fix: cost the device prefix exactly rather than approximately

The budget subtracted the prefix length plus two, on the assumption of a
leading and a trailing slash. That is right for /Music and wrong for an empty
prefix, where there is only one slash -- losing a character at the card root,
which is exactly where the longest paths sit.

Computed from the prefix as it will actually appear instead: /Music/ costs
seven characters and gives a mirror-relative budget of 253, the root costs one
and gives 259.

Worth being exact about because the reverse error is worse. A checker
comparing mirror-relative paths against the flat 260 passes everything between
253 and 260, and those are precisely the paths closest to the edge.

README.md

@@ -241,7 +241,16 @@ run would move rather than what it would delete.
 Rockbox's `MAX_PATH` is 260, from `firmware/include/fs_defines.h`, and it bounds
 the path *as the device sees it*. The directory the mirror is copied into comes
 out of the same budget, so `--device-prefix` (default `/Music`) is subtracted
-from `--max-path` to get what a mirror-relative path may spend.
+from `--max-path` to get what a mirror-relative path may spend:
+
+| Destination on the device | Mirror-relative budget |
+| ------------------------- | ---------------------- |
+| `/Music/` | 253 |
+| the card root | 259 |
+
+Worth being exact about, because a checker that measures mirror-relative paths
+against the flat 260 quietly passes everything from 253 to 260 — and those are
+the paths most likely to be near the edge in the first place.
 
 Over-budget paths are shortened from the **deepest component outward**: the
 track name carries the least navigational value and the artist directory the

music_mirror.py

@@ -167,6 +167,18 @@ def fat32_safe(component):
     return cleaned or "_"
 
 
+def device_prefix_length(prefix):
+    """Return the on-device prefix as it will actually appear, with slashes.
+
+    "/Music" costs seven characters -- the leading slash, the name, and the
+    separator before the mirror's own path -- while an empty prefix costs one.
+    Approximating that loses a character at the root, which is precisely where
+    the longest paths are.
+    """
+    cleaned = prefix.strip("/")
+    return f"/{cleaned}/" if cleaned else "/"
+
+
 def shorten_component(component, budget):
     """Return a component of at most `budget` characters, cut from the middle.
 
@@ -783,7 +795,7 @@ def main(argv=None):
 
     # The device's limit covers the whole path it will see, so what the mirror
     # may spend is that less the directory it gets copied into.
-    budget = max(0, args.max_path - len(args.device_prefix.strip("/")) - 2)
+    budget = max(0, args.max_path - len(device_prefix_length(args.device_prefix)))
     if args.fat32_safe:
         logger.info(
             "paths are limited to %d characters, from --max-path %d less the %r prefix",

tests/test_music_mirror.py

@@ -764,3 +764,25 @@ def test_a_long_title_keeps_both_ends():
     assert len(str(fitted)) <= 253
     assert fitted.name.startswith("The Beatles - Abbey Road - 09 - The Long One")
     assert fitted.name.endswith("The End.mp3")
+
+
+@pytest.mark.parametrize(
+    ("prefix", "expected"),
+    [("/Music", 253), ("Music", 253), ("/Music/", 253), ("", 259), ("/", 259)],
+)
+def test_the_device_prefix_is_costed_exactly(prefix, expected):
+    """A mirror-relative path of 253 characters becomes 260 on the device once
+    /Music/ is in front of it, which is the whole of the limit. Approximating
+    the prefix loses a character at the root, where the longest paths are."""
+    assert 260 - len(music_mirror.device_prefix_length(prefix)) == expected
+
+
+def test_the_budget_is_reported_so_it_can_be_checked(tmp_path, make_flac, caplog):
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "a.flac")
+
+    with caplog.at_level("INFO"):
+        run(source, mirror, "--fat32-safe")
+
+    assert "limited to 253 characters" in caplog.text

tools/check_fat32.py

@@ -28,6 +28,18 @@ PATH_LIMIT = 260
 DEVICE_PREFIX = "/Music"
 
 
+def device_prefix_length(prefix):
+    """Return the on-device prefix as it will actually appear, with slashes.
+
+    "/Music" costs seven characters -- the leading slash, the name, and the
+    separator before the mirror's own path -- while an empty prefix costs one.
+    Approximating that loses a character at the root, which is precisely where
+    the longest paths are.
+    """
+    cleaned = prefix.strip("/")
+    return f"/{cleaned}/" if cleaned else "/"
+
+
 def problems_with(relative, budget=PATH_LIMIT):
     """Return every reason this relative path is unfit for FAT32."""
     found = []
@@ -68,7 +80,7 @@ def main(argv=None):
         f" of the budget (default {DEVICE_PREFIX})",
     )
     args = parser.parse_args(argv)
-    budget = max(0, args.max_path - len(args.device_prefix.strip("/")) - 2)
+    budget = max(0, args.max_path - len(device_prefix_length(args.device_prefix)))
 
     root = Path(args.root)
     if not root.is_dir():