fix: read each file's digests once per run

tests/test_hashing.py

@@ -0,0 +1,45 @@
+"""compute_hashes reads a file once per process and change."""
+
+from __future__ import annotations
+
+import shutil
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+from unittest import mock
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(REPO_ROOT / "scripts"))
+
+import hashing  # noqa: E402
+
+
+class HashesAreReadOncePerContent(unittest.TestCase):
+    """verify --all hashed GB_dmg.bin 13 times and scph5501.bin 12, once
+    per platform and entry, in one process."""
+
+    def setUp(self):
+        self.dir = Path(tempfile.mkdtemp(dir=REPO_ROOT / "tmp"))
+        self.addCleanup(shutil.rmtree, self.dir, True)
+        self.file = self.dir / "rom.bin"
+        self.file.write_bytes(b"a" * 64)
+
+    def test_a_second_ask_reads_nothing(self):
+        first = hashing.compute_hashes(self.file)
+        with mock.patch.object(hashing, "_compute_hashes", side_effect=AssertionError):
+            self.assertEqual(hashing.compute_hashes(self.file), first)
+            self.assertEqual(
+                hashing.compute_hashes(self.file, frozenset({"md5"})),
+                {"md5": first["md5"]},
+            )
+
+    def test_a_rewrite_in_place_is_read_again(self):
+        first = hashing.compute_hashes(self.file)
+        with open(self.file, "r+b") as handle:
+            handle.write(b"b" * 64)
+        self.assertNotEqual(hashing.compute_hashes(self.file)["sha1"], first["sha1"])
+
+
+if __name__ == "__main__":
+    unittest.main()