fix: check the beebem teletext font size

tests/test_profile_checks.py

@@ -0,0 +1,56 @@
+"""A profile's checks are the ones the emulator's code runs.
+
+Each case reads a profile from emulators/ and offers its entry files of
+chosen sizes or contents, then compares the verdict with what the code at
+the profile's pin does with the same file. The cases come from reading that
+code: a check the code does not run rejects a file the emulator loads, and a
+check left out lets through a file the emulator refuses.
+"""
+
+from __future__ import annotations
+
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+
+import yaml
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(REPO_ROOT / "scripts"))
+
+from validation import _build_validation_index, check_file_validation  # noqa: E402
+
+
+def _profile(name: str) -> dict:
+    with open(REPO_ROOT / "emulators" / f"{name}.yml", encoding="utf-8") as fh:
+        return yaml.safe_load(fh)
+
+
+class _Probe(unittest.TestCase):
+    """Offer a profile entry a file and read the verdict."""
+
+    def setUp(self):
+        self._tmp = tempfile.TemporaryDirectory()
+        self.addCleanup(self._tmp.cleanup)
+
+    def accepts(self, emulator: str, name: str, data: bytes) -> bool:
+        index = _build_validation_index({emulator: _profile(emulator)})
+        path = Path(self._tmp.name) / name
+        path.write_bytes(data)
+        return check_file_validation(str(path), name, index, emulators={emulator}) is None
+
+
+class BeebEmTeletextFont(_Probe):
+    """BuildMode7Font reads 96 characters of 18 two-byte rows, 3456 bytes,
+    and returns false at the first EOF, which stops BeebEm at startup;
+    bytes past them are never read (Src/Video.cpp:167-215 at a982ba6)."""
+
+    def test_short_is_refused_and_longer_is_read(self):
+        self.assertFalse(self.accepts("beebem", "Teletext.fnt", b"\0" * 100))
+        self.assertTrue(self.accepts("beebem", "Teletext.fnt", b"\0" * 3456))
+        self.assertTrue(self.accepts("beebem", "Teletext.fnt", b"\0" * 4000))
+
+
+if __name__ == "__main__":
+    unittest.main()