keeps both entries and names
+    the contradiction, with the emulators that read each file."""
+
+    PC88 = b"\x01" * 2048
+    COCO = b"\x02" * 8194
+
+    def setUp(self):
+        import hashlib  # noqa: PLC0415
+
+        self._tmp = tempfile.TemporaryDirectory(dir=ROOT / "tmp")
+        root = Path(self._tmp.name)
+        files, by_md5, by_name = {}, {}, {}
+        self.md5 = {}
+        for label, data, rel in (
+            ("pc88", self.PC88, "bios/NEC/PC-88/N88SUB.ROM"),
+            ("coco", self.COCO, "bios/Tandy/CoCo/disk.rom"),
+        ):
+            path = root / rel
+            path.parent.mkdir(parents=True)
+            path.write_bytes(data)
+            sha1 = hashlib.sha1(data).hexdigest()
+            md5 = hashlib.md5(data).hexdigest()
+            self.md5[label] = md5
+            files[sha1] = {"path": str(path), "name": path.name, "md5": md5,
+                           "sha1": sha1, "crc32": "", "size": len(data)}
+            by_md5[md5] = sha1
+            by_name.setdefault(path.name, []).append(sha1)
+        self.db = {"files": files, "indexes": {
+            "by_md5": by_md5, "by_name": by_name, "by_crc32": {}, "by_path_suffix": {},
+        }}
+        self.scraped = {"cores": ["pcemu", "cocoemu"], "systems": {
+            "nec-pc-88": {"files": [
+                {"name": "disk.rom", "destination": "bios/disk.rom",
+                 "md5": self.md5["pc88"], "native_system": "pc88",
+                 "component": "retroarch"},
+            ]},
+            "coco": {"files": [
+                {"name": "disk.rom", "destination": "bios/disk.rom",
+                 "md5": self.md5["coco"], "native_system": "coco",
+                 "component": "xroar"},
+            ]},
+        }}
+        self.profiles = {
+            "pcemu": {"emulator": "PC Emu", "type": "libretro", "systems": ["nec-pc-88"],
+                      "files": [{"name": "n88sub.rom", "aliases": ["disk.rom"],
+                                 "size": 2048, "source_ref": "libretro.c:120"}]},
+            "cocoemu": {"emulator": "CoCo Emu", "type": "libretro", "systems": ["coco"],
+                        "files": [{"name": "disk11.rom", "source_ref": "xroar.c:714"}]},
+        }
+
+    def tearDown(self):
+        self._tmp.cleanup()
+
+    def _doubled(self) -> list[dict]:
+        from exporter.doubled import doubled_destinations  # noqa: PLC0415
+
+        return doubled_destinations(self.scraped, self.profiles, self.db)
+
+    def test_each_declaration_is_named_with_its_reader(self):
+        doubled = self._doubled()
+        self.assertEqual([d["destination"] for d in doubled], ["bios/disk.rom"])
+        pc88, coco = doubled[0]["declarations"]
+        self.assertEqual((pc88["system"], pc88["component"]), ("pc88", "retroarch"))
+        self.assertEqual(pc88["hashes"], {"md5": [self.md5["pc88"]]})
+        self.assertTrue(pc88["file"].endswith("bios/NEC/PC-88/N88SUB.ROM"))
+        self.assertEqual(pc88["readers"], [{
+            "emulator": "PC Emu", "profile": "pcemu", "entry": "n88sub.rom",
+            "source_ref": "libretro.c:120",
+        }])
+        # A size the profile states rules out the CoCo ROM, and XRoar's own
+        # profile never opens disk.rom.
+        self.assertEqual((coco["system"], coco["component"]), ("coco", "xroar"))
+        self.assertEqual(coco["hashes"], {"md5": [self.md5["coco"]]})
+        self.assertEqual(coco["readers"], [])
+
+    def test_the_platform_entries_are_left_as_written(self):
+        import copy  # noqa: PLC0415
+
+        before = copy.deepcopy(self.scraped)
+        self._doubled()
+        self.assertEqual(self.scraped, before)
+
+    def test_a_destination_both_hashes_accept_is_not_doubled(self):
+        self.scraped["systems"]["coco"]["files"][0]["md5"] = (
+            f"{self.md5['coco']},{self.md5['pc88']}"
+        )
+        self.assertEqual(self._doubled(), [])
+
+    def test_the_run_counts_it_and_the_report_carries_it(self):
+        source = (ROOT / "scripts" / "export_native.py").read_text(encoding="utf-8")
+        body = source[source.index("def export_platform("):source.index("def write_report(")]
+        self.assertIn('"doubled": doubled,', body)
+        self.assertIn("declared twice for two files", body)
+
+    def test_the_page_names_each_declaration(self):
+        page = generate_exports_page(
+            [_report("retrodeck", ["a.json"], doubled=self._doubled())], {})
+        self.assertIn("1 destination of the original is declared twice", page)
+        self.assertIn("- `bios/disk.rom`", page)
+        self.assertIn(
+            f"  - `pc88` (`retroarch`) md5 `{self.md5['pc88']}`: read by PC Emu "
+            "(`libretro.c:120`)", page)
+        self.assertIn(
+            f"  - `coco` (`xroar`) md5 `{self.md5['coco']}`: no emulator this "
+            "platform runs is known to read this file", page)
+
+
 if __name__ == "__main__":
     unittest.main()