# Test Case: Nonzero 48-bit Root NID

**Test ID**: TC019-48bit-root-nid

## Objective

Verify the `48BIT && rootnid_8b != 0` selector uses the complete 64-bit root
field and interprets the two-byte union as `blocks_hi`.

## Fixture

Use `images/root8-48bit.erofs`. The structured transformer requires incompat
bit `0x80`, nonzero image-bounded `rootnid_8b`, `blocks_hi=0`, valid CRC32C,
and unchanged provider-sized `blocks_lo`. Production fsck.erofs 1.8.6 does not
recognize this feature; FreeBSD mount/read is mandatory.

## Procedure

Host field check:

```sh
python3 tests/erofs_fixture.py inspect /work/build/repo22-g1/images/root8-48bit.erofs --path=/root.txt
od -An -tu2 -j 1038 -N 2 /work/build/repo22-g1/images/root8-48bit.erofs
od -An -tx4 -j 1104 -N 4 /work/build/repo22-g1/images/root8-48bit.erofs
od -An -tu8 -j 1136 -N 8 /work/build/repo22-g1/images/root8-48bit.erofs
```

Mount the same hashed image in FreeBSD, then run:

```sh
stat -f 'root_ino=%i type=%HT' "$mnt"
cmp /tmp/repo22-g1/source/compact/root.txt "$mnt/root.txt"
sha256 /tmp/repo22-g1/source/compact/root.txt "$mnt/root.txt"
./statfs_probe "$mnt"
```

## Expected Results

- `feature_incompat & 0x80` is set, `rootnid_8b` is nonzero, and observed root
  inode equals that complete field.
- Root content matches the source and statfs uses `blocks_lo` plus zero
  `blocks_hi`; mount and cleanup succeed.
- TC150's zero-`rootnid_8b` fallback is a separate selector and is not accepted
  as evidence for this TC.